Gene

Gene, unit of inheritance, a piece of the genetic material that determines the inheritance of a particular characteristic, or group of characteristics. Genes are carried by chromosomes in the cell nucleus and are arranged in a line along each chromosome. Every gene occupies a place, or locus, on the chromosome. Consequently, the word locus has become loosely interchangeable with the word gene.

The genetic material is deoxyribonucleic acid, or DNA (see Nucleic Acids), a molecule that forms the “backbone” of the chromosome. Because the DNA in each chromosome is a single, long, thin, continuous molecule, the genes must be parts of that molecule; and because DNA is a chain of minute subunits known as nucleotide bases, each gene includes many bases. Four different kinds of bases exist in the chain—adenine, guanine, cytosine, and thymine—and their sequence in a gene determines its properties.

Genes exert their effects through the molecules they produce. The immediate products of a gene are molecules of ribonucleic acid (RNA); these are copies of the DNA, except that RNA has the base uracil instead of thymine. The RNA molecules from some genes play a direct part in the metabolism of the organism, but most are used to make protein. Proteins are chains of subunits known as amino acids, and the sequence of bases in the RNA determines the sequence of amino acids in the protein by means of the genetic code (see Genetics: The Genetic Code). The sequence of amino acids in a protein dictates whether it will become part of the structure of the organism, or whether it will become an enzyme for promoting a particular chemical reaction. Thus, changes in the DNA can produce changes that affect the structure or the chemistry of an organism. Due to the complexity of living organisms, usually a number of genes will influence a process or major feature, but sometimes just a few or even one gene will affect an organism considerably. For example, it has been demonstrated in mice that just one gene can significantly affect memory. Inserting an extra copy of the gene that produces a component of the neuron receptor N-methyl-D-aspartate (NMDA) into the mouse genome causes a higher NMDA activity in the mouse’s brain and the mouse learns quicker and has a better memory than non-modified mice. Conversely, mice lacking this NR2B gene have impaired learning and memory. Scientists believe NMDA facilitates the creation of bonds between neurons that permit the association of two distinct stimuli, such as touching something hot and the sensation of pain. This ability is regarded by many scientists to be at the core of memory and learning. It is hoped that such work will aid drug design or gene-based therapies for memory loss in humans if there is sufficient similarity with human brain chemistry.

The nucleotide bases in DNA that code the structure of RNAs and proteins are not the only components of genes; groups of bases adjacent to the coding sequences affect the quantities and dispositions of gene products. In higher organisms (animals and plants, rather than bacteria and viruses), the noncoding sequences outnumber the coding ones by a factor of ten or more, and the functions of these noncoding regions are largely unknown. This means that geneticists cannot yet set precise limits on the sizes of animal and plant genes in general. However, with the current advances in genetic mapping (most notably in the Human Genome Project) more and more information on the nature of genes is being gathered. For example, it is now known that the genome of the fruit fly Drosophila melanogaster contains 13,601 genes, and the human chromosome 22 contains an estimated 34.5 million building blocks of DNA, which comprise at least 545 genes and 134 pseudogenes (DNA sequences that resemble genes but do not instruct the cell to produce proteins). Further work on mapped genomes should reveal how certain genetic sequences determine specific protein synthesis and hence structure, metabolic functions, and processes.