DNA replication is the process by which a cell makes an exact copy of its DNA before cell division. The process occurs in three major steps:

1. Initiation: The double helix structure of the DNA molecule is "unzipped" by an enzyme called helicase, which breaks the hydrogen bonds holding the complementary bases of DNA together. The separation of the two single strands of DNA creates a "Y" shape called a replication "fork". A short piece of RNA called a primer, produced by an enzyme called primase, comes along and binds to the end of the leading strand. The primer acts as the starting point for DNA synthesis.

2. Elongation: DNA polymerase binds to the leading strand and then "walks" along it, adding new complementary nucleotide bases (A, C, G, and T) to the strand of DNA in the 5 to 3 direction. As DNA polymerase makes its way down the unwound DNA strand, it relies upon the pool of free-floating nucleotides surrounding the existing strand to build the new strand. The nucleotides that make up the new strand are paired with partner nucleotides in the template strand; because of their molecular structures, A and T nucleotides always pair with one another, and C and G nucleotides always pair with one another. Thus, as a result of complementary base pairing, the replication process proceeds as a series of sequence and anti-sequence copying that preserves the coding of the original DNA.

3. Termination: The replication process continues until the entire DNA molecule has been copied. The result of DNA replication is two DNA molecules consisting of one new and one old chain of nucleotides. This is why DNA replication is described as semi-conservative, half of the chain is part of the original DNA molecule, and half is brand new.

During DNA replication, the DNA molecule separates into two strands, and then produces two new complementary strands following the rules of base pairing. The lagging strand is made in small pieces called Okazaki fragments, while the leading strand is made as a continuous piece. The process occurs quickly, with replication occurring at a rate of 1,000 nucleotides per second in prokaryotic bacteria and 50 nucleotides per second in eukaryotic human DNA.