Replication

Given the DNA structure, three questions immediately arose: How is DNA copied to allow faithful inheritance of genes, how does DNA store information, and how does DNA use that information to determine the properties of the cell? Watson and Crick addressed the theoretical underpinnings of each of these issues in their next paper, published in May 1953. Regarding copying, they wrote:

Now our model for deoxyribonucleic acid is, in effect, a pair of templates, each of which is complementary to the other. We imagine that prior to duplication the hydrogen bonds are broken, and the two chains unwind and separate. Each chain then acts as a template for the formation onto itself of a new companion chain, so that eventually we shall have two pairs of chains where we only had one before. Moreover, the sequence of the pairs of bases will have been duplicated exactly.

In outline, this is precisely correct, as confirmed in 1957 by Matthew Meselson (b. 1929) and Franklin Stahl (b. 1930). They fed bacteria base pairing linking of two nucleotides (either DNA or RNA) linked by weak bonds oooc ,oc radioactive nucleotides, so that both DNA chains would be labeled with radioactivity. They then removed the radioactive nucleotide source and measured the dilution of the radioactivity in each round of DNA copying. After one round, each DNA molecule had half the amount of radioactivity. According to the Watson-Crick prediction, this meant that one strand of each was completely new. After the second round, half the DNA molecules maintained this level of radioactivity, and half had none at all, just as expected from the Watson-Crick model.

This process, in which one parental DNA strand is conserved, unchanged, and acts as a template to synthesize a new partner, is called semi-conservative replication. The details of the copying process, called replication, are much more complex than this simple outline, though, and the entire process is still not fully understood in all its particulars. Central to it is DNA polymerase enzyme polymerase, a large multiprotein complex first discovered in 1957 by Arthur complex that synthe- Kornberg (born 1918) and Severo Ochoa (1905-1993).

individual nucleotides

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