Pharmacogenetics
Appendix 1: How genes work
Genes are made of the chemical substance DNA (deoxyribonucleic acid). This substance encodes genetic information. DNA can be found in most cells in a human body, where it is assembled into units called chromosomes. On every chromosome there are between 200 and several thousand genes. Humans have 23 pairs of chromosomes on which 30,000 - 40,000 genes are located. The total complement of this genetic information is called the genome of an individual.
A DNA molecule consists of two strands that wrap around each other to resemble a twisted ladder – the famous double helix. Each strand of DNA is made up of a string of smaller units called nucleotides, or bases. There are four different bases: adenine (A), thymine (T), guanine (G) and cytosine (C). These bases pair together: A with T, and C with G. Each base pair forms a rung of the ladder. The way these pair together causes the strands to coil up into the spiral twisted ladder. It also allows the DNA to replicate or copy itself.
Every gene contains the instructions for making a specific protein or ribonucleic acid (RNA). Each set of three base letters, for example ACG, is part of a code providing the instructions to assemble a protein. Proteins carry out the work of a cell. Each three-letter code is specific to one of the 22 amino acids, the chemical building blocks of proteins. The sequence of the gene determines the order that these blocks assemble together, and hence which protein is made. Different proteins have different specialised functions, such as making muscle, binding oxygen from the air, transmitting nerve impulses, and breaking down food or other substances. Many proteins are enzymes, with the specialised function of synthesising, breaking down or altering other chemical molecules. The chemical molecules that are processed by enzymes may be produced by other cells of the body. They can also be introduced externally, for example, when a patient takes a medicine. The activity of the enzymes will then influence the concentration of the medicine in the blood and other important locations in the body over time. Thus, as specific genes determine which form of proteins or enzymes are produced in a cell, genetic analysis may reveal how a patient is likely to respond to a medicine.