Candidates should be able to analyse, interpret and evaluate data concerning early experimental work relating to the role and importance of DNA.

One experiment performed when investigating the nature of DNA was one to find out if its replication is conservative or semi conservative.

Conservative means there would be one completely new piece of DNA made and one completely old one left.
Semi-conservative means there are two pieces of DNA that are both made up of half old and half new DNA.

DNA contains nitrogen.
There are two isotopes of nitrogen 14N and 15N.
15N is heavier and 14N lighter.
If DNA contains 15N it would sink towards the bottom of a tube when centrifuged.
If DNA contains 14N it would be towards the top of a tube when centrifuged.

Scientists grew bacteria on 14N, they then removed the DNA and centrifuged it- it came out at the top of the tube, showing that the DNA of the bacteria would have in it the DNA it grew on.

Scientists then grew bacteria on 15N. This DNA came out at the bottom of the tube.

Scientists moved the bacteria from the 15N to the 14N and left them long enough that the DNA would replicate once.

They found that DNA from these bacteria came out in the middle of the tube. This was because they had one light strand, and one heavy strand. This showed that the DNA included one old strand of DNA from when it was on 14N and one new strand of DNA from when it was on 15N.

So they could conclude that DNA replicated semi-conservatively, making two strands of half-new-half-old DNA.

The semi-conservative replication of DNA in terms of • breaking of hydrogen bonds between polynucleotide strands • attraction of new DNA nucleotides to exposed bases and base pairing • role of DNA helicase and of DNA polymerase.

The replication of DNA is called semi-conservative because each new piece of DNA is half old DNA and half new.

1. DNA helicase breaks the hydrogen bonds between the base pairs
2. The two strands of the DNA separate, leaving 2x polynucleotide strands
3. Free activated nucleotides are attracted to the complimentary nucleotides on the polynucleotide strand
4. They are then joined together by DNA polymerase

Good to visualise with a animation: http://www.youtube.com/watch?v=zdDkiRw1PdU