DNA Replication

DNA, short for deoxyribonucleic acid, is a double-stranded, helical molecule that forms the molecular basis for heredity. For DNA replication to occur, this molecule must first unwind, or "unzip," itself to allow the information-encoding bases to become accessible. The base pairing within DNA is of a complementary nature and, consequently, when the molecule unzips, due to the action of enzymes, two strands are temporarily produced, each of which acts as a template. A replication fork is first made—the DNA molecule separates at a small region and then the enzyme DNA polymerase adds complementary nucleotides to each side of the freshly separated strands. The DNA polymerase adds nucleotides only to one end of the DNA. As a result, one strand (the leading strand) is replicated continuously, while the other strand (the lagging strand) is replicated discontinuously, in short bursts. Each of these small sections is finally joined to its neighbor by the action of another enzyme, DNA ligase, to give a complete strand. This whole process gives rise to two completely new and identical daughter strands of DNA.

In the semi-conservative method, two strands of the parent molecule unwind and each becomes a template for the synthesis of the complementary strand of the daughter molecule. A competing hypothesis, which would eventually be disproved, was the conservative hypothesis that states no unzipping occurs and a new DNA molecule is formed alongside the original parent molecule. Consequently, of the two molecules of DNA produced after a round of replication, one of them is the intact parent molecule. By using radioactively labeled nitrogen to produce new DNA over several generations of cell replication by a bacillus species, all of the DNA in the daughter cells contained labeled nitrogen. The bacilli were then placed in media containing unlabeled nitrogen. After a further round of DNA replication the DNA was examined and it was found to contain equal amounts of labeled and unlabeled nitrogen. In the second generation two types of DNA were found—half was identical to the DNA from the first generation and the remaining half was found to consist of entirely unlabeled nitrogen. These results are consistent with the zip fastener model of the semi-conservative hypothesis, but not at all consistent with the conservative hypothesis. Thus, it was shown that DNA replication proceeds via the semi-conservative replication method.

This method of replication, known as the semi-conservative hypothesis, was proposed from the outset of the discovery, with the description of the structure of DNA by biochemists James D. Watson and Francis Harry Compton Crick in 1953. In 1957, biochemist Arthur Kornberg first produced new DNA from the constituent parts and a parent strand, forming synthetic but not biologically active molecules of DNA outside the cell. However, not until the work of Matthew Meselson and Franklin W. Stahl in the late 1950s was the semi-conservative hypothesis conclusively proven true.