Genes: Definition and Structure

Abstract

The earliest notion of heredity was that inherited characteristics were transmitted in parental fluids that intermixed in the offspring. Mendel demonstrated that hereditary ‘factors’ (later dubbed ‘genes’) behaved as particles remaining intact from one generation to another. Chemically, genes are deoxyribonucleic acid (DNA), strings of nucleotides (adenine, A; thymine, T; cytosine, C and guanine, G) whose order determines the order of the 20 amino acids in proteins. Proteins in turn determine the chemistry of the cell by catalysing reactions. And proteins constitute much of the cell's structure. Protein production comprises two steps: (1) Transcription, copying the nucleotide sequence of the gene into an message ribonucleic acid (mRNA) and (2) Translation, using the message in the mRNA to direct the synthesis of proteins. We now know the genetic code and understand the machinery that makes proteins. We also know how gene expression is controlled so that genetically identical cells of an individual can differentiate to serve diverse functions.

Key Concepts:

  • Individuals of a species vary considerably in appearance and function. These differences are to a large extent heritable. The observable appearance and physiology of an organism (i.e. its phenotype) result from the combined influence of its genes and its environment.

  • The factors (genes) responsible for inherited properties of organisms are passed from one generation to the next as if they were particles. Although the phenotypes of offspring may be intermediate between the phenotypes of their parents, the genes do not ‘blend’ within the offspring but maintain their chemical and functional integrity.

  • Genes are made of nucleic acids, linear molecules consisting of a string of four nucleotides, which, in deoxyribonucleic acid (DNA), are adenine (A), thymine (T), guanine (G) and cytosine (C).

  • The expression of genes requires two steps. The first is transcription of the DNA sequence of the gene into a messenger RNA (mRNA) molecule. The second is the translation of the mRNA into protein. There are specific sequences of bases that mark the start and stop points for both transcription and translation.

  • Most genes contain the recipe for making a specific polypeptide (protein) product. The proteins are linear molecules consisting of a string of amino acids.

  • The genetic code, the ‘language’ in which the information in the gene is written, is made up of triplets of nucleotides. Of the 64 possible triplets of A, T, C and G, 61 encode one of the 20 amino acids found in proteins and 3 signify ‘stop’, marking the end of the protein.

  • The catalysts that control the chemistry of living things are (almost always) proteins. A given enzyme may be the polypeptide product of a single gene or a complex of polypeptides encoded by one or several genes.

  • A typical gene may be a stretch of 1000 or more bases. Changes in the base composition (mutations) at any point in the gene can cause the gene product to be abnormal or inactive. Thus, mutations may occur at many different places within a given gene.

  • A functional test is used to determine whether two independently isolated, phenotypically similar recessive mutations lie in same gene. If a hybrid containing the two genes has the mutant phenotype, the mutations are said to be in the same gene (or ‘cistron’).

  • Determination of the base sequence of whole genomes is a powerful method for identifying mutations that affect specific functions and for finding sequences involved in the regulation of gene expression. Regions of the DNA that serve vital functions tend to be conserved over long evolutionary distances.

Keywords: allele; complementation; gene; genetic code; genotype; one gene–one polypeptide; phenotype; transcription; translation

Figure 1.

Mendel's demonstration of the particulate nature of the gene. Two true‐breeding strains of peas, one tall and the other short, were crossed to one another. All of their offspring were tall. At the level of their genes, the tall plants were TT homozygotes, and the short plants were tt. When a tall plant is crossed to another tall plant, the gametes are all T, and the offspring produced are all TT homozygotes. Like the parents, they are tall. Similarly, tt homozygotes, when crossed to one another, produce only short, tt homozygotes. A cross of a tall plant with a short one produces Tt heterozygotes, which are tall because the T allele of the tallness gene is dominant to the t allele. When these Tt heterozygotes make gametes, half of the gametes are T and half are t. In a cross between two Tt heterozygotes (or a self‐cross, which is genetically the same) T and t gametes join at random to form zygotes. A T egg is equally likely to be fertilised by a T pollen nucleus or a t pollen nucleus. Similarly, a t egg is equally likely to be fertilised by a T pollen nucleus or a t pollen nucleus. Thus, the progeny of a cross between two heterozygotes is 1/4 TT, 1/2 Tt and 1/4 tt. The homozygous TT offspring are tall, and so are the heterozygous Tt offspring (because T is the dominant allele). However, the tt homozygotes are short. This experiment shows that the t allele remains unchanged in the Tt heterozygote and reappears in the offspring of the Tt heterozygote when an appropriate cross is done.

Figure 2.

The genetic code. RNA contains four bases, adenine (A), guanine (G), cytosine (C) and uracil (U). One difference between DNA and RNA is that RNA contains U instead of T. These four bases can be assembled into 64 possible triplet codons, every one of which has a meaning in the genetic code. The codons are traditionally written in their mRNA form. The amino acid translations of the codons are shown in italics. The abbreviations stand for phenylalanine, leucine, isoleucine, methionine, valine, serine, proline, threonine, alanine, tyrosine, histidine, glutamine, asparagine, lysine, aspartate, glutamate, cysteine, tryptophan, arginine and glycine. The three ‘stop’ codons, UAA, UAG and UGA are given the whimsical names ochre, amber and opal, respectively.

Figure 3.

A two‐step biochemical process produces brown pigment in our hypothetical gerbil. The process starts with the conversion of a colourless compound L to a colourless compound M, a process catalysed by Enzyme 1. Then colourless compound M is converted to a brown pigment, Compound N. The second step is catalysed by Enzyme 2. If b1 is a mutation in the gene that encodes Enzyme 1, a homozygous b1b1 gerbil will be white because it cannot convert Compound L to Compound M. If b2 is a mutation in the gene that encodes Enzyme 2, a b2b2 homozygous gerbil will be white because it cannot convert Compound M to Compound N. A cross between the two homozygotes would produce a brown hybrid because the b1b1 homozygote has a wild‐type (functional) gene for Enzyme 2 and the b2b2 homozygote has a wild‐type gene for Enzyme 1.

Figure 4.

Recombination. The figure shows a short portion of a chromosome of a diploid organism that is the hybrid offspring of a male (blue chromosome) carrying mutation m1 and a female (red chromosome) carrying mutation m2. The two mutations lie at different positions within the same gene so that the hybrid whose genotype is shown at the top is unable to produce any functional enzyme from this gene. During the meiotic cell divisions that produce germ cells (eggs or sperms) from this hybrid, the two homologous chromosomes will pair and may break and rejoin so as to produce recombinant chromosomes containing some regions descended from the paternal chromosome and some descended from the maternal chromosome. Rarely the break will occur between the two mutations in the bracketed gene, and the product of this crossover will be one chromosome that carries both mutations and a reciprocal product that carries neither of the mutations. A germ cell will carry only one copy of the chromosome that bears this gene. If the hybrid shown in the figure is mated to either of its parents (the m1m1 homozygote or the m2m2 homozygote), the offspring receiving a mutation‐free recombinant chromosome will have the wild‐type phenotype. The offspring receiving a double‐mutant recombinant chromosome will have the mutant phenotype. This diagram simplifies the process by omitting two copies of the chromosomes that are present at this stage but that do not participate in the recombination event shown.

Figure 5.

The cistrans complementation test. A wild‐type gene (+) makes a functional protein, which is shown in the figure as a purple line. A mutant gene (m) makes a nonfunctional protein, which is shown in the figure as a blue line. If the two mutations, m1 and m2, affect different genes (top frames), a heterozygous cell that contains both mutations and their wild‐type (+) counterparts will contain some functional protein product from each gene. This will be true whether the mutations are in the cis configuration or the trans configuration. If the two mutations affect the same gene (bottom frames), then the cis heterozygote will make some functional protein and will have the wild‐type phenotype, but the trans heterozygote will make only nonfunctional protein and will have the mutant phenotype.

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Further Reading

Alberts B, Johnson A, Lewis J et al. (2007) Molecular Biology of the Cell, 5th edn. New York, NY: Garland.

Benzer S (1956) The elementary units of heredity. In: McElroy WD and Glass B (eds) A Symposium on the Chemical Basis of Heredity, pp. 70–93. Baltimore, MD: The Johns Hopkins Press.

Cairns J, Stent GS and Watson JD (eds) (2007) Phage and the Origins of Molecular Biology. The Centennial Edition. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press.

Carlson EA (2004) Mendel's Legacy: The Origin of Classical Genetics. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press.

Griffiths AJF, Gelbart WM, Miller JH and Lewontin RC (2002) Modern Genetic Analysis: Integrating Genes and Genomes, 2nd edn. New York, NY: WH Freeman and Company.

Hartl D (2012) Essential Genetics: A Genomic Perspective, 6th edn. Boston, MA: Jones and Bartlett.

Judson HF (1996) The Eighth Day of Creation: Makers of the Revolution in Biology. Plainview, NY: Cold Spring Harbor Laboratory Press.

Lodish H, Berk A, Kaiser CA et al. (2012) Molecular Cell Biology, 7th edn. New York, NY: Scientific American Books.

Mendel GJ (1866) Experiments in Plant Hybridisation. http://www.mendelweb.org/

National Center for Biotechnology Information, U.S. National Library of Medicine. Bethesda, MD, USA. http://www.ncbi.nlm.nih.gov/

Snustad DP and Simmons MJ (2011) Principles of Genetics, 6th edn. New York, NY: John Wiley and Sons.

Sturtevant AH (2001) A History of Genetics. New York, NY: Harper and Row.

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Susman, Millard(Jul 2014) Genes: Definition and Structure. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001494.pub3]