Chromosomal DNA replicates through a process that begins at a specific point called the origin of replication and proceeds bidirectionally until the entire chromosome is copied.
Here's a breakdown of the process:
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Initiation: Replication begins at specific sites on the chromosome called origins of replication. These origins are recognized by initiator proteins that bind and unwind the DNA double helix.
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Replication Fork Formation: Once the DNA is unwound, two replication forks form at the origin. A replication fork is a Y-shaped structure where the DNA is actively being replicated.
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Bidirectional Replication: DNA replication proceeds in both directions from the origin of replication. This means that each replication fork moves away from the origin, unwinding and replicating the DNA as it goes.
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Key Enzymes and Proteins: Several enzymes and proteins are crucial for DNA replication:
- DNA Helicase: Unwinds the DNA double helix at the replication fork.
- Single-Strand Binding Proteins (SSBPs): Prevent the separated DNA strands from re-annealing.
- DNA Primase: Synthesizes short RNA primers, providing a starting point for DNA polymerase.
- DNA Polymerase: Adds nucleotides to the 3' end of the RNA primer, synthesizing new DNA strands.
- DNA Ligase: Joins the Okazaki fragments (short DNA fragments synthesized on the lagging strand) together to create a continuous strand.
- Topoisomerases: Relieve the torsional stress caused by unwinding the DNA.
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Leading and Lagging Strands: Because DNA polymerase can only add nucleotides to the 3' end of a DNA strand, one strand (the leading strand) is synthesized continuously in the direction of the replication fork. The other strand (the lagging strand) is synthesized discontinuously in short fragments called Okazaki fragments, which are later joined together by DNA ligase.
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Speed of Replication: Replication forks move relatively quickly, replicating DNA at a rate of approximately 500-1000 nucleotides per second.
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Termination: Replication continues until the replication forks meet each other on the opposite side of the chromosome. The newly synthesized DNA strands are then sealed together, resulting in two identical copies of the original chromosome.
In summary, chromosomal DNA replication is a highly coordinated and efficient process that involves the unwinding of DNA, the synthesis of new DNA strands by DNA polymerase, and the joining of Okazaki fragments to create continuous strands. The process begins at a single origin of replication and proceeds bidirectionally until the entire chromosome is duplicated.
