Within the nucleus of every cell are long strings of DNA, the code that holds all the information needed to make and control every cell within a living organism.
DNA, which stands for deoxyribonucleic acid, resembles a long, spiraling ladder. It consists of just a few kinds of atoms: carbon, hydrogen, oxygen, nitrogen, and phosphorus. Combinations of these atoms form the sugar-phosphate backbone of the DNA — the sides of the ladder, in other words.
Other combinations of the atoms form the four bases: thymine (T), adenine (A), cytosine (C), and guanine (G). These bases are the rungs of the DNA ladder. (It takes two bases to form a rung — one for each side of the ladder.)
A sugar molecule, a base, and a phosphate molecule group together to make up a nucleotide. Nucleotides are abundant in the cell's nucleus. Nucleotides are the units which, when linked sugar to phosphate, make up one side of a DNA ladder.
During DNA replication, special enzymes move up along the DNA ladder, unzipping the molecule as it moves along. New nucleotides move in to each side of the unzipped ladder. The bases on these nucleotides are very particular about what they connect to. Cytosine (C) will "pair" to guanine (G), and adenine (A) will "pair" to thymine (T). How the bases are arranged in the DNA is what determines the genetic code.
When the enzyme has passed the end of the DNA, two identical molecules of DNA are left behind. Each contains one side of the original DNA and one side made of "new" nucleotides. It is possible that mistakes were made along the way — in other words, that a base pair in one DNA molecule doesn't match the corresponding pair in the other molecule. On average, one mistake may exist in every billion base pairs. That's the same as typing out the entire Encyclopaedia Britannica five times and typing in a wrong letter only once!