The general shape of the molecule is like a ladder, one end of which has been repeatedly twisted, so that each of the uprights is a helix. Each of the rungs of the ladder consists of a pair of bases, either with adenine at one end and thymine at the other, or with cytosine at one end and guanine at the other.
Any base can occur at either end of a rung.
The directions for constructing proteins are encoded in the sequence of bases attached to one of the two uprights of this molecular ladder. Each group of three adjacent bases along one upright of a DNA molecule is a codon . A codon encodes the instruction to place some particular amino acid at a position in the protein that corresponds to the position of that codon. The remaining lines contain the bases, denoted by the lower-case letters a , c , g , and t , fifty bases to a line, except that the last line has only six bases on it. At a few positions in the sequence, the base has not yet been determined.