DNA(deoxyribose nucleic acid) is a macro-molecule that consists of repeating units, called nucleotides. Nucleotides are made up of a sugar molecule, a phosphate and one out of four nitrogenous bases (Adenine, Thymine, Guanine, Cytosine). DNA is always found as double chains of nucleotides forming a structure that resembles that of a twisted ladder. Repeating sugar and phosphate molecules are forming the sides or backbone of the ladder and pairs of bases the rungs of the ladder. Adenine always pairs with Thymine and Guanine always pairs with Cytosine (A-T, C-G); they are complimentary base pairs. Consequently there are always as many "A's" than there are "T's" in a DNA molecule and equal numbers of "C's" and "G's" (Chargoff rule).

DNA carries the genetic information of the cell. When cells divide, they have to copy that genetic information and pass on one copy to each daughter cell. DNA replication is "semi-conservative": The DNA double helix unravels and unzips (the weak hydrogen bonds between the bases are broken by an enzyme (helicase). Each original DNA strand now serves as a template for the new. Free-floating nucleotides, that are made in other parts of the cell, match with their complimentary nucleotide "partner" and link up. In this way, a single DNA molecules becomes two. One strand of each of the two DNA molecules is the original (conserved), and one strand is freshly assembled (hence: Semi (half) conservative replication).