DNA Replication Copying DNA when you need to make new chromosomes.

When do you make a new set of chromosomes?

When the cell is dividing. The process we are interested in is Mitosis. Cell Division which makes two cells with an exact copy of chromosomes.

It’s a simple process. The DNA unzips, new nucleotides match up with the old bases. The DNA zips up again.

We have to add in some control processes. Enzymes that untwist the DNA and relieve the strain – helicase and gyrase. Enzyme that unzips – DNA Polymerase – causes the hydrogen bonds between the bases to break open so new bases can join.

Hydrogen bonds hold the two strands together 3 hydrogen bonds between G and C Only 2 between A and T

DNA copies are made so that cells can divide. Many protein enzymes are required.

The two strands build up in different ways. The leading strand builds up in order, in the 5- 3 direction (5 to 3 is a good time of the day) The lagging strand build up in chunks, with the gaps filled in later. The chunks are called Okazaki fragments. Another enzyme called DNA Ligase joins the fragments together. Manual pg 82, 83.

Once the DNA has made two lots of chromosome material, the whole cell goes through mitosis. Manual pg 81 Mitosis and the cell cycle.