1. Replication Initiation and termination a. What are sites of replication initi

Question

1.      Replication Initiation and termination

a.      What are sites of replication initiation?

b.      Why is the timing and regulation of replication initiation important?

c.       When do pre-initiation complexes form in eukaryotic cells? What triggers their firing?

d.      How does replication differ for circular vs. linear chromosomes?

e.      What is the end replication problem?

f.        How are the ends of linear chromosomes protected? Why is this important?

g.       How are the ends of linear chromosomes extended?

Explanation / Answer

a. Replication starts at a particular sites called origins of DNA replications i.e. Ori C in the case of E. coli.

b. Replication is a highly regulated process and strictly controlled by the time dependent expression and regulation of cyclins, once the time is passed the cyclins were degraded rapidly and controls the cell cycles under stringent situation.

c. The replication initiation proteins identify and bind to specific modifications to the nucleosomes in the origin region followed by certain proteins that recognize and bind to the origin of replication and then allow the other proteins necessary for DNA replication to bind the same region followed by two copies of an enzyme called helicase proteins which are recruited to the origin. Each helicase unwinds and separates the DNA helix into single-stranded DNA. As the DNA opens up, Y-shaped structures called replication forks.

d. DNA replication in circular chromosomes basically has single replication initiation sites compared to multiple initiation sites in linear chromosome

5. After the removal of last piece of 3’ primer an overhanging 3’ end is obvious. So that particular portion of overhanging template DNA is not replicated. Thus, it important to recover this shortage in the newly synthesised daughter strand, if not the resulted daughter strand will eventually become critically short of successive replication.

6. The ends of linear chromosomes protected by a short tandem repeated sequence called telomere sequences i.e. TTAGGG in humans. Telomeres are the caps at the end of each strand of DNA that protect our chromosomes, the only part of the chromosome that is lost, is the telomere, and the DNA is left undamaged. Without telomeres, important DNA would be lost every time a cell divides.

7. The ends of linear chromosomes extended with a specialized enzyme called telomerase which is a type of reverse transcriptase which is having its own RNA template for DNA synthesis.

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