DNA replication and gene mutations

Last time

Before we start: last time, in Stem cells, telomeres and cancer, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A photocopier with a proofreader

Before a cell divides, it must copy three billion base pairs. And it must copy them almost perfectly. The copying is done by a set of enzymes, working at a moving fork, and sometimes a mistake slips through. That mistake is a mutation.

Semi-conservative replication

D N A replication is semi conservative. Each new molecule contains one original strand and one newly made strand. Both strands of the parent molecule act as templates. Bases are added by complementary base pairing, so the copy is exact. After three rounds of replication from one molecule there are eight molecules, but only two of them contain an original strand.

The enzymes in order

D N A helicase breaks the hydrogen bonds between bases and unwinds the helix. This makes a replication fork. Free nucleotides pair with the exposed bases. D N A polymerase joins them to the new strand, by forming phosphodiester bonds. D N A polymerase can only add nucleotides to the three prime end, so it builds every new strand in the five prime to three prime direction. D N A ligase joins the fragments together, making phosphodiester bonds in the gaps.

Leading and lagging strands

The two template strands run in opposite directions. On one, the new strand grows towards the fork and can be made continuously. This is the leading strand. On the other template, the new strand must grow away from the fork. It is made in short pieces, the Okazaki fragments, each started from a primer. D N A ligase then joins the Okazaki fragments into one continuous strand.