DNA has three main functions: storing genetic information, copying that information during cell division, and transmitting the information to offspring. DNA is made up of nucleotides containing a sugar, phosphate, and one of four nitrogenous bases. Rosalind Franklin discovered that DNA has a double helix structure with the bases on each strand bonding with their complement on the other strand in a regular pattern. Watson and Crick were the first to model DNA as a double helix with antiparallel strands held together by hydrogen bonding between complementary nucleotide base pairs. DNA replication copies the DNA before cell division, unwinding the double helix and using DNA polymerase to add complementary bases to each strand.