 What is a gene?  This question is dealt with at further length in section 4 of this article, but a quick answer suffices for present purposes: genes are linear sequences of nucleotides in DNA molecules. Each DNA molecule consists of a double chain of nucleotides. There are four kinds of nucleotides in DNA: guanine, cytosine, thymine, and adenine. The pair of nucleotide chains in a DNA molecule twist around one another in the form of a double helix. The two chains in the helix are bound by hydrogen bonds between nucleotides from adjacent chains. The hydrogen bonding is specific so that a guanine in one chain is always located next to cytosine in the adjacent chain (and vice-versa) and thymine in one chain is always located next to adenine (and vice-versa). Hence, the linear sequence of nucleotides in one chain of nucleotides in a DNA molecule is complimentary to the linear sequence of nucleotides in the other chain of the DNA molecule. A gene is a segment of nucleotides in one of the chains of a DNA molecule. Of course, not every string of nucleotides in DNA is a gene; segments of DNA are identified as genes according to what they do (see below).