Two Identical Dna Molecules

Action at the Replication Fork

DNA synthesis occurs in different directions on each strand, as shown by the arrows near the replication fork in step © of Figure 10-10. As the replication fork moves along the original DNA, synthesis of one strand follows the movement of the replication fork. Synthesis on the other strand, however, moves in the opposite direction, away from the replication fork, which leaves gaps in the newly synthesized strand. The gaps are later joined together by an enzyme called DNA ligase.

Prokaryotic and Eukaryotic Replication

In prokaryotic cells, which have one circular chromosome, replication begins at one place along the chromosome. Two replication forks are formed and proceed in opposite directions, like two zippers opening in opposing directions. Replication continues along each fork until they meet and the entire molecule is copied.

In eukaryotic cells, each chromosome is long, but not circular. At the rate that DNA polymerase adds nucleotides (about 50 nucleotides per second in eukaryotic cells), it would take 53 days to replicate the largest human chromosome. Instead, replication begins at many points or origins along the DNA. As with prokaryotes, at each origin, two replication forks move in opposite directions. For example, in a fruit fly chromosome, replication begins simultaneously at about 3,500 sites in a DNA molecule. Only simultaneous replication along chromosomes could allow rapid enough copying of the organism's entire DNA.

figure 10-10

During DNA replication, the two strands separate, and each strand serves as a template. DNA polymerases add complementary nucleotides. At the end of replication, there are two identical copies of the original DNA molecule.

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