These bases are the rungs of the DNA 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. 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. On average, one mistake may exist in every billion base pairs.