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Table of Contents

Replication

Chromosomes must make a copy of themselves so that when cells divide, each daughter cell must receive an exact copy of the genetic information. This copying of DNA is called replication and takes place in a cell during interphase.

Replication occurs as follows: - The hydrogen bonds holding the base pairs break and the two halves of the molecule separate.

- DNA unwinds and as the strands separate, DNA polymerase catalyses the addition of free nucleotides to the exposed bases.

- Each chain acts as a template so that free nucleotides can be joined to their complementary bases by the enzyme, DNA polymerase.

- The result is two DNA molecules, each made up of one new synthesised chain and one original chain (that has been conserved from the original molecule).

- This is called the semi conservative hypothesis. This theory was proposed by Watson and Crick in 1954 and was confirmed shortly after by the evidence provided by Meselson and Stahl's experiments.

The Experiment

The experiments involved the use of an ultra-centrifuge. This rotates centrifuge tubes containing liquid suspensions at very high speeds which results in the denser particles separating out at a lower point in the tube than the lighter particles. The light particles would stay at the bottom while the larger particles would raise to the top.

1. The scientists cultured the bacterium, Escherichia coli, for several generations on a medium containing amino acids made with the heavy isotope of nitrogen 15N. The bacteria incorporated the 15N into their nucleotides and then into their DNA so that all the DNA contained the heavy isotope of nitrogen. They extracted the bacteria and centrifuged it and allowed the DNA to settle at different points of the test tube.

2. The bacteria with the heavy isotope of nitrogen were washed, then transferred to a medium containing the normal, lighter form of nitrogen, 14N, and were allowed to divide once more.

3. When extracts of DNA from this first generation culture were centrifuged it was shown to have a mid point density since half the strand was made up of the original strand of 15N (Heavy isotope) DNA and the other half was made up of the new strand containing 14N.

4. When extracts were taken from the second generation grown in the light isotope of nitrogen, the DNA settles at the mid point and high points of the test tube. This was enough evidence to conclude that the semi conservative hypothesis was indeed true and was a source of reliable information.

Biology


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