DNA is made up of nucleotides that each contain a sugar, phosphate, and nitrogen base. There are four nitrogen bases: adenine, guanine, cytosine, and thymine. The order of these bases determines DNA's genetic code. DNA has a double helix structure with two antiparallel strands bonded together via complementary base pairing between adenine and thymine and cytosine and guanine. DNA replication is semi-conservative and relies on this base pairing to produce two new DNA strands from the existing strands.