The message set out sequentially along the human CHROMOSOMES. The human gene map (the genome) was constructed through the work of the international, collaborative HUMAN GENOME PROJECT. Genomic sequencing is offered by a number of US laboratories and private companies as well as some European counties. In the UK, 2016 saw the start of implementing the 100,000 Genome Project which will map the genome of patients with rare diseases or common cancers and their families, with the aim of linking any genetic anomalies found in their medical histories with their future health outcomes. The process is sufficiently cost effective that this initial focus may progress to routine DNA sequencing in routine healthcare. Genes are responsible for the PROTEIN synthesis of the cell (see CELLS): they instruct the cell how to make a particular polypeptide chain for a particular protein. They carry, in coded form, the detailed specifications for the thousands of kinds of protein molecules required by the cell for its existence, for its ENZYMES, for its repair work and for its reproduction. These proteins are synthesised from the 20 natural AMINO ACIDS, which are uniform throughout nature and which exist in the cell cytoplasm as part of the metabolic pool. The protein molecule consists of amino acids joined end to end to form long polypeptide chains. An average chain contains 100–300 amino acids. The sequence of bases in the nucleic acid chain of the gene corresponds in some fundamental way to the sequence of amino acids in the protein molecule, and hence it determines the structure of the particular protein. This is the genetic code. Deoxyribonucleic acid (see DNA) is the bearer of this genetic information.

DNA has a long backbone made up of repeating groups of phosphate and sugar deoxyribose. To this backbone, four bases are attached as side groups at regular intervals. These four bases are the four letters used to spell out the genetic message: they are adenine, thymine, guanine and cytosine. The molecule of the DNA is made up of two chains coiled round a common axis to form what is called a double helix. The two chains are held together by hydrogen bonds between pairs of bases. Since adenine only pairs with thymine, and guanine only with cytosine, the sequences of bases in one chain fixes the sequence in the other. Several hundred bases would be contained in the length of DNA of a typical gene. If the message of the DNA-based sequences is a continuous succession of thymine, the RIBOSOME will link together a series of the amino-acid phenylalanine. If the base sequence is a succession of cytosine, the ribosome will link up a series of prolines. Thus, each amino acid has its own particular code of bases. In fact, each amino acid is coded by a word consisting of three adjacent bases. In addition to carrying genetic information, DNA is able to synthesise or replicate itself and so pass its information on to daughter cells.

All DNA is part of the chromosome and so remains confined to the nucleus of the cell (except in the mitochondrial DNA). Proteins are synthesised by the ribosomes which are in the cytoplasm. DNA achieves control over protein production in the cytoplasm by directing the synthesis of ribonucleic acid (see RNA). Most of the DNA in a cell is inactive, otherwise the cell would synthesise simultaneously every protein that the individual was capable of forming. When part of the DNA structure becomes ‘active’, it acts as a template for the ribonucleic acid, which itself acts as a template for protein synthesis when it becomes attached to the ribosome.

Ribonucleic acid exists in three forms. First, ‘messenger RNA’ carries the necessary ‘message’ for the synthesis of a specific protein, from the nucleus to the ribosome. Second, ‘transfer RNA’ collects the individual amino acids which exist in the cytoplasm as part of the metabolic pool and carries them to the ribosome. Third, there is RNA in the ribosome itself. RNA has a similar structure to DNA but the sugar is ribose instead of deoxyribose and uracil replaces the base thymine. Before the ribosome can produce the proteins, the amino acids must be lined up in the correct order on the messenger RNA template. This alignment is carried out by transfer RNA, of which there is a specific form for each individual amino acid. Transfer RNA can not only recognise its specific amino acid, but also identify the position it is required to occupy on the messenger RNA template. This is because each transfer RNA has its own sequence of bases and recognises its site on the messenger RNA by pairing bases with it. The ribosome then travels along the chain of messenger RNA and links the amino acids, which have thus been arranged in the requisite order, by peptide bonds and protein is released.

Proteins are important for two main reasons. First, all the enzymes of living cells are made of protein. One gene is responsible for one enzyme. Genes thus control all the biochemical processes of the body and are responsible for the inborn difference between human beings. Second, proteins also fulfil a structural role in the cell, so that genes controlling the synthesis of structural proteins are responsible for morphological differences between human beings.