DNA replication

42 DNA replication





Questions



image How is DNA synthesis achieved against the two DNA strands?

image How does a cell ensure rapid replication of its genome during cell division?

image How may nucleotide analogues interrupt the cell cycle?


Replication of strands


During S phase (6–8 h), each of the 46 chromosomes are replicated to form a sister chromatid joined to the original molecule through the centromere. DNA replication commences at specific origin-of-replication sites with ATP-dependent unwinding of the chromatin structure by DNA helicase to expose binding sites for a priming RNA polymerase. DNA topoisomerase also acts to break and rejoin the unwound strands to prevent tangling of the structure through super-coiling. Single-stranded-DNA-binding protein binds to the unwound strands to protect them from degradation before DNA duplication begins. One of the unwound strands will have 3′–5′ polarity (leading strand) and the other will have 5′–3′ polarity (lagging strand) Because DNA polymerase only catalyses synthesis in the 5′–3′ direction, a different process is required for the lagging strand.



Replication against the 3′–5′ leading strand


Initiation requires the 5′–3′ synthesis of a small RNA fragment (10–20 nucleotides) against the template strand (leading strand) reading in the 3′ to 5′ direction (Fig. 3.42.1

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Jun 18, 2016 | Posted by in BIOCHEMISTRY | Comments Off on DNA replication

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