How is DNA structured?

DNA stands for deoxyribonucleic acid. Two polynucleotide (which means nucleotide polymer) strands are twisted together and form what looks like a ladder - the DNA double helix. These two strands run antiparallel to each other (parallel but in opposite directions) with the nitrogenous bases pointing into the middle. These bases are where the information for coding proteins is stored. Hydrogen bonds between the nitrogenous bases hold the strands together and form the 'rungs' of the ladder. This base pairing is specific - a pyrimidine always binds with a purine. Specifically, guanine (G) always binds with cytosine (C), and adenine (A) always binds with thymine (T). This is called complementary base pairing. This ensures that the two polynucleotide chains are always equally spaced apart; purines (G and A) are larger (double ring structure) and so need to always bond to a pyrimidine (single ring structure). The root of complementary base pairing is the number of hydrogen bonds each base can form - there are three hydrogen bonds between G and C but only two between A and T.