5.05:_DNA_Replication

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<figure><img class="internal" alt="Diagram of DNA replication fork, showing template strands, leading and lagging strands, DNA polymerase, DNA ligase, and Okazaki fragments. Arrows indicate direction of replication." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6524/ReplicationFork.gif?revision=1" /><figcaption>Figure 5.5.3: Control of Replication</figcaption></figure> <p class="lt-bio-4743" style="background-color: unset;">With their multiple origins, how does the <strong>eukaryotic cell</strong> know which origins have been already replicated and which still await replication?</p> <p class="lt-bio-4743" style="background-color: unset;"><strong>An observation:</strong> When a cell in G<sub><font size="2">2</font></sub> of the cell cycle is fused with a cell in S phase, the DNA of the G<sub><font size="2">2</font></sub> nucleus does not begin replicating again even though replication is proceeding normally in the S-phase nucleus. Not until mitosis is completed, can freshly-synthesized DNA be replicated again.</p> <p class="lt-bio-4743" style="background-color: unset;">Two control mechanisms have been identified &mdash; one <strong>positive</strong> and one <strong>negative</strong>. This redundancy probably reflects the crucial importance of precise replication to the integrity of the genome.</p> <span id="Licensing:_positive_control_of_replication"></span><span id="Licensing:_positive_control_of_replication"></span><h2 style="background-color: unset;" class="lt-bio-4743">Licensing: positive control of replication</h2> <p class="lt-bio-4743" style="background-color: unset;">In order to be replicated, each origin of replication must be bound by:</p> <ul> <li class="lt-bio-4743" style="background-color: unset;">an <strong>O</strong>rigin <strong>R</strong>ecognition <strong>C</strong>omplex of proteins (<strong>ORC</strong>). These remain on the DNA throughout the process.</li> <li class="lt-bio-4743" style="background-color: unset;">Accessory proteins called <strong>licensing factors</strong>. These accumulate in the nucleus during G<sub><font size="2">1</font></sub> of the cell cycle. They include: <ul> <li class="lt-bio-4743" style="background-color: unset;">Cdc-6 and Cdt-1, which bind to the <strong>ORC</strong> and are essential for coating the DNA with</li> <li class="lt-bio-4743" style="background-color: unset;">MCM proteins. Only DNA coated with MCM proteins (there are 6 of them) can be replicated.</li> </ul> </li>
</ul> <p class="lt-bio-4743" style="background-color: unset;">Once replication begins in S phase,</p> <ul> <li class="lt-bio-4743" style="background-color: unset;">Cdc-6 and Cdt-1 leave the ORCs (the latter by ubiquination and destruction in proteasomes).</li> <li class="lt-bio-4743" style="background-color: unset;">The MCM proteins leave in front of the advancing replication fork.</li>
</ul>

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<hr class="autoattribution-divider" /><div class="autoattribution"><p>This page titled <a target="_blank" class="internal mt-self-link" href="/Sandboxes/johnnyphung/biology/05:_DNA/5.05:_DNA_Replication">5.5: DNA Replication</a> is shared under a <a rel="nofollow" href="https://creativecommons.org/licenses/by/3.0" target="_blank">CC BY  3.0</a>  license and was authored, remixed, and/or curated by <a rel="nofollow" target="_blank" href="http://www.biology-pages.info/">John W. Kimball</a> via <a rel="nofollow" href="https://www.biology-pages.info/" target="_blank">source content</a> that was edited to the style and standards of the LibreTexts platform.</p></div>

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