DNA Replication

Before a cell can divide, it must duplicate all its DNA. In eukaryotes, this occurs during S phase of the cell cycle.

The Steps:

• a portion of the double helix is unwound by a helicase.
• a molecule of DNA polymerase binds to one strand of the DNA and begins moving along it in the 3' to 5' direction, using it as a template for assembling a leading strand of nucleotides and reforming a double helix.
• because DNA synthesis can only occur 5' to 3', a second DNA polymerase molecule is used to bind to the other template strand as the double helix opens. This molecule must synthesize discontinuous segments of polynucleotides (called Okazaki fragments). Another enzyme, DNA ligase then stitches these together into the lagging strand.

Speed of Replication

Prokaryotes

The single molecule of DNA that is the E. coli genome contains 4.7 x 10⁶ nucleotide pairs. DNA replication begins at a single, fixed location in this molecule, the replication origin, proceeds at about 1000 nucleotides per second, and thus is done in no more than 40 minutes. And thanks to the precision of the process (which includes a "proof-reading" function), the job is done with only about one incorrect nucleotide for every 10⁹ nucleotides inserted. In other words, more often than not, the E coli genome (4.7 x 10⁶) is copied without error!

Eukaryotes

The average human chromosome contains 150 x 10⁶ nucleotide pairs which are copied at about 50 nucleotides per second. The process would take a month (rather than the hour it actually does) but for the fact that there are many replication origins on the eukaryotic chromosome. Replication begins at some origins earlier in S phase than at others, but the process is completed for all by the end of S phase. As replication nears completion, "bubbles" of newly replicated DNA meet and fuse, finally forming two new molecules.