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Biology · 术语表

什么是 The replication fork and its enzymes？

Definition 18.2 University Biology — Year 1 · 第 18 章 — DNA Replication and Mitosis

Replication begins at an origin — one on the bacterial chromosome, tens of thousands on a eukaryotic genome — where the helix is opened and two replication forks move away in opposite directions. At each fork: a helicase unwinds the helix (one ATP per base pair), single-strand binding proteins keep the separated strands apart, and a topoisomerase ahead of the fork relieves the twist that unwinding builds up. DNA polymerase adds nucleotides only to the $3'$ hydroxyl of an existing chain, so that every new strand grows $5' \to 3'$ and none can start from nothing: a primase first lays down a short RNA primer that the polymerase extends. On the template read $3' \to 5'$ the new strand grows continuously toward the fork: the leading strand. On the other template the new strand must grow away from the fork, in pieces of $1000\text{ to }2000\,$ nucleotides in bacteria and $100\text{ to }200\,$ in eukaryotes, the Okazaki fragments, each with its own primer: the lagging strand. A second polymerase replaces the primers with DNA and a ligase seals the fragments into one strand.

A replication fork. The helicase opens the parental duplex; the leading strand is copied continuously toward the fork, the lagging strand backward in Okazaki fragments, because polymerases build only in the 5' 3' direction. — A replication fork. The helicase opens the parental duplex; the leading strand is copied continuously toward the fork, the lagging strand backward in Okazaki fragments, because polymerases build only in the $5' \to 3'$ direction.

示例

Example 18.4 (Speed and starting points)

A bacterial fork moves at $1000\,$ base pairs a second: two forks copy the $4.6\,\mathrm{Mb}$ of E. coli in $40\,\mathrm{min}$ , and in rich medium, where the cell divides every twenty, a new round starts before the last has finished. A eukaryotic fork, slowed by chromatin, moves at $50\,$ a second: copying a human chromosome of $250\,\mathrm{Mb}$ from one origin would take a month, so the genome is copied from some $40\,000$ origins in eight hours, in an order fixed for each cell type.

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