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I'm currently reading a little about DNA replication, and have come accross the following statement;

Replication starts from a fixed point and is bi-directional ... In Eukaryotes, there are multiple replication forks, each progressing in a bi-directional fashion.

If there is a single, long strand of DNA in a Eukaryotic cell, I see potential problems with this:

These forks involve opening up a section of double-stranded DNA, and each strand becoming a double strand in a newly synthesised piece of DNA. At some point, before any single fork has become two new double-stranded molecules, another fork could 'collide' with this, causing it to attempt to replicate the non-finished section.

Simply, how can one replication fork meet another without either exponentially increasing the number of strands being replicated?

Also, on a more general level, I would be quite interested to know the actual benefit of this, when, typically, only a single copy of the double-stranded molecule needs making.

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To get to the exponentially increasing number of strands, the replication would have to be started on the already replicated strand, and not on the original strand. Replication in eukaryotes is tighly controlled and such an event is prevented by the regulation.

How can the cell prevent re-replication of the strand that is currently synthesized? By timing the steps of the replication in strict order and preventing any replication outside of that order.

Replication starts at defined origins of replication. In the G1 phase, pre replication complexes (pre-RCs) are assembled at the origins of replications. At these pre-RCs the replication can be initiated, but pre-RCs itself cannot be assembled in the later phases of the cell cycle. When the replication starts, the pre-RC is converted to a post-RC which can't initiate the replication anymore, preventing re-replication.

The benefit of multiple origins of replication is replication speed. Eukaryotic genomes can be much, much larger than those of simple bacteria. Replicating them using a single origin of replication would take a very long time.

Some further reading material:

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