<P> DNA is made up of a double helix of two complementary strands . During replication, these strands are separated . Each strand of the original DNA molecule then serves as a template for the production of its counterpart, a process referred to as semiconservative replication . Cellular proofreading and error - checking mechanisms ensure near perfect fidelity for DNA replication . </P> <P> In a cell, DNA replication begins at specific locations, or origins of replication, in the genome . Unwinding of DNA at the origin and synthesis of new strands, accommodated by an enzyme known as ligase, results in replication forks growing bi-directionally from the origin . A number of proteins are associated with the replication fork to help in the initiation and continuation of DNA synthesis . Most prominently, DNA polymerase synthesizes the new strands by adding nucleotides that complement each (template) strand . DNA replication occurs during the S - stage of interphase . </P> <P> 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 applies such artificial synthesis to amplify a specific target DNA fragment from a pool of DNA . </P> <P> DNA 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 abbreviated as A, C, G and T. Adenine and guanine are purine bases, while cytosine and thymine are pyrimidines . These nucleotides form phosphodiester bonds, creating the phosphate - deoxyribose backbone of the DNA double helix with the nucleobases pointing inward (i.e., toward the opposing strand). Nucleobases are matched between strands through hydrogen bonds to form base pairs . Adenine pairs with thymine (two hydrogen bonds), and guanine pairs with cytosine (three hydrogen bonds). </P>

Where does the dna molecule separate during replication