Dna Replication

Replication - DNA Synthesis

DNA is copied, or replicated, before every cell division, so that one identical copy can go to each daughter cell. The method of DNA replication is obvious from its structure: the double helix unzips and two new strands are built up by complementary base-pairing onto the two old strands.

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1. Replication starts at a specific sequence on the DNA molecule called the replication origin.

2. An enzyme unwinds and unzips DNA, breaking the hydrogen bonds that join the base pairs, and forming two separate strands.

3.  The new DNA is built up from the four nucleotides (A, C, G and T) that are abundant in the nucleoplasm.

4.  These nucleotides attach themselves to the bases on the old strands by complementary base pairing. Where there is a T base, only an A nucleotide will bind, and so on.

5.  The enzyme DNA polymerase joins the new nucleotides to each other by strong covalent bonds, forming the sugar-phosphate backbone.

6.  A winding enzyme winds the new strands up to form double helices.

7.  The two new molecules are identical to the old molecule.

DNA replication can take a few hours, and in fact this limits the speed of cell division. One reason bacteria can reproduce so fast is that they have a relatively small amount of DNA. 

The Meselson-Stahl Experiment    

This replication mechanism is sometimes called semi-conservative replication, because each new DNA molecule contains one new strand and one old strand. This need not be the case, and alternative theories suggested that a "photocopy" of the original DNA could be made, leaving the original DNA conserved (conservative replication). The evidence for the semi-conservative method came from an elegant experiment performed in 1958 by Meselson and Stahl. They used the bacterium E. coli together with the technique of density gradient centrifugation, which separates molecules on the basis of their density.

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