DNA

DNA (deoxyribonucleic acid) is a long molecule that stores in most cells the instructions for heredity. These instructions tell a cell how to make proteins and how to function.

DNA is found in bacteria, fungi, plants, and animals. In cells with a nucleus, such as human cells, DNA is mainly located in the nucleus, with a small amount in the mitochondria. Some viruses use DNA; others use RNA.

Structure

DNA consists of two strands twisted around each other, a form known as the double helix. Each strand is a chain of nucleotides. Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and a nitrogenous base. The bases are four: adenine (A), thymine (T), guanine (G), and cytosine (C).

The two strands are linked by hydrogen bonds between the bases: A always pairs with T, and G pairs with C. For this reason, when the two strands are separated, each can serve as a template for making a new strand. This mechanism allows DNA to copy itself before a cell divides.

Information storage

The sequence of bases is the hereditary code. A segment of DNA that carries instructions for making a particular protein is called a gene. When a gene is used, DNA is first copied into RNA (transcription), and then the RNA is translated into a chain of amino acids that form a protein (translation). Three consecutive bases stand for one amino acid, a pattern known as the genetic code.

Mutation and inheritance

Small errors can occur when DNA copies itself, or they can be caused by radiation and chemicals. These changes (mutations) sometimes have no effect, and sometimes they alter how a protein functions. Cells have repair systems that correct most errors. Mutations in reproductive cells can pass to offspring and are one of the foundations of biological variation.

Scientific and medical use

Techniques for reading DNA sequences (sequencing) are used to investigate genetic diseases, to study relationships between species, and to examine evidence in criminal cases. DNA taken from two individuals can show whether they are related. Genetic technology is also used to modify genes in bacteria and plants so they produce desired substances.