A possible mechanism for replication

A chemical that carries inherited information must be able to copy itself exactly. Complementary base pairing between adenine and thymine and between cytosine and guanine makes this possible.

Watson and Cricks description of DNA suggested that, during replication, the hydrogen bonds connecting base pairs are disrupted allowing the two polynucleotide chains to unwind from one another. Each chain then acts as a template for the synthesis of a new complementary polynucleotide chain. It was suggested that the DNA molecule 'unzips' from one end and new nucleotides already present in the nucleus bind with their complementary bases in each exposed chain. This therefore forms two identical molecules of DNA from the single parent molecule.

Experimental evidence

Arthur Korrnberg and his colleagues were the first to successfully replicate DNA in a test tube. They used the following ingredients:

§ intact DNA (to act as a template)

§ a mixture containing all four nucleotides

§ DNA polymerase (an enzyme which catalyses the synthesis of DNA)

§ ATP (as a source of energy).

New DNA molecules were formed, which contained the same proportions of the four bases as the original parent DNA. This was a strong indication that DNA can copy itself by complementary base pairing.

Semiconservative replication

The idea that DNA unzips before replication is an attractively simple one. This mechanism is called semiconservative replication, because each new molecule of DNA (daughter DNA) contains one intact strand from the original DNA (parental DNA) and one newly synthesised strand. However, semiconservative replication is not the only means by which DNA might replicate by complementary base pairing (figure 1).