Gene

While all genes are made of DNA, not all stretches of DNA act as genes. Indeed, in eukaryotic organisms, most of the DNA does not function as genes, meaning it is not the code for making proteins or RNA. Some DNA outside of genes has a structural role, some are remnants of old genes that DNA nucleotides pair up across the double helix. Schematic structure of a gene. Only the exons (shaded) code for the final product, which may be protein or RNA. now are functionless, and much of it appears to be "junk," inserted and copied by viruslike sequences. Within a gene, usually only one side of the double helix actually codes for product; the other side is silent. Which side of the helix acts as code varies from gene to gene.

Almost all genes code for proteins. Proteins are strings of amino acids, and the sequence of nucleotides in the gene dictates the sequence of amino acids in the protein. Proteins perform almost all the functions in cells, and can be grouped into four major classes: they act as enzymes that control the rate of chemical reactions in the cell; they form structural components of organelles, membranes, and other cell components; they receive and transmit signals between and within cells; or they act as regulators of genes by latching onto DNA, thereby increasing or decreasing the rate at which the gene is used, or "expressed."

Genes vary in length. The largest human gene is 2.5 million base pairs in length, and codes for the muscle protein named dystrophin, which is more than 3,500 amino acids long. Eukaryotic genes generally produce proteins of about 150 to 3,000 amino acids in length. Some genes are relatively small, as in prokaryotes, which produce proteins of 50 to 300 amino acids. Most eukaryotic protein-coding genes are present in only two copies per genome, occurring in the same position on homologous chromosomes, one of which is received from each parent. If the two copies differ slightly they are called alleles. Changes in nucleotide sequences are termed mutations or polymorphisms, depending on their effect.

Some genes code not for protein but for RNA molecules that have their own functions within the cell. These include the transfer RNAs, ribosomal RNAs, and a variety of other smaller RNAs with roles in the nucleus. RNA-coding genes are usually present in multiple copies per eukaryotic genome.