Deoxyribonucleic acid replication
المؤلف:
Hoffman, R., Benz, E. J., Silberstein, L. E., Heslop, H., Weitz, J., & Salama, M. E.
المصدر:
Hematology : Basic Principles and Practice
الجزء والصفحة:
8th E , P186-187
2025-11-05
52
The initiation of DNA replication represents a commitment to cell proliferation and is a central event in the growth and division of all organisms. The assembly of replication machineries is coordinated by multiple proteins, which ensure that DNA synthesis begins at the correct chromosomal locus. Strict regulation of initiation is crucial to viability because inappropriate replication start is linked to genetic instability, including alterations in gene copy number and DNA dam age. DNA replication starts with the assembly of prereplication complexes (pre-RCs) at multiple DNA replication origins during the G1 phase. At the transition from G1 to S phase, the replicative helicase is activated, leading to a change from pre-RCs to preinitiation complexes, which unwind DNA and initiate DNA synthesis. The recognition of pre-RC sites is known as replication origin licensing, and the activation of DNA synthesis is known as origin firing. The separation of these two steps is critical for preventing re-replication within the same cell cycle.
Replication origin licensing in the G1 phase involves the sequential assembly of different proteins. The origin recognition complex (ORC) consists of the six subunits ORC1–ORC6 and initially binds to replication origins. Then, CDC6 and CDT1 associate with the ORC, leading to the recruitment of the minichromosome maintenance (MCM) complex, which consists of six subunits: MCM2 MCM7. The MCM complex is a helicase that forms a double hexamer and encircles double-stranded DNA. Together, the inactive assembly of ORC, CDC6, CDT1, and MCM is known as the pre-RC. Notably, the Meier-Gorlin syndrome, a form of primordial dwarfism, is linked to mutation or depletion of pre-RC components, which impairs cell cycle progression.
During the transition from G1 to S phase, the MCM com plex is phosphorylated by CDC7 and cyclin E–CDK2, leading to the recruitment of CDC45 and the four-protein DNA replication complex GINS. Together, CDC45, MCM, and GINS represent the CMG complex, and dissociation of the two MCM hexamers enables the formation of two preinitiation complexes at a replication origin. Cyclin E–CDK2 phosphorylates Treslin (TICRR) and RECQL4 that, with TOPBP1 and MCM10, facilitate the formation of the preinitiation complex. After replication origins are licensed, ORC1 phosphorylation by cyclin A–CDK1 inhibits rebinding of ORC1 to replication origins, and CDT1 is blocked by Geminin (GMNN) and sent to proteasomal degradation by SCFSKP2 to prevent relicensing during the S phase.
In the S phase, initiation of DNA replication by CDK2 promotes helicase activation, which leads to the unwinding of DNA and recruitment of DNA polymerases alpha (POLA), delta (POLD), and epsilon (POLE), together with PCNA (proliferating cell nuclear antigen) and RFC (replication factor C). The MCM complex translocates from replication origins to unwind double-stranded DNA, whereas POLA binds to single-stranded DNA and initiates DNA synthesis on both the leading and lagging strands by providing an RNA primer and synthesizing the first bases of DNA. POLE and POLD elongate these primers with a base substitution error rate of approximately 10−5. When they occasionally incorporate a false nucleotide, it is usually removed by an exonuclease associated with these DNA polymerases. This so-called proofreading, together with DNA mismatch repair mechanisms, leads to mutation rates of as low as 10−9 per base and per cell division cycle.
An immediate consequence of DNA replication is the disruption of chromatin in front of the replication fork, which results in the release of parental histones. When the replication fork has passed, chromatin is rapidly reassembled onto old as well as newly replicated DNA, and in the reassembled chromatin, half of the histones are recycled from the parental chromatin, whereas the other half are newly synthesized. Then, the multiprotein complex cohesin mediates cohesion between replicated sister chromatids in the S phase, which is essential for chromosome segregation in the M phase.
In addition to DNA replication, the centrosome also needs to be replicated to allow the formation of two daughter cells. The centrosome is the major microtubule-organizing center and consists of two centrioles. Phosphorylation by cyclin E–CDK2 promotes the formation of one procentriole at each centriole. During S and G2 phases, procentrioles elongate until they reach the length of the older centrioles. In the early M phase, the two centrosomes separate to promote cytokinesis.
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