In a cell, DNA replication begins at specific locations, or origin of replication, in the genome. Unwinding of DNA at the origin, and synthesis of new strands, forms a replication fork. A number of proteins are associated with the fork and assist in the initiation and continuation of DNA synthesis. Most prominently, DNA polymerase synthesizes the new DNA by adding matching nucleotides to the template strand.
DNA replication can also be performed in vitro (artificially, outside a cell). DNA polymerases isolated from cells and artificial DNA primers can be used to initiate DNA synthesis at known sequences in a template DNA molecule. The polymerase chain reaction (PCR), a common laboratory technique, cyclically apply such artificial synthesis to amplify a specific target DNA fragment from a pool of DNA.
[hide] 1 Background on DNA structure
2 DNA polymerase
3 Replication process 3.1 Replication origin
3.3 DNA replication proteins
3.4 Replication fork 3.4.1 Leading strand
3.4.2 Lagging strand
3.4.3 Dynamics at the replication fork
4 Polymerase chain reaction
Background on DNA structure 
DNA usually exists as a double-stranded structure, with both strands coiled together to form the characteristic double-helix. Each single strand of DNA is a chain of four types of nucleotides. Nucleotides in DNA contain a deoxyribose sugar, a phosphate, and a nucleobase. The four types of nucleotide correspond to the four nucleobases adenine, cytosine, guanine, and thymine, commonly notated as A,C, G and T. These nucleotides form phosphodiester bonds, creating the phosphate-deoxyribose backbone of the DNA double helix with the nucleobases pointing inward. Nucleotides (bases) are matched between strands through hydrogen bonds to form base pairs. Adenine pairs with thymine (two hydrogen bonds), and cytosine pairs with guanine (three hydrogen bonds) because a purine must pair with a pyrimidine.
DNA strands have a directionality, and the different ends of a single strand are called the "3' (three-prime) end" and the "5' (five-prime) end" with the direction of the naming going 5 prime to the 3 prime region. The strands of the helix are anti-parallel with one being 5 prime to 3 then the opposite strand 3 prime to 5. These terms refer to the carbon atom in deoxyribose to which the next phosphate in the chain attaches. Directionality has consequences in DNA synthesis, because DNA polymerase can synthesize DNA in only one direction by adding nucleotides to the 3' end of a DNA strand.
The pairing of bases in DNA through hydrogen bonding means that the information contained within each strand is redundant. The nucleotides on a single strand can be used to reconstruct nucleotides on a newly synthesized partner strand.
DNA polymerase 
Main article: DNA polymerase
DNA polymerases adds nucleotides to the 3' end of a strand of DNA. If a mismatch is accidentally incorporated, the polymerase is inhibited from further extension. Proofreading removes the mismatched nucleotide and extension continues. DNA polymerases are a family of enzymes that carry out all forms of DNA replication. However, a DNA polymerase can only extend an existing DNA strand paired with a template strand; it cannot begin the synthesis of a new strand. To begin synthesis,...