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Definition: How does the S phase of the cell cycle ensure the replication of DNA?

The S phase, or synthesis phase, is a crucial stage in the cell cycle where DNA replication takes place. During this phase, the cell ensures the accurate duplication of its genetic material to ensure the proper functioning and survival of the organism.

Initiation of DNA Replication

The S phase begins with the initiation of DNA replication. This process is tightly regulated to ensure that DNA is replicated only once and that each daughter cell receives an identical copy of the genetic material. The initiation of DNA replication involves the assembly of a protein complex known as the pre-replication complex (pre-RC) at specific sites on the DNA called origins of replication.

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Unwinding of DNA

Once the pre-RC is assembled, it recruits additional proteins that unwind the DNA double helix, creating a replication fork. The replication fork is the site where DNA replication occurs. The unwinding of DNA is essential for exposing the template strands that will be used as a blueprint for synthesizing new DNA strands.

Primer Synthesis

Before DNA replication can begin, a short RNA primer is synthesized by an enzyme called primase. The RNA primer provides a starting point for DNA synthesis and is later replaced by DNA.

DNA Synthesis

The actual synthesis of new DNA strands occurs during the S phase. DNA polymerases, enzymes specialized in DNA replication, catalyze the addition of nucleotides to the growing DNA strands. DNA polymerases can only add nucleotides in the 5′ to 3′ direction, resulting in the continuous synthesis of one DNA strand (leading strand). The other DNA strand (lagging strand) is synthesized discontinuously in small fragments called Okazaki fragments.

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Proofreading and Repair

During DNA synthesis, DNA polymerases have built-in proofreading mechanisms that ensure the accuracy of DNA replication. These mechanisms detect and correct errors in nucleotide incorporation, minimizing the occurrence of mutations. Additionally, various DNA repair pathways are activated during the S phase to fix any damage or errors that may have occurred during DNA replication.

Conclusion

The S phase of the cell cycle plays a vital role in ensuring the replication of DNA. Through a series of tightly regulated processes, including initiation, unwinding, primer synthesis, DNA synthesis, proofreading, and repair, the cell ensures the accurate duplication of its genetic material, maintaining the integrity and stability of the genome.

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