 The modern working definition of a gene is a portion ( or sequence ) of DNA that codes for a known cellular function or process ( e.g. the function " make melanin molecules " ) . A single ' gene ' is most similar to a single ' word ' in the English language . The nucleotides ( molecules ) that make up genes can be seen as ' letters ' in the English language . Nucleotides are named according to which of the four nitrogenous bases they contain . The four bases are cytosine , guanine , adenine , and thymine . A single gene may have a small number of nucleotides or a large number of nucleotides , in the same way that a word may be small or large ( e.g. ' cell ' vs. ' electrophysiology ' ) . A single gene often interacts with neighboring genes to produce a cellular function and can even be ineffectual without those neighboring genes . This can be seen in the same way that a ' word ' may have meaning only in the context of a ' sentence . ' A series of nucleotides can be put together without forming a gene ( non coding regions of DNA ) , like a string of letters can be put together without forming a word ( e.g. udkslk ) . Nonetheless , all words have letters , like all genes must have nucleotides . 
