DNA Replication: Helicase and Polymerase

0:29Why copy DNA?

First, why copy DNA at all? Before a cell divides, it must give each daughter cell a full set of genes. Copying the DNA exactly means the genes pass on unchanged from one generation of cells to the next. That is called genetic continuity. A mistake in copying could change a gene, so the process has to be accurate.

The method is called semi-conservative replication. Half of each new molecule is kept from the old one. Hold that thought.

1:02Helicase unwinds the helix

Step one. The double helix has to open up. The enzyme DNA helicase unwinds the helix. As it moves along, it breaks the hydrogen bonds between the complementary base pairs. Now you have two separate strands, with their bases exposed.

Each hydrogen bond is weak on its own, so the enzyme can open the helix without damaging the strands. You will often hear this called unzipping. In the exam, use the proper words: unwinding, and breaking hydrogen bonds. Notice, helicase breaks hydrogen bonds. It does not break the backbone. The sugar-phosphate backbones stay intact.

1:43Each strand is a template

Each of these old strands now acts as a template. A template is a pattern. The order of bases on the old strand decides the order of bases on the new one. And this is a favourite mark: both strands are used as templates, not just one.

2:00Free nucleotides pair up

Step two. Free nucleotides are available, ready to be used. They are attracted to the exposed bases on the template strand. Each one lines up by complementary base pairing. A pairs with T, and C pairs with G. Hydrogen bonds form between each new base and the template base opposite it.

Try one. If the template reads A, T, G, C, the new strand must read T, A, C, G. Always check your pairs. A with G, or C with T, is the classic slip. Because the pairing is so specific, the new strand has exactly the right base sequence.

2:40DNA polymerase joins them

Step three. The new nucleotides are lined up, but they are not joined to each other yet. DNA polymerase does that job. It catalyses a condensation reaction between adjacent nucleotides on the new strand, forming a phosphodiester bond. Remember condensation from earlier lessons. Two molecules join, and a molecule of water is released.

That builds the sugar-phosphate backbone of the new strand. So remember the two kinds of bond. Hydrogen bonds hold the new base to the template. Phosphodiester bonds join the nucleotides within the new strand.

3:19Two identical molecules

Both templates are copied at the same time. The result is two DNA molecules. Each one contains one original strand and one newly made strand. That is why it is called semi-conservative. Half of each molecule is conserved from the parent.

So the original molecule is not kept whole, and it is not thrown away. Its strands are shared out, one to each new molecule. And because of base pairing, both copies have the same base sequence as the original.

4:28Examiner trap and recap

The traps. Saying helicase unzips the DNA is not enough. Say that it breaks hydrogen bonds. And do not say that DNA polymerase joins the new bases to the template. It joins adjacent nucleotides to each other, in the new strand.

Recap. Helicase unwinds and breaks hydrogen bonds. Bases pair up on both templates. Polymerase joins nucleotides. Each copy keeps one old strand.

Next lesson: how did scientists actually prove that it works this way?