Semi-Conservative Replication of DNA

When Watson and Crick discovered the structure of DNA they wanted to use the information to help them identify how DNA is replicated. Three different theories of replication were proposed by Watson and Crick. The semi-conservative model, where the DNA strand splits into two halves, which will then create a new DNA strand consisting of the old original half and a new half. The conservative model where the whole of the original DNA strand acts as a template and is replicated to make a completely new strand of DNA. The dispersive model where DNA strands were comprised of some new parts and some old parts spread throughout. Meselon and Stahl ran an experiment to confirm which theory of replication was correct, they did this by growing E. coli in differing isotopes of nitrogen, and found evidence that proved Watson and Cricks theory of semi-conservative replication of DNA. DNA replication is necessary for cell division because it means resulting cell will get a complete set of genes from the original cell. The result of DNA replication will be that the chromosomes within the cell will be made out of two chromatids and each chromatid contains half of the parent cell DNA and half the new daughter cell DNA. These two chromatids will become separated when the cell divides and eventually form 2 separate chromosomes. Many processes happen simultaneously during DNA replication while enzymes are constantly controlling and the process.

Stages of replication:

- In the first stage, no replication has taken place but the strands of DNA in the parent chromosomes are being unwound by the enzyme helicase. Another enzyme known as DNA gyrase relieves the strain caused by unwinding and cutting the strands.
- In the second stage, new DNA for the daughter cell is formed by an enzyme complex called DNA polymerase and other proteins that cause the two strands to break apart
- In the third stage, the new double helix DNA contains one of the original DNA strands from the parent and the newly synthesised strand. The newly joined DNA strands are rewound into the double-helix shape; they are then recoiled around histone proteins and further wrapped up to form chromatids

Image: http://en.wikipedia.org/wiki/Semiconservative_replication
Details on replication models: Biozone student workbook, AQA biology AS CGP