7.2 – DNA Replication
7.2.1 – State that DNA replication occurs in a 5′ → 3′ direction
The 3’ end of the nucleotide is a free -OH group. The 5’ end of the free DNA nucleotide is added to the 3’ end of the chain of nucleotides that is already synthesised.
7.2.2 – Explain the process of DNA replication in prokaryotes, including the role of enzymes (helicase, DNA polymerase, RNA primase and DNA ligase), Okazaki fragments and deoxynucleoside triphosphates
In prokaryotes, the initiation spot on the DNA is called the ori, or origin point. Replications finishes at the ter spot, or termination point.
DNA Helicase
This binds to the double helix to stimulate the separation of the strands. The hydrogen bonds between base pairs break to form replication forks. The helicase is located at these replication forks.
DNA Polymerase
Polymerase III replicates DNA in a 5’ to 3’ direction along the leading strand. It starts at the RNA primer, adding nucleotides using complementary base pairing and moving in the direction of the replication fork. On the other hand, along the lagging strand, polymerase III moves away from the replication fork. This results in the formation of Okazaki fragments. Polymerase I replaces the RNA primers with DNA. However, there is still a gap where two nucleotides have not been connected.
It is possible that errors may occur during replication, but the polymerase has mechanisms of back-checking for mutations.
Okazaki Fragments
These are short strands of DNA that are formed on the lagging strand. Each one is initiated at the replication fork, and is later joined to form one continuous length by DNA ligase. The leading strand is replicated in one continuous length
RNA Primase
For replication to occur, a free 3’ hydroxyl group is required. Primase synthesises at the initiation sites.
DNA Ligase
Gaps a made in the DNA from where the primer is removed. Ligase closes the gap by forming a covalent bond between the phosphate groups and the neighbouring fragments are joined.
Deoxynucleoside Triphosphates
The free nucleotides have three phosphates. During polymerisation, the condensation reaction, two are removed so that only one remains to form the backbone
7.2.3 – State that DNA replication is initiated at many points in eukaryotic chromosomes
Prokaryotic DNA is replicated in a continuous loop. On the other hand, eukaryotic DNA is replicated at multiple points to speed up the reaction. The DNA is unwound at multiple points along the helix into bubbles that expand, allowing replication to continue in both direction. The bubbles eventually fuse.