 The molecular basis for genes is deoxyribonucleic acid ( DNA ) . DNA is composed of a chain of nucleotides , of which there are four types : adenine ( A ) , cytosine ( C ) , guanine ( G ) , and thymine ( T ) . Genetic information exists in the sequence of these nucleotides , and genes exist as stretches of sequence along the DNA chain . Viruses are the only exception to this rule — sometimes viruses use the very similar molecule , RNA , instead of DNA as their genetic material . Viruses cannot reproduce without a host and are unaffected by many genetic processes , so tend not to be considered living organisms . 
