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Base pairing by H bonds the separated parental strands of DNA serves as a template strand for the synthesis of a new complementary strand. The incoming nucleotides for the new strand recognize their partners by base pairing and so are lined up on the template strand (Fig. 5.01). Since A pairs only with T, and since G pairs only with C, the sequence of each original strand dictates the sequence of the new complementary strand.

Synthesis of both new strands of DNA occurs at the replication fork that moves along the parental molecule. Amazingly, in E. coli, DNA is made at nearly 1,000 nucleotides per second. The replication fork consists of the zone of DNA where the strands are separated, plus an assemblage of proteins that are responsible for synthesis, sometimes referred to as the replisome. The result of replication is two double stranded DNA molecules, both with sequences identical to the original one. One of these daughter molecules has the original left strand and the other daughter has the original right strand. The pattern of replication is semi-conservative, since each of the progeny conserves half of the original DNA molecule (Fig. 5.02).

Replication is similar, but not exactly the same, in prokaryotes and eukaryotes. DNA replication in bacteria will be covered initially, as this process is better understood and is less complicated than the process in eukaryotes.

The strands of the parent DNA molecule cannot be separated until the supercoils and helical twisting have been removed.

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