Biology, 26.07.2019 10:00 kdawg203

What are the three main steps of dna replication and what happens in each

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1. The double helix must "unwind"
2.Nucleotides must be added in the correct order to create a complentary strand
3. New strand must be sealed up

Answer from: tressasill

The three steps are:
1) Initiation
This is when the enzyme helicase breaks the hydrogen bonds between the two DNA strands, allowing the DNA to unwind. Proteins called SSB (single-strand binding proteins) prevent the DNA from joining back together. When the DNA untwists, another enzyme called RNA primase forms a short RNA sequence that the DNA Polymerase III needs to attach to, to begin synthesizing the new DNA strand in the next step, elongation.

2) Elongation
DNA Polymerase III begans synthesizing the new DNA strand by reading the template strand (the original DNA) from 3' to 5' and creating the daughter strand (the new DNA) from 5' to 3'. The leading strand (line b in the picture) is created towards the replication fork (the v part of the Y) and is synthesized continuously, meaning the DNA Polymerase III moves in one motion without lifting off the DNA strand. The lagging strand (the broken parts making up c in the picture) is synthesized discontinuously, meaning in little fragments. These little fragments are called Okazaki Fragments. This happens because DNA Polymerase III synthesizes away from the replication fork, and has to lift up every time the replication fork opens up more.

3) Termination
Another enzyme called ligase joins the Okazaki fragments of the lagging strand together into one large DNA strand. The RNA created by the RNA primase in the initiation step is replaced with DNA by an enzyme called DNA Polymerase I (careful! this is a different enzyme than the DNA Polymerase III that synthesized the DNA in step 2). The ends of the DNA strand have repeated DNA sequences called telomeres that were synthesized by an enzyme called telomerase. These telomeres protect the end of chromosomes and shorten with each time the cell divides. The SSBs detach allowing the DNA strand to join back together, creating two new DNA strands.

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