Archive for the ‘GENETICS’ Tag

VIRUSES   Leave a comment

DEFINITION:1 orig., venom, as of a snake 2 a) any of a kingdom (Virus) of prokaryotes, usually ultra microscopic, that consist of nucleic acid, either RNA or DNA, within a case of protein: they infect animals, plants, and bacteria and can reproduce only within living cells so that they are considered as being either living organisms or inert chemicals b) a disease caused by a virus 3 anything that corrupts or poisons the mind or character; evil or harmful influence 4 an unauthorized, disruptive set of instructions placed in a computer program, that leaves copies of itself in other programs and disks.

1981: US AIDS diagnosed. A new fatal, infectious disease was diagnosed in 1981. Called Acquired Immunodeficiency Syndrome (AIDS), it began appearing in major cities among homosexual men and intravenous drug users. Other high-risk groups were haemophiliacs and other recipients of blood or blood products, babies born of AIDS-infected women, bisexual men, and prostitutes and their customers. AIDS was soon recognized as a worldwide health emergency: a fatal disease with no known cure that quickly became an epidemic. It was especially widespread in Africa, the apparent land of its origin.

By 1983 the virus that causes the disease had been isolated. Some medicines, notably AZT (azidothymidine), slowed the disease’s progress for a few months or more; but the spread of AIDS continued relentlessly, with more than 3,000 new cases being reported each month by 1991.

The federal government had committed more than 1.6 billion dollars to research, while the homosexual community and other special interest groups sought more federal funding and greater assistance from the health insurance industry. Educational programs on safe sexual practices, such as the use of condoms, seemed the best means of slowing the epidemic. Meanwhile, more than 70,000 persons in the United States had died from AIDS by the end of the decade.

1981: WORLD AIDS identified. A strange, new, and deadly disease made its appearance in 1980. Physicians in such large cities as Los Angeles, New York, and San Francisco noticed that homosexual men were dying from rare lung infections or from a cancer known as Kaposi’s sarcoma. By 1981 the disease was identified and given a name: AIDS, or acquired immunodeficiency syndrome.

The virus that causes AIDS, human immunodeficiency virus (HIV), was identified by Dr. Luc Montagnier of the Pasteur Institute in Paris in research done during the years 1981-84. The results of Dr. Montagnier’s studies were released in 1984. Since its discovery, AIDS has become one of the world’s major health problems. Within certain populations it has become an epidemic: male homosexuals, haemophiliacs, and intravenous drug users in the United States, for example, and heterosexual men and women in Sub-Saharan Africa. Many people were infected through blood transfusions before HIV screening was introduced. An individual infected with the virus may not show the symptoms of AIDS for several years, but the condition is eventually fatal.

The search for a successful vaccine was pursued in laboratories around the world, with no success by the early 1990s. Meanwhile, the disease continued to spread to different parts of the world. Already rife in the United States, Europe, and sub-Saharan Africa by the mid-1980s, it quickly spread to Central and East Asia. The disease also began to spread to larger portions of the heterosexual community throughout the world.

The composition of a virus is relatively simple, and its size is extremely small. It cannot even properly be called an organism because it is unable to carry on life processes outside a living cell of an animal, plant, or bacteria. Yet its method of entering and “enslaving” a living cell is so ingenious that the virus is humankind’s deadliest enemy and resists the most advanced efforts of modern science to eliminate it.

Millions of people throughout the world suffer each year from viral diseases such as polio, measles, chicken pox, mumps, acquired immunodeficiency syndrome (AIDS), and the common cold. Viruses also produce such illnesses as foot-and-mouth disease in livestock, distemper in dogs, panleukopenia in cats, and hog cholera. The viruses that infect bacteria are called bacteriophages.

Structure and Composition

Nucleic acid, any of substances comprising genetic material of living cells; divided into two classes: RNA (ribonucleic acid) and DNA (deoxyribonucleic acid); directs protein synthesis and is vehicle for transmission of genetic information from parent to offspring.

Viruses are exceedingly small; they range in size from about 0.02 to 0.25 micron in diameter (1 micron = 0.000039 inch). By contrast, the smallest bacteria are about 0.4 micron in size. As observed with an electron microscope, some viruses are rod-shaped, others are roughly spherical, and still others have complex shapes consisting of a multi sided “head” and a cylindrical “tail.” A virus consists of a core of nucleic acid surrounded by a protein coat called a capsid; some viruses also have an outer envelope composed of fatty materials and proteins. The nucleic-acid core is the essential part of the virus it carries the virus’s genes. The core consists of either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), substances that are essential to the transmission of hereditary information. The protein capsid protects the nucleic acid and may contain molecules that enable the virus to enter the host cell that is, the living cell infected by the virus.

Cycle and Patterns of Infection

Outside of a living cell, a virus is a dormant particle. It exhibits none of the characteristics generally associated with life namely, reproduction and metabolic processes such as growth and assimilation of food. Unlike bacteria and other micro-organisms, viruses remain dormant in body fluids. Thus, great numbers of viruses may be present in a body and yet not produce a disease because they have not invaded the body’s cells. Once inside a host cell, however, the virus becomes an active entity capable of taking over the infected cell’s metabolic machinery. The cellular metabolism becomes so altered that it helps to produce thousands of new viruses.

The virus’s developmental cycle begins when it succeeds in introducing its nucleic acid, and in some cases its protein coat, into a host cell. Bacteriophages attach to the surface of the bacterium and then penetrate the rigid cell wall, transmitting the viral nucleic acid into the host. Animal viruses enter host cells by a process called endocytosis. Plant viruses enter through wounds in the cell’s outer coverings through abrasions made by wind, for example, or through punctures made by insects.

Virion, an entire virus particle the extracellular infective form of a virus consisting of an outer protein shell (capsid) and an inner core of nucleic acid (either ribonucleic or deoxyribonucleic acid); in some, the capsid further enveloped by a fatty membrane.

Once inside the host cell, the virus’s genes usually direct the cell’s production of new viral protein and nucleic acid. These components are then assembled into new, complete, infective virus particles called virions, which are then discharged from the host cell to infect other cells.

In the case of bacteriophages, the new virions are usually released by bursting the host cell a process called lysing, which kills the cell. Sometimes, however, bacteriophages form a stable association with the host cell. The virus’s genes are incorporated into the host cell’s genes, replicate as the cell’s genes replicate, and when the cell divides, the viral genes are passed on to the two new cells.

In such cases no virions are produced, and the infecting virus seems to disappear. Its genes, however, are being passed on to each new generation of cells that stem from the original host cell. These cells remain healthy and continue to grow unless, as happens occasionally, something triggers the latent viral genes to become active. When this happens, the normal cycle of viral infection results: the viral genes direct viral replication, the host cell bursts, and the new virions are released. This pattern of infection is called lysogeny.

Closely related to lysogeny is the process known as transduction, whereby a virus carries bacterial genes from one host to another. This transduction process occurs when genes from the original host become incorporated into a virion that subsequently infects another bacterium.

Viral infections of plant and animal cells resemble those of bacterial cells in many ways. The release of new virions from plant and animal cells does not, however, always involve the bursting of the host cell as it does in bacteria. Particularly among animal viruses, the new virions may be released by budding off from the cell membrane, a process that does not necessarily kill the host cell.

In general, a viral infection produces one of four effects in a plant or animal cell: in apparent effect, in which the virus remains dormant in the host cell; cytopathic effect, in which the cell dies; hyperplastic effect, in which the cell is stimulated to divide before its death; and cell transformation, in which the cell is stimulated to divide, take on abnormal growth patterns, and become cancerous.

Cold sore (or fever blister, or Herpes simplex), a virus infection of the borders of the mouth, lips and nose, or genitals; marked by watery blisters; may be due to illness, emotional upset, or other stress.

Viral infections in animals can be localized or can spread to various parts of the body. Some animal viruses produce latent infections: the virus remains dormant much of the time but becomes active periodically. This is the case with the herpes simplex viruses that cause cold sores.

Natural Defences, Immunization, Treatment

Fever, a condition in which the body temperature rises above normal.

Animals have a number of natural defences that may be triggered by a viral infection. Fever is a general response; many viruses are inactivated at temperatures just slightly above the host’s normal body temperature. Another general response of infected animal cells is the secretion of a protein called interferon. Interferon inhibits the reproduction of viruses in non infected cells.

Fever and interferon production are general responses to infection by any virus. In addition, humans and other vertebrates can mount an immunological attack against a specific virus. The immune system produces antibodies and sensitized cells that are tailor-made to neutralize the infecting virus. These immune defenders circulate through the body long after the virus has been neutralized, thereby providing long-term protection against reinfection by that virus.

Such long-term immunity is the basis for active immunization against viral diseases. A weakened or inactivated strain of an infectious virus is introduced into the body. This virus does not provoke an active disease state, but it does stimulate the production of immune cells and antibodies, which then protect against subsequent infection by the virulent form of the virus.

Active immunizations are routine for such viral diseases as measles, mumps, poliomyelitis, and rubella. In contrast, passive immunization is the injection of antibodies from the serum of an individual who has already been exposed to the virus. Passive immunization is used to give short-term protection to individuals who have been exposed to such viral diseases as measles and hepatitis. It is useful only if provided soon after exposure, before the virus has become widely disseminated in the body.

The treatment of an established viral infection usually is restricted to relieving specific symptoms. There are few drugs that can be used to combat a virus directly. The reason for this is that viruses use the machinery of living cells for reproduction. Consequently, drugs that inhibit viral development also inhibit the functions of the host cell. Nonetheless, a small number of antiviral drugs are available for specific infections.

The most successful controls over viral diseases are epidemiological. For example, large-scale active immunization programs can break the chain of transmission of a viral disease. Worldwide immunization is credited with the eradication of smallpox, once one of the most feared viral diseases. Because many viruses are carried from host to host by insects or contaminated food, insect control and hygienic food handling can help eliminate a virus from specific populations.

History of Virus Research

Historic descriptions of viral diseases date back as far as the 10th century BC. The concept of the virus, however, was not established until the last decade of the 19th century, when several researchers obtained evidence that agents far smaller than bacteria were capable of causing infectious diseases.

Mosaic disease, highly infectious virus disease affecting many plants including cucumber, potato, tomato, bean, and turnip; dwarfs growth and mottles leaves.

The existence of viruses was finally proved when bacteriophages were discovered by independent researchers in 1915 and 1917. The question of whether viruses are actually micro-organisms (similar to very tiny bacteria) was resolved in 1935, when the virus responsible for causing mosaic disease in tobacco was isolated and crystallized; the fact that it could be crystallized proved that the virus was not a cellular organism.

Bacteriophages are a valuable research tool for molecular biologists. Studies of bacteriophages have helped to illuminate such basic biological processes as genetic recombination, nucleic-acid replication, and protein synthesis.

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GENETICS (Part 1 of 3)   Leave a comment

DEFINITION:1 the branch of biology that deals with heredity and variation in similar or related animals and plants 2 the genetic features or constitution of an individual, group, or kind.

April 28, 1994: Biological clock found in mice. Evidence for a so-called biological clock in mice was announced by scientists at Northwestern University in Illinois. It was the first time that a gene governing the daily cycle of waking and sleeping, called the circadian rhythm, had been found in mammals. Previously, genes governing circadian rhythms had been found only in fruit flies and bread mould The biological clock gene in mice was found on mouse chromosome number 5. The chromosomes of all living things hold the DNA, which determines the genetic make-up of each individual. Scientists hoped that this research would someday help them find a similar gene governing the biological clock in humans.

Why do human children resemble their parents? Why do the offspring of any species resemble their parents? Biologists have shown that the factors which cause such resemblances are passed on relatively unaltered from generation to generation by a process called heredity. Resemblances, they say, are transmitted by genes, cell units too tiny to be seen even with a microscope. The branch of biology that deals with genes is called genetics.

Through the ages men have speculated about heredity. In ancient Greece, for example, it was thought that the blood was in some way responsible for the transmission of hereditary traits, and the word “blood” is still often used to mean ancestry. Since the beginning of the 20th century, however, genes have been known to be the carriers of traits, though until the 1940s very little was known about them. Scientists recognized that genes were directly responsible for the characteristics of an organism and that genes were transmitted from parents to offspring. However, they had little idea of the gene structure and composition that made these actions possible.

By the 1950s scientists had learned a great deal about the chemistry of genes. Genes were found to be segments of certain complex molecules located in the cell nucleus. The molecules have the unique ability to duplicate themselves and, in so doing, to pass on body-building instructions to the next generation of a species.

THE ORIGINS OF MODERN GENETICS

Even before the beginnings of written history people were aware of some of the ways in which heredity takes place. The domesticated animals and plants of today are proof of this. Today’s domesticated horses, cattle, dogs, corn, wheat, and cotton differ greatly from their primitive, “wild” ancestors. They are products of the ancient breeders’ art, an art that included the proper selection of parents, well-controlled matings, and the careful choice of the best offspring to further improve a breed.

Early Theories of Heredity

Over the centuries more and more became known about the control of heredity for practical purposes. However, scientists remained baffled about the actual processes of trait transmission. All sorts of what proved to be erroneous explanations were advanced. In the 17th century, for example, a group of biologists called the ovists held that the ovaries of females contained the hereditary material and that the male sperm merely triggered embryonic development. Other scientists were of the opinion that tiny but fully formed creatures were present in the sperm.

Early in the 19th century the French biologist Jean Baptiste Lamarck suggested that traits and abilities acquired during the lifetime of an organism could be transmitted to future generations. This theory was termed “the inheritance of acquired characteristics.” Long before Lamarck, notions of this kind had led expectant mothers to practice the piano, gaze at beautiful pictures, or think “kind” thoughts in the hope that this would affect the character of their unborn children. For similar reasons, many breeders exposed plants and animals to the environmental conditions their breeding programs were intended to combat. Genetic discoveries in the mid-1800s proved Lamarck’s view to be mistaken.

1859: Darwin’s theory of evolution. A heated debate that continues to this day was sparked in 1859 with the publication of Charles Darwin’s ‘On the Origin of Species by Means of Natural Selection’. This work was immediately recognized by the scientific community as a landmark treatise on biology and evolution, but some Christians saw it as a threat to their theology.

Charles Darwin began his observations in December 1831 when, at age 22, he left England for South America aboard the exploratory ship HMS Beagle. During this five-year voyage Darwin observed many species of animals and birds and collected many fossils. His observations on the differences and similarities of species, both living and extinct, led him to ask many questions: Why did some species survive and others die out? Why did certain species live in certain places and not in others? These questions preoccupied him when he returned to England in 1836.

Darwin’s observations led him to doubt the commonly held belief that all the species had been created at once and had remained unchanged through time. The problem was to find out what forces made organisms change. Darwin’s answer was his theory of natural selection: certain members of a species have traits that make them better adapted to their environment. These animals are more successful and therefore have more surviving offspring that inherit these traits. Animals that are not well adapted do not have as many offspring and eventually die out. In this way, species change and certain groups become extinct.

Although Darwin devoted much of his time to his theory of natural selection, he did not publish it for more than 20 years. He knew that his explanation of the species would anger many people, since it did not agree with the dominant Christian theology of the time. Despite early scientific and religious opposition, Darwin’s theory of natural selection is now accepted as the explanation of evolution, at least within the scientific community. However, arguments continue between evolutionists and creationists (those who believe all species were created by God in their present form). Darwin’s theories have indeed changed the way most people view the world, from the evolution of humans to the philosophical bases of science itself.

1865: The birth of genetics. It was unfortunate for the biological sciences that Gregor Mendel was an obscure Austrian monk. His pioneering work in the field of genetics was being done at the time that Charles Darwin’s publications on evolution were beginning to create worldwide controversy, but Mendel’s work would remain unknown for years.

Mendel became an Augustinian monk in 1843, but his abilities in mathematics and the sciences were evident. His experiments on the principles of heredity were begun in about 1856 in what is now Czechoslovakia. By crossing various strains of peas with one another, Mendel found that traits were passed on from generation to generation in what he called “discrete hereditary elements” in sex cells, or gametes.

Mendel reported the results of his experiments to a local society for the study of natural science in 1865 and published his findings in the society’s journal. They were as good as buried there for the next 35 years. Although the journal found its way to libraries in Europe and North America, few paid any attention to his writings. When other botanists obtained results similar to Mendel’s, they began searching through earlier writings on the subject. Only then was Mendel’s 1865 research revealed. His “discrete hereditary elements” are now called genes, and the new science once called Mendelism is known as genetics.

Two Pioneers of Genetics

In 1859 the English biologist Charles Darwin published his epic ‘The Origin of Species’, an attempt to demonstrate that all living things are related through the common bond of evolution. Darwin assumed that all species produce more offspring than reach maturity. Those offspring that survive and reproduce, he reasoned, do so because they are better suited to the existing environment. Because environment changes with time, he argued, species must either adapt to the new conditions or become extinct. Darwin did not know just what mechanisms made it possible for such changes in species to take place. He recognized, however, that if his theory were correct, changeable or mutable units of heredity must exist and that variations in species must arise as a result of an accumulation of small changes in these units of heredity.

In 1865 Gregor Mendel, a monk in an Austrian Roman Catholic monastery, wrote a paper that laid the foundation for modern genetics. Mendel was the first to demonstrate experimentally the manner in which specific traits are passed on from one generation to the next. He concluded that “discrete hereditary elements” (not called genes until the 1900s) in the sex cells are responsible for the transmission of traits. Mendel was ahead of his time, however. The significance of his work was not realized until 1900.

Mendel’s Contributions to Genetics

Pea, a climbing pod-bearing plant (Pisum sativum), or its seed.

In the monastery garden where he conducted his experiments, Mendel observed the inheritance of traits in the easily available garden pea, Pisum sativum. The plant is an ideal genetic working material because a number of progeny can be produced in a short time and because its reproductive parts are so constructed that accidental fertilization is nearly impossible.

Mendel began by tracing the inheritance of one or two contrasting traits at a time. Thus, he crossed tall peas with short peas or red-flowered peas with white-flowered peas. Then he recorded how many of the progeny developed each of the contrasting traits. He used the progeny in subsequent matings to follow the progress of the traits under study through a number of generations.

Somatic cells (or body cells), cells of the body that compose the tissues, organs, and parts of that individual other than the germ cells.

Gamete (or germ cell), sex cell that fuses with a cell of the opposite sex to form new life.

From the evidence obtained in this way, Mendel reasoned that contrasting traits are governed by units of inheritance existing in pairs in somatic, or body, cells but singly in gametes, or sex cells. If the genotype R stands for red and the genotype r for white, then homozygous red-flowered peas have RR somatic cells and R gametes. The somatic cells and gametes of homozygous white-flowered peas are, by contrast, rr and r, respectively.

Allele, in genetics; an alternate form of gene located on a specific site on a chromosome.

The separation of alleles (R from r, for example) in gamete formation is called the principle of segregation. Mendel correctly assumed that chance determines which gene of a pair finds its way into a given gamete. A red-flowered pea may be a heterozygous, or hybrid, Rr. That is, in some way the allele for red flowers (R) “dominates” the allele for white flowers (r). However, the R and r alleles of the hybrid segregate during sex-cell division to produce an equal number of R and r gametes. This is proved by test crossing the hybrid with a homozygous white (rr) plant. Since the homozygous white produces only r gametes and the hybrid produces both R and r gametes, the ratio of red plants to white plants is one to one.

Mendel also demonstrated that non allelic genes (for tall or short and red or white phenotypes, for example) segregate independently of one another into the gametes. This phenomenon is called the principle of independent assortment. For example, a cross between pure strains of tall plants with red flowers (TTRR) and short plants with white flowers (ttrr) produces hybrid progeny that are all tall with red flowers (TtRr). A test cross between these tall, red hybrids (TtRr) and short, white pure strains (ttrr) results in four equally distributed types of progeny 25 percent tall, red TtRr, 25 percent short, red ttRr, 25 percent tall, white Ttrr, and 25 percent short, white ttrr. Modern geneticists have learned, however, that independent assortment does not always hold true because non alleles located side by side on the same chromosome tend to be inherited as a package.

1953: Discovery of DNA structure. The full name of DNA is deoxyribonucleic acid. It carries the codes of genetic information that transmit inherited characteristics to successive generations of living things.

DNA was discovered in 1869 by Friedrich Miescher. In 1943 its role in inheritance was demonstrated. In 1953 its structure was determined by an American biochemist, James D. Watson, and an English physicist, Francis H.C. Crick. Watson and Crick showed the structure to be two strands of a phosphoryl-deoxyribose polymer arranged as a double helix. Watson and Crick were awarded the Nobel prize in physiology or medicine in 1962.

1973: Biotechnology. Two American biochemists, Stanley H. Cohen and Herbert W. Boyer, inaugurated the science of genetic engineering and its associated field of biotechnology in 1973. They showed that it was possible to break down DNA into fragments and combine them into new genes, which could in turn be placed in living cells. There they would reproduce each time a cell divided into two parts.

Genetic engineering makes it possible to modify existing organisms or create organisms that already exist in the human body but that are difficult to isolate. For example, one early product was a genetically engineered form of insulin, used in the treatment of diabetes. Other genetically engineered products include interferons, which are used in the treatment of viral infections and showed promise in the treatment of various forms of cancer. Scientists hope that genetically engineered products will someday prevent or cure such genetic disorders as muscular dystrophy and cystic fibrosis.

Genetic engineering also opens the possibility of creating entirely new organisms. In 1980 the United States Supreme Court ruled that newly developed organisms could be patented, thus giving ownership rights to the companies that made them.

Posted 2012/04/19 by Stelios in Education

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GENETICS (Part 2 of 3)   Leave a comment

Genetic Research After Mendel

Chromosome, microscopic, threadlike part of the cell that carries hereditary information in the form of genes; among simple organisms, such as bacteria and algae, chromosomes consist entirely of DNA and are not enclosed within a membrane; among all other organisms chromosomes are contained in a membrane-bound cell nucleus and consist of both DNA and RNA; arrangement of components in the DNA molecules determines the genetic information; every species has a characteristic number of chromosomes, called the chromosome number; in species that reproduce asexually the chromosome number is the same in all the cells of the organism; among sexually reproducing organisms, each cell except the sex cell contains a pair of each chromosome.

Weismann, August (1834-1914), German biologist; advanced theory that changes in the characteristics of a species are due to changes in germ plasm.

Sutton, Walter S. (1876-1916), U.S. geneticist and physician; noted for studies of chromosomes.

Boveri, Theodor Heinrich (1862-1915), German scientist whose work with roundworm eggs proved that chromosomes are separate, continuous entities within the nucleus of a cell.

Chromosomes, structures in the cell nucleus that carry genes, were discovered after Mendel’s work was published. However, accurate accounts of their behaviour were not generally available until about 1885. Earlier the German biologist August Weismann had suggested that heredity depends on a special material called germ plasma that is transmitted unaltered from one generation to another. In the 1880s Weismann and other scientists advanced the idea that the germ plasm was located in the chromosomes. In 1902 Walter S. Sutton of the United States and Theodor Boveri of Germany independently recognized the connection between the segregation of alleles as described by Mendel and the segregation of homologous pairs of chromosomes in the division of sex cells.

Morgan, Thomas Hunt (1866-1945), U.S. zoologist, born in Lexington, Ky.; professor Columbia University 1904-28; director of biological laboratories, California Institute of Technology; received 1933 Nobel prize for work on role of chromosomes in heredity; wrote books on embryology, evolution, and heredity.

In 1910 the American geneticist Thomas H. Morgan and his associates discovered that genes occur on chromosomes and that those genes lying close together on the same chromosome form linkage groups that tend to be inherited together. They also showed that linkage groups often break apart naturally as a result of a phenomenon called crossing over.

Beadle, George Wells (1903-89), U.S. biologist, born near Wahoo, Neb.; professor and chairman of biology division California Institute of Technology 1946-60, acting dean of faculty 1960-61; president University of Chicago 1961-68; director Institute of Biomedical Research, AMA, 1968-70; received 1958 Nobel prize for work in biochemical and microbial genetics.

Tatum, Edward Lawrie (1909-75), U.S. biochemist, born in Boulder, Colo.; professor Yale University 1946-48, Stanford University 1948-57, and Rockefeller University 1957-75; received 1958 Nobel prize for discovery that genes act by controlling specific chemical processes.

Avery, Oswald Theodore (1877-1955), U.S. bacteriologist who determined that deoxyribonucleic acid (DNA) is the basic genetic material of the cell.

Watson, James Dewey (born 1928), U.S. biochemist, born in Chicago, Ill.; on staff Harvard University 1955-68, professor 1961-68; director Cold Spring Harbor Laboratory from 1968; received 1962 Nobel prize for discovery of molecular structure of DNA.

Crick, Francis Harry Compton (born 1916), British biochemist; on staff Cavendish Laboratory, Cambridge University 1949-77; professor Salk Institute for Biological Studies from 1977; received 1962 Nobel prize for discovery of molecular structure of DNA; elected to U.S. National Academy of Sciences 1969.

Jacob, Francois (born 1920), French biologist, born in Nancy; with Pasteur Institute from 1950, College de France from 1964; received 1965 Nobel prize for work in genetics.

Monod, Jacques (1910-76), French biologist, born in Paris; with Pasteur Institute from 1945, director from 1971; received 1965 Nobel prize for work in genetics; researched protein metabolism and RNA.

In the 1940s George W. Beadle and Edward L. Tatum of the United States began to investigate the role played by genes in the production of enzymes. By 1944 Oswald T. Avery had discovered that deoxyribonucleic acid (DNA) was the basic genetic material of the cell. The precise molecular structure of DNA was determined in 1953 by James D. Watson of the United States and Francis H.C. Crick of England. By 1961 the French geneticists Francois Jacob and Jacques Monod had developed a model for the process by which DNA directs the synthesis of proteins, thereby deciphering, in principle, the genetic code of the DNA molecule. In 1988 an international team of scientists began a project to devise a map of the human genome, all the genes that determine the make-up of a human being.

Recombinant DNA, genetically engineered DNA prepared in vitro by cutting up DNA molecules and splicing together specific DNA fragments; usually uses DNA from more than one species of organism.

Clone, process of biologically purifying a gene from one species by inserting it into the DNA of another species where it is replicated along with the host DNA; used to manufacture insulin.

Since the 1970s the techniques of recombinant DNA have allowed researchers to biologically purify, or clone, a gene from one species by inserting it into the DNA of another species, where it is replicated along with the host DNA. In this manner human hormones, such as insulin and growth hormone, have been manufactured economically by colonies of bacteria.

CHROMOSOMES AND CELL DIVISION

Chromosomes are mainly aggregates of deoxyribonucleic acid (DNA) and protein. All but the simplest kinds of plants and animals inherit two sets of chromosomes (the diploid number), one set (the haploid number) from each parent. In humans, each somatic cell has a haploid set of 23 chromosomes from each parent, for a total of 46.

The chromosomes within each set vary in appearance. However, each has a homologous partner in the other set, which resembles it in both appearance and genetic characteristics. A given gene is found on only a particular chromosome in each set. Its allele is on that chromosome’s homologue in the other set. The alleles are passed on to new cells during mitosis, the division of somatic cells.

Mitosis takes place as soon as a sperm fertilizes an egg. It continues throughout the life of the organism. Prior to mitosis, the cell chromosomes make exact copies of themselves. At this point, twice the diploid number of chromosomes exist in the cell. As mitosis proceeds, one set of the doubled chromosomes goes into each of the two daughter cells. Each thus acquires a full diploid set of chromosomes. This process is repeated again and again as cells divide and the body grows. Sex cells, however, divide in a different way.

Sex cells in the adult reproductive organs produce gametes by meiosis. This process consists of two divisions. As the first division proceeds, the homologous chromosomes in the nucleus of the sex cell seek each other out and join, or synapse. They are called bivalents at this point.

Then the bivalents duplicate themselves to form a bundle, or tetrad, or four intertwined chromatids. The tetrads then thicken and separate, and a pair of homologous chromatids pass into each of two daughter cells.

Meiosis does not stop at this stage, however. The two daughter cells, still with a diploid number of chromosomes, undergo a second division, the reduction division. In this division, the homologous chromatids do not duplicate themselves but merely separate and pass randomly into two additional cells, where they thicken into chromosomes. In meiosis, each sex cell produces four gametes, each with a haploid number of chromosomes (only one allele is in each gamete). When a male gamete fertilizes an egg, the diploid number of chromosomes is restored.

Chromosomes are fully visible under a microscope during the four stages of cell division prophase, metaphase, anaphase, and telophase. However, between the telophase and the next prophase a lengthy period called the interphase occurs, during which the chromosomes are too thin and strung out to be seen. Important chemical activities take place during the interphase. Ribonucleic acid (RNA), chemically related to DNA, and proteins are synthesized during the lengthy interphase as well as during the relatively short period of cell division.

Late in the interphase, DNA is synthesized and daughter chromosomes are created. First, DNA is made. Soon afterwards, in a burst of activity, chromosomal DNA, RNA, and protein are fitted together, the chromosomes begin to take shape, and cell division begins. During sex cell division, however, an important gene exchange between homologous chromosomes takes place.

Linked Non alleles and Crossing Over

As meiosis takes place, homologous chromosomes exchange some of their genes. This phenomenon is known as crossing over. Although the process is not well understood, it is thought that a reciprocal breakage and rejoining of homologous chromatids occurs while the tetrads are intertwined during early meiosis.

Geneticists began to investigate crossing over when they noted that the traits actually inherited did not always adhere to the principle of independent assortment. Test crosses between AaBb and aabb parents A, a, B, and b representing the dominant and recessive genes of non alleles did not always produce equal numbers of AaBb, aaBb, Aabb, and aabb progeny but a greater number of the parental types AaBb and aabb and a smaller number of the recombinant types Aabb and aaBb. Geneticists concluded that the dominant non alleles A and B were linked together on one homologous chromosome and that the recessive non alleles a and b were linked together on the other. If this linkage were unbreakable, in meiosis the hybrid AaBb would form only AB and ab gametes. In fact, however, Ab and aB gametes were also formed the frequency varying for different linked non alleles It was therefore surmised that an exchange, or crossing over, took place.

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GENETICS (Part 3 of 3)   Leave a comment

Sex Linkage

Linked genes occur on the sex chromosomes as well as on the non sex chromosomes, or autosomes. In humans, a woman carries two X chromosomes and 44 autosomes in each body cell and one X chromosome and 22 autosomes in each egg. A man carries one X and one Y chromosome and 44 autosomes in each body cell and either an X or a Y chromosome and 22 autosomes in each sperm cell.

Only sons inherit traits carried by genes located on the Y chromosome, because a boy (XY) develops whenever a Y sperm fertilizes an egg. Traits carried on genes located on an X chromosome of the father are transmitted only to daughters (XX).

GENES AND THE GENETIC CODE

Genes, the arbiters of body form and organ function, work with precision. They transmit to each cell a genetic code that determines the cell’s purpose.

Nucleic Acids The Key to Heredity

Nucleic acid, any of substances comprising genetic material of living cells; divided into two classes: RNA (ribonucleic acid) and DNA (deoxyribonucleic acid); directs protein synthesis and is vehicle for transmission of genetic information from parent to offspring.

The structure of DNA makes gene transmission possible. Since genes are segments of DNA, DNA must be able to make exact copies of itself to enable the next generation of cells to receive the same genes.

Adenine, a purine base that codes hereditary information in the genetic code in DNA and RNA.

Cytosine, pyrimidine base that codes genetic information in DNA or RNA.

The DNA molecule looks like a twisted ladder. Each “side” is a chain of alternating phosphate and deoxyribose sugar molecules. The “steps” are formed by bonded pairs of purine-pyrimidine bases. DNA contains four such bases the purines adenine (A) and guanine (G) and the pyrimidines cytosine (C) and thymine (T).

The RNA molecule, markedly similar to DNA, usually consists of a single chain. The RNA chain contains ribose sugars instead of deoxyribose. In RNA, the pyrimidine uracil (U) replaces the thymine of DNA.

DNA and RNA are made up of basic units called nucleotides. In DNA, each of these is composed of a phosphate, a deoxyribose sugar, and either A, T, G, or C. RNA nucleotides consist of a phosphate, a ribose sugar, and either A, U, G, or C.

Nucleotide chains in DNA wind around one another to form a complete twist, or gyre, every ten nucleotides along the molecule. The two chains are held fast by hydrogen bonds linking A to T and C to G A always pairs with T (or with U in RNA); C always pairs with G. Sequences of the paired bases are the foundation of the genetic code. Thus, a portion of a double-stranded DNA molecule might read: A-T C-G G-C T-A G-C C-G A-T. When “unzipped,” the left strand would read: ACGTGCA; the right strand: TGCACGT.

DNA is the “master molecule” of the cell. It directs the synthesis of RNA. When RNA is being transcribed, or copied, from an unzipped segment of DNA, RNA nucleotides temporarily pair their bases with those of the DNA strand. In the preceding example, the left hand portion of DNA would transcribe a strand of RNA with the base sequence: UGCACGU.

Genes and Protein Synthesis

A genetic code guides the assembly of proteins. The code ensures that each protein is built from the proper sequence of amino acids.

Genes transmit their protein-building instructions by transcribing a special type of RNA called messenger RNA (mRNA). This leaves the cell nucleus and moves to structures in the cytoplasm called ribosomes, where protein synthesis takes place.

Cell biologists believe that DNA also builds a type of RNA called transfer RNA (tRNA), which floats freely through the cell cytoplasm. Each tRNA molecule links with a specific amino acid. When needed for protein synthesis, the amino acids are borne by tRNA to a ribosome.

For years biologists wondered how amino acids were guided to fit together in the exact sequences needed to produce the thousands of kinds of proteins required to sustain life. The answer seems to lie in the way the four genetic “code letters” A, T, C, and G are arranged along the DNA molecule.

The Genetic Code

Experimental evidence indicates that the genetic code is a “triplet” code; that is, each series of three nucleotides along the DNA molecule orders where a particular amino acid should be placed in a growing protein molecule. Three-nucleotide units on an mRNA strand for example UUU, UUG, and GUU are called codons. The codons, transcribed from DNA, are strung out in a sequence to form mRNA.

According to the triplet theory, tRNA contains anti codons, nucleotide triplets that pair their bases with mRNA codons. Thus, AAA is the anti codon for UUU. When a codon specifies a particular amino acid during protein synthesis, the tRNA molecule with the anti codon delivers the needed amino acid to the bonding site on the ribosome.

The genetic code consists of 64 codons. However, since these codons order only some 20 amino acids, most, if not all, of the amino acids can be ordered by more than one of them. For example, the mRNA codons UGU and UGC both order cysteine. Because mRNA is a reverse copy of DNA the genetic code for cysteine is ACA or ACG. Some codons may act only to signal a halt to protein synthesis.

To illustrate the operation of the genetic code, assume that one protein is responsible for the development of brown hair and that this protein is composed of three amino acid molecules arranged in linear sequence for example, cysteine-cysteine-cysteine. (This is a much simplified example, since proteins actually incorporate from 100 to 300 amino acid molecules.) The gene (DNA segment) specifying formation of this protein reads: ACAACAACA. It produces the mRNA segment UGUUGUUGU. This segment then drifts to a ribosome. Three tRNA molecules, each with the cysteine-bearing anti codon ACA, line up in order on the ribosome and deposit their cysteine to make the brown-hair protein.

Since code transmission from DNA to mRNA is extremely precise, any error in the code affects protein synthesis. If the error is serious enough, it eventually affects some body trait or feature.

Mutations

Down’s syndrome (or mongolism), a congenital condition with moderate to severe mental retardation; characteristic features include: broad flat faces, slanted eyes, small ears and noses; heart defects and other abnormalities.

Certain chemicals and types of radiation can cause mutations changes in the structure of genes or chromosomes. The simplest type of mutation is a change in the DNA or RNA nucleotide sequence. Mutations may also involve the number of chromosomes or the gain, loss, or rearrangement of chromosome segments. If a mutation occurs in parental sex cells, the change is passed on to the offspring. In humans, an extra chromosome in body cells (47 instead of 46) has been implicated in Down’s syndrome, a serious mental abnormality.

Most mutations are considered harmful and are, therefore, eventually eliminated. Some, however, enable an organism to adapt to a changing environment. Biologists believe that mutations have caused the many genetic changes involved in the evolution of species.

Assisted by Val W. Woodward

Genetic Terms

allele. One of the members of a gene pair, each of which is found on chromosomes; the pair of alleles determines a specific trait.

chromosome. A structure in the cell nucleus containing genes.

dominance. The expression of one member of an allelic pair at the expense of the other in the phenotypes of heterozygotes.

gene. One of the chromosomal units that transmit specific hereditary traits; a segment of the self-reproducing molecule, deoxyribonucleic acid.

genotype. The genetic make-up of an organism, which may include genes for the traits that do not show up in the phenotype.

heterozygous. Containing dissimilar alleles.

homozygous. Containing a pair of identical alleles.

phenotype. The visible characteristics of an organism (for example, height and colouration).

recessiveness. The masking of one member of an allelic pair by the other in the phenotypes of heterozygotes.

Posted 2012/04/19 by Stelios in Education

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HUMAN DISEASES (Part 1 of 7)   Leave a comment

A disease is a condition that impairs the proper function of the body or of one of its parts. Every living thing, both plants and animals, can succumb to disease. People, for example, are often infected by tiny bacteria, but bacteria, in turn, can be infected by even more minute viruses.

Hundreds of different diseases exist. Each has its own particular set of symptoms and signs, clues that enable a physician to diagnose the problem. A symptom is something a patient can detect, such as fever, bleeding, or pain. A sign is something a doctor can detect, such as a swollen blood vessel or an enlarged internal body organ.

Every disease has a cause, although the causes of some remain to be discovered. Every disease also displays a cycle of onset, or beginning, course, or time span of affliction, and end, when it disappears or it partially disables or kills its victim.

Endemic disease (also called childhood disease), disease continually prevalent in a region.

An epidemic disease is one that strikes many persons in a community. When it strikes the same region year after year it is an endemic disease.

An acute disease has a quick onset and runs a short course. An acute heart attack, for example, often hits without warning and can be quickly fatal. A chronic disease has a slow onset and runs a sometimes years-long course. The gradual onset and long course of rheumatic fever makes it a chronic ailment.

How Germs Invade the Body

Humans live in a world where many other living things compete for food and places to breed. The pathogenic organisms, or pathogens, often broadly called germs, that cause many diseases are able to invade the human body and use its cells and fluids for their own needs. Ordinarily, the body’s defence system can ward off these invaders.

Pathogenic organisms can enter the body in various ways. Some such as those that cause the common cold, pneumonia, and tuberculosis are breathed in. Others such as those that cause venereal diseases enter through sexual contact of human bodies. Still others such as those that cause bacillary dysentery, cholera, and typhoid fever get in the body through contaminated food, water, or utensils.

Insects can spread disease by acting as vectors, or carriers. Flies can carry germs from human waste or other tainted materials to food and beverages. Germs may also enter the body through the bite of a mosquito, louse, or other insect vector.

Kinds of Disease

Infectious, or communicable, diseases are those that can be passed between persons such as by means of airborne droplets from a cough or sneeze. Tiny organisms such as bacteria and fungi can produce infectious diseases. So can viruses. So can tiny worms. Whatever the causative agent might be, it survives in the person it infects and is passed on to another. Or, its eggs are passed on. Sometimes, a disease-producing organism gets into a person who shows no symptoms of the disease. The asymptomatic carrier can then pass the disease on to someone else without even knowing he has it.

Non-infectious, or non-communicable, diseases are caused by malfunctions of the body. These include organ or tissue degeneration, erratic cell growth, and faulty blood formation and flow. Also included are disturbances of the stomach and intestine, the endocrine system, and the urinary and reproductive systems. Some diseases can be caused by diet deficiencies, lapses in the body’s defence system, or a poorly operating nervous system.

Disability and illnesses can also be provoked by psychological and social factors. These ailments include drug addiction, obesity, malnutrition, and pollution-caused health problems.

Furthermore, a thousand or more inheritable birth defects result from alternations in gene patterns. Since tiny genes are responsible for producing the many chemicals needed by the body, missing or improperly operating genes can seriously impair health. Genetic disorders that affect body chemistry are called inborn errors of metabolism. Some forms of mental retardation are hereditary.

HOW THE BODY FIGHTS DISEASE

Mucous membrane (or mucosa), membrane that secretes mucus and lines the mouth, nose, throat, windpipe, lungs, eyelids, and the alimentary canal.

As a first line of defence, a healthy body has a number of physical barriers against infection. The skin and mucous membranes covering the body or lining its openings offer considerable resistance to invasion by bacteria and other infectious organisms. If these physical barriers are injured or burned, infection resistance drops. In minor cases, only boils or pimples may develop. In major cases, however, large areas of the body might become infected.

Cilia (plural of cilium), hairlike, vibratory appendages found in some plants and animals.

Breathing passages are especially vulnerable to infection. Fortunately, they are lined with mucus-secreting cells that trap tiny organisms and dust particles. Also, minute hairs called cilia line the breathing passages, wave like a field of wheat, and gently sweep matter out of the respiratory tract. In addition, foreign matter in the breathing passages can often be ejected by nose blowing, coughing, sneezing, and throat clearing. Unfortunately, repeated infection, smoking, and the repeated use of strong chemicals (including alcohol and drugs) can damage the respiratory passageways and make them more susceptible to infection.

Scavenger cells are present too in the walls of the bronchi, the branched air tubes to the lungs. Foreign matter reaching the bronchi after evading the other defences can be “eaten” by the scavengers and disposed of in the lymph glands of the lungs.

Many potential invaders cannot stand body temperature (98.6° F or 37° C). Even those that thrive at that temperature may be destroyed when the body assumes higher, fever temperatures.

Wax in the outer ear canals and tears from eye ducts can slow the growth of some bacteria. And stomach acid can destroy certain swallowed germs.

Lymph, a colourless liquid exuded through capillaries to nourish tissues of the body.

The body’s second line of defence is in the blood and lymph. Certain white blood cells flock to infected areas and try to localize the infection by forming pus-filled abscesses. Unless the abscess breaks and allows the pus to drain, the infection is likely to spread. When this happens, the infection is first blocked by local lymph glands. For example, an infection in the hand travels up the arm, producing red streaks and swollen, tender lymph glands in the armpit. Unless the infection is brought under control, it will result in blood poisoning.

Scavenger cells, or phagocytes, are located at various sites to minimize infection. One type in the spleen and liver keeps the blood clean. Others in such high-risk areas as the walls of the bronchi and the intestines remove certain bacteria and shattered cells.

How We Become Immune to Disease

The body has a special way of handling infection. It has a system that fends off the first traces of an infectious substance and then, through a “memory,” gives the body a long-lasting immunity against future attacks by the same kind of invader.

Antigen, a substance in blood that causes production of antibodies against itself.

Antibody, the protective substance produced in body fluids in response to exposure to foreign antigen in blood.

Many substances could harm the body if they ever entered it. These substances, or antigens, range from bacteria and pollen to a transplanted organ (viewed by the body as an invader). To fight them the body makes special chemicals known as antibodies.

Antibodies are a class of proteins called immuno-globulins. Each antibody is made of a heavy chain of chemical subunits, or amino acids, and a light chain of them. The light chain has special sites where the amino acids can link with their complements on the antigen molecule. When an antibody hooks up with an antigen, it often puts the antigen out of action by inactivating or covering a key portion of the harmful substance. In some cases, through the process of opsonization, antibodies “butter” the surface of some antigens and make them “tastier” to phagocytes, which engulf the antigens. Sometimes an antibody hooks to a bacterial antigen but needs an intermediate, or complement, to actually destroy the bacterium. As the antibody-antigen complex circulates in the blood, the complex “fixes” complement to it. In turn, the complement causes powerful enzymes to eat through the bacterial cell wall and make the organism burst.

There are several kinds of immuno-globulins IgM, the largest; IgG, the most plentiful and versatile; and IgA, the next most plentiful and specially adapted to work in areas where body secretions could damage other antibodies. Other immuno-globulins are tied in with allergic reactions. IgM is made at the first signs of an antigen. It is later supplanted by the more effective IgG.

When infection first strikes, the immunity system does not seem to be working. During the first day or so, antibodies against the infection cannot be found in the blood. But this is only because the basic cells involved in antibody production have been triggered by the presence of antigen to multiply themselves. The antibody level starts to rise on about the second day of infection and then zooms upward. By the fifth day the antibody level has risen a thousandfold.

The first antibodies, the large IgM type, are not the best qualified to fight a wide range of antigens, but they are particularly effective against bacteria. The more versatile IgG is circulating in the blood on about the fourth day of infection. Its production is stimulated by the rising level of IgM in the blood. At this time, IgM production drops off and the immunity system concentrates on making IgG. The IgG type of antibody sticks well to antigens and eventually covers them so that the antigens can no longer stimulate the immune response and IgG production is switched off. This is an example of negative feedback control.

Antibody Production

Thymus, organ, located behind the breastbone and above the heart; participates in the production of white blood cells or lymphocytes; because it attains maximum size at puberty and becomes smaller in adults, researchers feel it may be an endocrine gland that affects growth and sexual maturation.

Antibodies are made by two kinds of cells plasma cells and a class of white blood cells, lymphocytes. Plasma cells actually originate from lymphocytes and are found throughout the lymphatic tissue. Lymphocytes stem from cells in the blood-forming sections of bone marrow. When the bone-marrow cells circulate to the thymus, a lymphatic structure in the chest, they receive “orders” to become lymphocytes and make antibodies. Most lymphocytes last for only a few hours, but a few wander through the blood and body tissues for years. These lymphocytes are responsible for “remembering” old antigens and for inducing the immunity system to produce antibodies against those or similar antigens if they ever again enter the body.

When people develop antibodies against a disease by the action of their own immunity system, they have active immunity. When they are given someone else’s antibodies, however, they just have passive immunity to a disease.

Passive immunity is only temporary. Some people may also get temporary relief from a disease through injections of serum containing gamma globulin, a portion of the blood rich in antibodies.

Without protective antibodies, we could die of the first disease that struck us. This would be true, too, of newborn babies, except that they receive passive immunity from their mothers. During her lifetime, a mother accumulates a wide variety of antibodies against a host of diseases. Enough of them are passed to the developing baby in her womb to give it a temporary immunity to many diseases during the early months of its life, until it can develop its own set of antibodies.

HUMAN DISEASES (Part 2 of 7)   Leave a comment

What Happens When Immunity Backfires

Paradoxically, a person’s immunity system can backfire and develop auto-antibodies against his own body tissue. In many diseases of unknown cause, doctors have found many unusual antibodies in the blood serum of patients.

Rheumatoid arthritis (RA), chronic disease of the connective tissue, causing painful sensations in joints and muscles.

Doctors think the patients become sensitive to something made by their own bodies. Only a slight change in certain proteins in normal body tissue is necessary for them to become antigens. Most diseases marked by the production of auto-antibodies cannot be traced to infection or drug allergy. In rheumatoid arthritis, for example, the rheumatoid factor is the presence of auto-antibodies in the victim’s blood. These auto-antibodies may stick to the membranes lining the bone joints and cause a reaction that destroys tissue in the joints. In other disorders associated with reversed immunity, auto-antibodies strike red blood cells, tissues surrounding small blood vessels, or other target areas. Ulcerative colitis, a disorder marked by an inflamed portion of the intestine, often with ulcers, is also believed to be an autoimmune disease.

In some cases, lymphocyte defects or discrepancies in antibody production lead to an immune deficiency. The victim is then helpless against recurring infections. A simple head cold can soon become pneumonia. Antibiotics or serums with antibody-rich gamma globulin offer temporary relief in such cases.

1796: Inoculation against disease. The simple medical procedure known as vaccination first came into use in about 1713 as a means of preventing smallpox. Such inoculation sometimes proved dangerous, because individuals sometimes caught a severe case of the disease instead of a mild one. This problem was solved by Edward Jenner, a British physician, in 1796. He realized that individuals inoculated with the much milder cowpox virus became immune to smallpox. Jenner tested his theory in May 1796.

This kind of inoculation earned the name vaccine, from the Latin word vaccinus, meaning “from cows.” Since Jenner’s day vaccines have been developed to fight polio, diphtheria, whooping cough, measles, typhoid fever, cholera, tetanus, and other diseases.

1928: Penicillin. In 1928 the Scottish bacteriologist Alexander Fleming was doing research on the Staphyloccus bacteria when he noticed that a growth of mould Penicillium notatium was contaminating the culture. There was no bacteria where the mould was present. Following up on this fact, Fleming found there was something in the mould that prevented bacterial growth. He named this substance penicillin.

By continued experiment Fleming learned that penicillin is capable of killing many common disease-causing bacteria. His discovery proved to be one of the first and most useful antibiotics used in medicine today. By 1940 penicillin had been turned into an injectable medicine. Its use grew dramatically during World War II as an infection-preventing agent.

HOW DRUGS FIGHT DISEASE

With the advent of drug therapy in the 20th century, doctors began to use lifesaving drugs to fight disease. The clinical use of sulphanilamide, the predecessor of sulphur drugs, in the 1930s and the mass production of penicillin, the first antibiotic, in the 1940s gave physicians extremely powerful tools with which to fight infection. A disease-fighting drug never acts by itself. It always works in conjunction with the body’s immunity system. Vaccines have also become available for the prevention of certain diseases.

How Certain Drugs Quell Infection

Such antibiotics as penicillin, streptomycin, and tetracycline are very effective against bacterial infections. The name “antibiotic” comes from antibiosis, or the use of substances made by one living thing to kill another. Antibiotics are made by bacteria and moulds that are specially cultured in commercial drug laboratories.

Antibiotics kill bacteria and other disease organisms in various ways. Some destroy the cell walls of bacteria. Others interfere with bacterial multiplication or fatally alter the way bacteria make vital proteins. Still others mix up the genetic blueprints of the bacteria.

Ordinarily, an antibiotic tricks bacteria into using the antibiotic’s chemicals instead of closely related ones that the organisms really need for making the key enzymes required for their growth and reproduction. With the antibiotic assimilated into their systems instead of the vital chemicals, an essential activity or structure of the pathogens is lacking and they die.

Sulphur drugs act in a somewhat similar but less effective way. Weakened but not killed by the sulphur drugs, the pathogens fall easy prey to the body’s scavenger cells. Drugs are also available against parasitic worms, infectious amoeba, and other pathogenic organisms.

Antibiotics are not very effective against viruses because the drugs cannot get into the body cells where viruses hide and multiply. However, the body produces a protein called interferon that inhibits viral reproduction.

A drug is sometimes recognized by the body’s immunity system as an antigen. It then triggers a severe reaction. In some cases, a person can suffer anaphylaxis, or extreme sensitivity, to penicillin after repeated injections. Without quick medical aid, severe cases of anaphylactic shock can be fatal.

How Bacteria Become Drug Resistant

Once in every several hundred million cell divisions a mutation makes a bacterium immune to an antibiotic drug. The mutation alters the bacterium’s genetic code and thus its ability to use certain chemicals for its life activities. Mutations can be caused by the radiations from outer space that stream into the Earth’s atmosphere, as well as by some atmospheric chemicals. As a result of the mutation, all bacteria that stem from the immune germ will be resistant to the drug unless any of them undergoes a mutation that makes the strain susceptible again. Hence, whenever a new antibiotic is developed, there will be a chance that bacteria will develop an immunity against it. But because mutations are fairly rare, doctors have a good chance of fighting a bacterial disease with the new drug before future strains become resistant.

Some members of a bacterial strain are resistant to certain drugs naturally. In the course of time they can eventually become selected through evolutionary forces to become the dominant drug-resistant forms of a pathogenic strain.

More importantly, some bacteria can pass on their drug resistance to bacteria of another strain by “infection.” Since the passing of resistance factors does not depend upon the lengthy process of mutation, it poses a much greater problem of drug immunity. As a consequence, doctors often must prescribe more than one antibiotic to fight certain diseases in the hope that this will slow bacterial resistance.

Use of Vaccines and Hormones

A person can become artificially immune to some diseases by means of a vaccine. Vaccines contain antigens that stimulate the production of protective antibodies. Immunity to smallpox, polio, measles, rabies, and certain other diseases, is induced by injecting a person with vaccines containing living but attenuated, or weakened, disease organisms.

A vaccine containing only dead organisms protects against typhoid fever and whooping cough, as well as against measles and polio. Vaccines containing toxins, or poisons, are used to prevent diphtheria and tetanus. When injected into a person, they trigger the production of special antibodies called antitoxins.

Some body disorders are caused by too much or too little hormone production. Hormones are body chemicals that influence many vital biochemical reactions. When someone suffers a hormone deficiency, a doctor usually can treat the deficiency with hormone shots.

1347: Black Death. The plague is one of the most devastating diseases that has ever afflicted mankind. It is a highly contagious fever caused by the bacillus Yersinia pestis, which is carried by fleas that infest rats.

The plague, commonly called bubonic plague or the Black Death, has been known since ancient times, but the best documented instance was its deadly appearance in Europe in 1347. It raged throughout all of Europe, killing at least one-fourth of the population probably 25 million people. Without understanding how it is spread, people had no defence against the disease. Poor sanitary conditions and the disruption of war only worsened the epidemic.

In Europe the epidemic started in Sicily and was spread by shipboard rats to other Mediterranean ports. It moved to North Africa, Italy, Spain, England, and France. By 1349 it made its way to Austria, Germany, Switzerland, and the Low Countries. By 1350 it reached Scandinavia and the Baltic states.

In general, the population of Europe did not recover to its size before the plague until the 16th century, and some towns never recovered. The immediate results of the plague a general collapse of economies, a breakdown of class relationships, and a halt to wartime hostilities forced a massive restructuring of society. It has had a lasting impact on art, literature, and religious thought.

INFECTIOUS DISEASES

Infectious diseases can be transmitted in many ways. They can be spread in droplets through the air when infected persons sneeze or cough. Whoever inhales the droplets can then become infected. Some diseases can be passed through contaminated eating or drinking utensils. Some can be spread through sexual activity. Others can even be spread in the course of medical or surgical treatment, or through the use of dirty injection equipment, especially by drug abusers.

Cold (also called common cold, or coryza), illness, acute inflammation of upper respiratory tract.

Once an infectious organism gains a foothold in the body, it begins to thrive and multiply. Its success is slow or fast, depending upon the nature of the pathogen. The symptoms of the common cold, for example, appear within a few days of infection. However, the symptoms of kuru, an uncommon disease of the nervous system, often appear three years or longer after infection.

Incubation period, length of time before the symptoms of a disease appear.

Every infectious disease has an incubation period. This is the length of time between the pathogen’s gaining a foothold in the body and the appearance of the first symptoms of the disease.

Several factors also determine whether a person will become the victim of a disease after being infected. The number of invading germs the dose of the infection influences the outbreak of disease. So does the virulence of the pathogens; that is, their power to do harm. In addition, the condition of the body’s immunological defences also affects the probability of catching a disease.

Contagious Disease

A great many infectious diseases are contagious; that is, they can easily be passed between people. To acquire certain contagious diseases someone need only be in the presence of someone with it, or come in contact with an infected part of the body, or eat or drink from contaminated utensils.

Someone can be a carrier of a contagious disease in several ways. He can be an asymptomatic carrier, or have a disease without ever developing its symptoms. He can be an incubationary carrier and pass on the pathogens at any time during the “silent” incubation period. He can be a convalescent carrier and transmit some of the infectious organisms remaining in the body even after recovery. Of course, anyone suffering the frank symptoms of a contagious disease can pass it on to others while the disease is running its course.

HEART AND BLOOD SYSTEM DISEASES

Disease of the heart or of the blood vessels, called cardiovascular disease, is the leading cause of death in the United States and Canada. It claims more than a million lives each year in the United States; more than 70,000 each year in Canada.

The heart is a muscular pump. When its own tissue or blood vessels become diseased, serious and sometimes fatal harm can follow.

Coronary Artery Disease

Disease of the coronary arteries that supply oxygen and nutrients to the heart is the most common heart ailment. Coronary artery disease accounts for more than a third of all deaths among males in the United States between the ages of 35 and 55. It also strikes many women past the age of 50. Hypertension (high blood pressure), overweight, cigarette smoking, diabetes mellitus, excess cholesterol, triglycerides and other fats in the blood, and probably lack of regular exercise contribute to the chance of getting coronary artery disease.

Coronary artery disease is characterized by an atheroma, a fatty deposit of cholesterol beneath the inner lining of the artery. The atheroma obstructs the passage of blood, thereby reducing the flow of nourishing blood to the heart muscle. It also sets up conditions for a blood clot in the coronary artery. Atheroma formation seems to run in families. Eating foods rich in saturated animal fat and cholesterol is also thought to contribute to atheroma formation.

Many persons with coronary artery disease do not experience symptoms. If the obstruction is bad enough, however, it may cause angina pectoris, myocardial infarction, or heart enlargement and failure.

Angina pectoris, brief paroxysm of severe chest pain with feeling of suffocation.

Angina pectoris is a chest pain that feels like something is squeezing or pressing the chest during periods of physical exertion. It takes place when the heart’s oxygen needs cannot be met because of a blocked coronary artery. Rest will relieve the pain. Some persons have angina pectoris for years and still live active lives.

Myocardial infarction is commonly called heart attack. Tissue death that results from a lack of blood is called infarction. When the coronary artery becomes so obstructed that the myocardium, or heart muscle, does not receive oxygen, it dies.

Heart attack (also called myocardial infarction, or coronary occlusion), an acute episode of heart disease.

Once, it was believed that a blood clot occluded the coronary artery and caused the infarction. This is why a heart attack is sometimes called a coronary occlusion. However, it now appears that most clots form in the artery after the infarction.

The first few hours after a heart attack are the most critical because abnormal heart rhythms may develop. Ventricular fibrillation is the most dangerous. The ventricles of the heart contract so fast that the pumping action is baulked Death follows in three or four minutes. Heart attack patients are usually treated in the coronary care unit of a hospital for a few days to enable electronic monitoring of the heart rate and rhythm.

Heart failure, condition that develops when repeated heart attacks occur.

Heart failure can occur when repeated heart attacks put too much strain on the remaining healthy heart muscle. As attacks destroy more and more heart muscle, the remaining muscle hypertrophies, or enlarges, to maintain effective pumping. Pressure builds up in a weakened heart, however, and causes fluid backup in the lungs. As a result, the heart output cannot keep pace with the body’s oxygen demands.

GENETIC ENGINEERING (Part 1 of 2)   Leave a comment

DEFINITION: the branch of biology dealing with the splicing, and recombining, of specific genetic units from the DNA of living organisms: it is used to modify the existing genetic codes to produce new, or improved, species, valuable biochemicals.

1865: The birth of genetics. It was unfortunate for the biological sciences that Gregor Mendel was an obscure Austrian monk. His pioneering work in the field of genetics was being done at the time that Charles Darwin’s publications on evolution were beginning to create worldwide controversy, but Mendel’s work would remain unknown for years.

Mendel became an Augustinian monk in 1843, but his abilities in mathematics and the sciences were evident. His experiments on the principles of heredity were begun in about 1856 in what is now Czechoslovakia. By crossing various strains of peas with one another, Mendel found that traits were passed on from generation to generation in what he called “discrete hereditary elements” in sex cells, or gametes.

Mendel reported the results of his experiments to a local society for the study of natural science in 1865 and published his findings in the society’s journal. They were as good as buried there for the next 35 years. Although the journal found its way to libraries in Europe and North America, few paid any attention to his writings. When other botanists obtained results similar to Mendel’s, they began searching through earlier writings on the subject. Only then was Mendel’s 1865 research revealed. His “discrete hereditary elements” are now called genes, and the new science once called Mendelism is known as genetics.

1953: Discovery of DNA structure. The full name of DNA is deoxyribonucleic acid. It carries the codes of genetic information that transmit inherited characteristics to successive generations of living things.

DNA was discovered in 1869 by Friedrich Miescher. In 1943 its role in inheritance was demonstrated. In 1953 its structure was determined by an American biochemist, James D. Watson, and an English physicist, Francis H.C. Crick. Watson and Crick showed the structure to be two strands of a phosphoryl-deoxyribose polymer arranged as a double helix. Watson and Crick were awarded the Nobel prize in physiology or medicine in 1962.

1973: Biotechnology. Two American biochemists, Stanley H. Cohen and Herbert W. Boyer, inaugurated the science of genetic engineering and its associated field of biotechnology in 1973. They showed that it was possible to break down DNA into fragments and combine them into new genes, which could in turn be placed in living cells. There they would reproduce each time a cell divided into two parts.

Genetic engineering makes it possible to modify existing organisms or create organisms that already exist in the human body but that are difficult to isolate. For example, one early product was a genetically engineered form of insulin, used in the treatment of diabetes. Other genetically engineered products include interferons, which are used in the treatment of viral infections and showed promise in the treatment of various forms of cancer. Scientists hope that genetically engineered products will someday prevent or cure such genetic disorders as muscular dystrophy and cystic fibrosis.

Genetic engineering also opens the possibility of creating entirely new organisms. In 1980 the United States Supreme Court ruled that newly developed organisms could be patented, thus giving ownership rights to the companies that made them.

Chromosome, microscopic, threadlike part of the cell that carries hereditary information in the form of genes; among simple organisms, such as bacteria and algae, chromosomes consist entirely of DNA and are not enclosed within a membrane; among all other organisms chromosomes are contained in a membrane-bound cell nucleus and consist of both DNA and RNA; arrangement of components in the DNA molecules determines the genetic information; every species has a characteristic number of chromosomes, called the chromosome number; in species that reproduce asexually the chromosome number is the same in all the cells of the organism; among sexually reproducing organisms, each cell except the sex cell contains a pair of each chromosome.

Each human cell holds a vast storehouse of genetic information in some 100,000 genes, which code for individual biochemical functions, strung out along 46 chromosomes. Collectively, this storehouse forms the human genome. The techniques of genetic engineering allow scientists to identify specific genes, to remove any one of those genes from an organism’s chromosome, to clone or make a large number of identical copies of that gene, to analyse a copy in detail, to modify it, and to reinsert it into the genetic material of the organism from which it was derived or into the genetic material of a similar or very different organism.

The development of genetic engineering has had a great influence on science and business and has begun to radically alter medicine and agriculture. One of the first steps in shedding new light on human evolution and in controlling or altogether eliminating many diseases was taken in the early 1990s. Scientists mapped, or took apart, the smallest human chromosomes: the Y chromosome and chromosome 21. Breaking these chromosomes into small pieces allowed researchers to reproduce these segments in large quantities. Researchers believe that this, in turn, will lay the groundwork for mapping and eventually controlling all genes, including those that may be responsible for certain diseases.

Recombinant DNA, genetically engineered DNA prepared in vitro by cutting up DNA molecules and splicing together specific DNA fragments; usually uses DNA from more than one species of organism.

Smith, Hamilton O. (born 1931), U.S. microbiologist, born in New York City; with U.S. Public Health Service 1962-67; at School of Medicine of Johns Hopkins University from 1967, professor from 1973; received 1978 Nobel prize for research on effect of restriction enzymes on DNA molecules.

Nathans, Daniel (born 1928), U.S. microbiologist, born in Wilmington, Del.; professor school of medicine of Johns Hopkins University from 1967, director microbiology department from 1972; received 1978 Nobel prize for research on effect of restriction enzymes on DNA molecules.

Posted 2012/02/25 by Stelios in Education

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