Each cell of a living organism contains a set of instructions that explains how to build the various components of the plant, animal, fungus or bacteria. DNA is the substance that living things use to carry these instructions. DNA groups itself in discrete clusters that supply the code for a given trait. Scientists refer to each of these clusters as genes.
DNA is a long molecule that, in all organisms except for viruses and some bacteria, takes the form of a double helix. This structure resembles a twisted ladder that has numerous rungs. One of four chemicals, namely adenine, thymine, guanine and cytosine, forms each side of each individual rung. The order of these four chemicals works as an alphabet, allowing organisms to create very long, complicated codes. The entire DNA strand is billions of base pairs, or rungs, long. Fortunately, organisms do not need to read and interpret the entire DNA strand each time they need to build some more proteins. Instead, the organism only needs to access a small part of the DNA, a gene, to build the protein for which it carries the code. Some genes are only a few hundred base pairs long while others may be several million base pairs long.