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.

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Posted 2012/04/19 by Stelios in Education

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