DNA is a double helix with both strands of the molecule complementary to the other in terms of base pairings. DNA can be envisioned as a ladder, with the sides consisting of a sugar called deoxyribose and a phosphate group and the rung consisting of one of four nitrogenous bases. They are adenine, guanine, cytosine and thymine. The rules for base pairings are adenine pairs with thymine and cytosine to guanine. The smallest unit of DNA is called a nucleotide. Two nucleotides bond together by weak hydrogen bonds between the nitrogenous base pair. When DNA is going to replicate, either side of the strand can serve as a template. This is known as semiconservative replication. At the origins site of the genome, DNA unwinds. The enzyme DNA polymerase is necessary for this step and to aid in adding nucelotides complementary to the template strand. Eventually, each half of the original DNA is copied, resulting in two complete sets of DNA. DNA has directionality. There is the 3' prime end and the 5' prime end. The two strands in the DNA are anti-parallel, with one strand being 5 prime to 3 prime and the other strand is from the 3 prime to the 5 prime direction. DNA polymerase can synthesize DNA in one direction by adding nucleotides to the 3 prime end of the strand.