Introduction to Genetics
Genetics, the study of the biological mechanisms by which an organism's traits are passed from generation to generation. These mechanisms involve complex molecules that constitute an organism's genetic material. This material is made up of a large number of subunits called genes.
The general study of the transmission of traits is known as heredity. Genetics is concerned not only with the transmission of traits, but also with other characteristics of genetic material. Genetics has become a science central to medicine, biology, and the study of evolution. Discoveries within genetics led to the development of the field of moecular biology.
A number of areas of study have developed within genetics. Population genetics studies the occurrence of different genes in different groups of people. Behavioral genetics looks at the effects of genes on traits or behavioral patterns. Statistical genetics develops and applies statistical methods to analyze genetic data. Genetic epidemiology looks at genetic influences in the amount and distribution of health and disease in groups of people.
Discoveries in genetics have been applied successfully in a number of fields, including medicine and agriculture. Knowledge about genetic factors in disease is crucial in diagnosing and treating illnesses, and genetics is used in finding ways to fight disease-producing viruses and bacteria. Improved methods of animal and plant breeding owe a great deal to the study of genetics.
Genetic Material
All organisms inherit a specific pattern of growth, which, for any sexually reproduced organism, comes from the organism's parents. The pattern of growth from one parent combines with that of the other, determining how the organism will develop. The information that governs the pattern of growth is found in the genetic material that exists in each living cell. In most cells, the genetic material is located inside the cell's nucleus, although in some single-celled organisms without nuclei, it lies in the cytoplasm Viruses, which do not have cells, are made up almost entirely of genetic material.
Chromosomes and GenesWithin each nucleus, the genetic material is arranged into tiny bodies called chromosomes. Each chromosome contains several types of proteins and deoxyribonucleic acid (DNA). DNA consists of two strands, each composed of a long chain of subunits called nucleotides, that form a double helix (the shape of two stretched, intertwined springs). The number of chromosomes in a cell and the size of the DNA (as determined by the number of nucleotides it contains) vary from species to species.
There are four kinds of nucleotides in DNA. Inheritable traits are determined by genes, which are DNA segments that contain specific sequences of the four kinds of nucleotides. The entire sequence of nucleotides in DNA is the genetic code. This code supplies the instructions by which an organism's cells make proteins, the chemical compounds that are necessary for all biological functions.
To make proteins from the genetic code's instructions, the code must be transferred from DNA to another type of nucleic acid, ribonucleic acid (RNA). RNA transports the genetic message from the DNA to the protein-making parts of the cell.
Sometimes a gene is missing or has an error in the coded instructions. Such abnormalities of the genetic material cause genetic diseases and birth defects.
All the genes in a cell of an organism make up the genome of that organism's species. The human genome contains about 1 billion nucleotides. In comparison, the genome of a bacterial cell contains about 4.5 million nucleotides. In 2001, researchers discovered that nucleotides in the human genome form about 30,000 genes, far fewer than had been previously estimated.
Some biologists believe that less than 5 per cent of the human genome is made up of genes with protein-coding information, and that half of the noncoding DNA consists of segments of DNA called transposons. Transposons can affect the transmission of traits. They sometimes change their location on the chromosome and, in doing so, can speed up, slow down, or block the function of specific genes. It is not known whether the properties of a given organism's transposons are inheritable.
Another characteristic of genetic material that may not be inheritable is called DNA methylation. In this process, a segment of DNA becomes inactivated when a methyl group (a compound formed by one atom of carbon and three atoms of hydrogen) becomes attached to it. Methylation plays an important role in regulating gene activity. Biologists believe defective methylation may play a role in the development of certain diseases, such as cancer.
Reproduction and Genetics
Mitosis and MeiosisThe somatic (body) cells of all organisms carry a number of paired, homologous (similar) chromosomes. With few exceptions, all organisms of the same species have the same number of chromosomes in each of their somatic cells. For example, each somatic cell of a normal human being has 46 chromosomes; each of an earthworm has 32; and each of a corn plant has 20. The number of such chromosomes in each somatic cell is called a diploid, or doubled, number.
When a somatic cell divides, by a process called mitosis, each chromosome duplicates, with exact copies of the original going to each new cell. Thus, each new cell has the same number of chromosomes as the parent cell.
Sexual reproduction comes about through the union of two specialized cells, one from each parent. These cells are called gametes. A gamete, such as an egg or sperm, contains only half the number of chromosomes as a somatic cell of the same species. Therefore, the gamete is said to contain a haploid, or halved, number of chromosomes. This reduction in number occurs through the process of meiosis, or reduction division. Meiosis involves two successive cell divisions called the first and second meiotic divisions.
Special diploid cells (such as certain cells in the testes and ovaries) undergo meiosis. In these cells, the parental chromosomes duplicate. Then, homologous parental chromosomes pair up with each other and become connected in a process called synapsis. The cell then divides (the first meiotic division) a into two diploid cells with mixed parental chromosomes.
Each of these diploid cells also divides (the second meiotic division) into two parts, with each chromosome of a pair going to a separate cell. The result is four haploid cells (gametes), each of which contains half the number of chromosomes of the parent cell. Without meiosis, the chromosomes in the cells of each new individual would double in number with each successive generation.
The union of male and female gametes at fertilization is called sexual recombination. The union results in a fertilized egg called a zygote, which has the combined number of chromosomes of both gametes. The zygote, therefore, has the same diploid number as a somatic cell. The zygote develops into a new organism, producing somatic cells by mitotic cell division. As the organism matures, certain cells are set aside for reproduction. These cells are diploid but divide through meiosis, producing haploid cells (gametes).
The ways that chromosome activity affects the inherited traits of organisms have been formulated into a number of principles, or laws. Some of these principles were perceived long before anything was known about chromosomes and genes.
History of Genetics
The first truly scientific study of heredity was conducted by Gregor Mendel (1822–1884), an Austrian monk. He published his findings in 1866. The principles he established—segregation and dominance, and independent assortment—became the basis for the science of genetics.
Mendel's work was neglected until 1900, when a number of biologists rediscovered his work and his papers were republished.
In 1906, Thomas H. Morgan and his associates at Columbia University began a long series of genetic studies on fruit flies, which produce a new generation every two weeks. These studies resulted in an understanding of linkage, sex determination, and sex linkage, and in the mapping of gene arrangements on chromosomes. One of Morgan's associates, H. J. Muller, showed that mutations could be produced by X rays.
In the 1940's and early 1950's, biologists discovered that the nucleic acid DNA (and RNA in some viruses) was the carrier of the hereditary pattern. In 1953, the structure of DNA was discovered by Francis H. C. Crick, a British molecular biologist, James D. Watson, a United States biochemist, and Maurice H. F. Wilkins, a British biophysicist. In 1957, Arthur Kornberg, a United States biochemist, discovered the enzyme that synthesizes DNA. By 1966, biologists knew which nucleotide sequences specify the genetic code for which amino acids, the compounds that make up proteins. In 1977, Richard J. Roberts and Phillip A. Sharp, United States biochemists, discovered that some genes have DNA segments with no apparent function; these segments are called introns. Introns occur between exons, the DNA segments that specify the genetic code for proteins. Researchers soon discovered that most genes are made up of introns and exons.
In the mid-1970's, scientists developed genetic engineering, a process in which some DNA is taken from cells of one organism and combined with DNA in the cells of another. Genetic engineering is used in the manufacture of a number of substances, including certain drugs and hormones. It is also used to produce desired characteristics in crops, livestock, and laboratory animals. In an experimental technique called gene therapy, human genes are manipulated to treat or cure genetic diseases.
In a process called cloning, genetically identical copies of individual organisms are produced. Since ancient times, cloning has been practiced with plants, as in growing a new plant from a cut stem. It has also been known that simple animals could be cloned; for example, when a worm is cut into pieces, each piece develops into a new worm.
The first vertebrates (animals with backbones) to be cloned were frogs, in the 1950's. Mammals were first cloned in the 1980's. In the principal technique used to clone animals, the nucleus of a cell of an embryo is removed and inserted into a fertilized egg whose nucleus has been removed. The egg develops into an animal genetically identical to the embryo. In this way, many identical individuals can be made from a single embryo. Laboratory mice and certain livestock are the animals most commonly cloned today.
In the 1980's, biochemists developed a process for readily duplicating segments of DNA. This process, called the polymerase chain reaction, or PCR, makes it possible to produce very large amounts of DNA from samples that would otherwise be too small to analyze for their DNA. The polymerase chain reaction is widely used in a number of biological fields, including forensic pathology and studies of evolution.
In 1990, an international undertaking called the Human Genome Project was begun to locate the positions of all of the genes on the human chromosomes. The project is also concerned with mapping these genes; that is, determining each gene's sequence of nucleotides. Among the genes that have been located and mapped are those for muscular dystrophy, Huntington's disease, cystic fibrosis, colon cancer, and amyotrophic lateral sclerosis. In 1998, scientists completed mapping the genetic sequence of the roundworm Caenorhabditis elegans, the first full mapping of a multicellular organism. In 2001, the first nearly complete mapping of a major food crop (rice) was announced.
By 2000 a number of countries, including Iceland and the United Kingdom, had begun projects to collect DNA samples from citizens and link this genetic information with their health records, information about their lifestyles, and their family health history.