Gene

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Gene

A gene is the fundamental physical and functional unit of heredity. Whether in a microorganism or in a human cell, a gene is an individual element of an organism’s genome and determines a trait or characteristic by regulating biochemical structure or metabolic process.

Genes are segments of nucleic acid, consisting of a specific sequence and number of the chemical units of nucleic acids, the nucleotides. In most organisms, the nucleic acid is DNA (deoxyribonucleic acid), though in retroviruses, the genetic material is composed of ribonucleic acid (RNA). Some genes in a cell are active more or less all the time, which means they are continuously transcribed and provide a constant supply of their protein product. These “housekeeping” genes are always needed for basic cellular reactions. Others may be rendered active or inactive depending on the needs and functions of the organism under particular conditions. The signal that masks or unmasks a gene can come from outside the cell, for example, from a steroid hormone or a nutrient, or it can come from within the cell itself because of the activity of other genes. In both cases, regulatory substances can bind to the specific

DNA sequences of the target genes to control the synthesis of transcripts.

In a paper published in 1865, Gregor Mendel (1823–1884), advanced a theory of inheritance dependent on material elements that segregate independently from each other in sex cells. Before Mendel’s findings, inherited traits were thought to be passed on through a blending of the mother and father’s characteristics, much like a blending of two liquids. The term “gene” was coined later by the Danish botanist Wilhelm Johannsen (1857-1927), to replace the variety of terms used up until then to describe hereditary factors. His definition of the gene led him to distinguish between genotype (an organism’s genetic makeup) and pheno-type (an organism’s appearance). Before the chemical and physical nature of genes were discovered they were defined on the basis of phenotypic expression and algebraic symbols were used to record their distribution and segregation. Because sexually reproducing, eukaryotic organisms possess two copies of an inherited factor (or gene), one acquired from each parent, the genotype of an individual for a particular trait is expressed by a pair of letters or symbols. Each of the alternative forms of a gene is also known as alleles. Dominant and recessive alleles are denoted by the use of higher and lower case letters. It can be predicted mathematically, for example, that a single allele pair will always segregate to give a genotype ratio 1AA:2Aa:1aa, and the phenotype ratio 2A:1aa (where A represents both AA and Aa since these cannot be distinguished phenotypically if dominance is complete).

The molecular structure and activity of genes can be modified by mutations and the smallest mutational unit is now known to be a single pair of nucleotides, also known as a muton. To indicate that a gene is functionally normal it is assigned a plus (+) sign, whereas a damaged or mutated gene is indicated by a minus (−) sign. A wild-type Escherichia coli able to synthesize its own arginine would thus, be symbolized as arg+ and strains that have lost this ability by mutation of one of the genes for arginine utilization would be arg~. Such strains, known as arginine auxotrophs, would not be able to grow without a supplement of arginine. At this level of definition, the plus or minus actually refer to an operon rather than a single gene, and finer genetic analysis can be used to reveal the exact location of the mutated gene.

The use of mutations in studying genes is well illustrated in a traditional genetic test called the “cistrans test” which also gave the gene the alternative name, cistron. This is a complementation test that can be used to determine whether two different mutations (m and m ) occur in the same functional unit, i.e., within the same gene or cistron. It demonstrates well how genes can be defined phenomenologically and has been performed successfully in microorganisms such as yeasts. It works on the principle that pairs of homologous chromosomes containing similar genes can complement their action. Two types of heterozygotes of the test organism are prepared. Heterozygotes are organisms having different alleles in the two homologous chromosomes each of which was inherited from one parent. One heterozygote contains the mutations under investigation within the same chromosome, that is in the cis — configuration, which is symbolically designated ++/m m (m and m are the two mutations under investigation and the symbol “+” indicates the same position on the homologous chromosome in the unmutated, wild type state). The second mutant is constructed to contain the mutations in such a way that one appears on each of the homologous chromosomes. This is called the trans — configuration and is designated, for example, by +/+m . If two recessive mutations are present in the same cistron, the heterozygous trans — configuration displays the mutant phenotype, whereas the cis — configuration displays the normal, wild type, phenotype. This is because in the cis — configuration, there is one completely functional, unmutated, cistron (++) within the system which masks the two mutations on the other chromosome and allows for the expression of the wild type phenotype. If one or both mutations are dominant, and the cis— and trans— heterozygotes are phe-notypically different, then both mutations must be present in the same cistron. Conversely, if the cis— and trans— heterozygotes are phenotypically identical, this is taken as evidence that the mutations are present in different cistrons.

In 1910, the American geneticist Thomas Hunt Morgan (1866-1945) began to uncover the relationship between genes and chromosomes. He discovered that genes were located on chromosomes and that they were arranged linearly and associated in linkage groups, all the genes on one chromosome being linked. For example the genes on the X and Y chromosomes are said to be sex-linked because the X and Y chromosomes determine the sex of the organisms, in humans X determining femaleness and Y determining maleness. Nonhomologous chromosomes possess different linkage groups, whereas homologous chromosomes have identical linkage groups in identical sequences. The distance between two genes of the same linkage group is the sum of the distances between all the intervening genes and a schematic representation of the linear arrangement of linked genes, with their relative distances of separation, is known as a genetic map. In the construction of such maps the frequency of recombination during crossing over is used as an index of the distance between two linked genes.

Resources

BOOKS

Chiu, Lisa Seachrist and Judith A. Seachrist. When a Gene Makes You Smell Like a Fish and Other Amazing Tales about the Genes in Your Body. New York: Oxford University Press, USA, 2006.

Dawkins, Richard. The Selfish Gene. 3rd ed. Oxford: Oxford University Press, 2006.

Lewin, Benjamin. Essential Genes. New York: Prentice Hall, 2005.