DNA Replication

DNA replication is the process by which a cell copies its entire genome, once per cell cycle, during the S (synthesis) phase, so that each of the two daughter cells produced by division receives a complete, identical set of chromosomes. Replication takes a single parent DNA molecule and produces two DNA molecules, each one identical both to the other and to the original parent. Historically, three competing hypotheses were put forward to explain exactly how this copying might work. In conservative replication, the original double helix would stay entirely intact, unchanged, while an entirely separate, brand-new double-stranded molecule would be built alongside it using the original only as a template to read from. In dispersive replication, the original molecule would be broken up into fragments, each fragment would act as a template for a new complementary fragment, and the two resulting daughter molecules would each end up as a patchwork mixture of old and new segments scattered throughout their length. The mechanism that Watson and Crick themselves proposed in 1953, and the one experimentally confirmed to be correct, is semi-conservative replication: the two original polynucleotide strands of the parent helix unwind and separate from one end, breaking only the hydrogen bonds that had held them together as complementary base pairs (the covalent backbone of each strand is left intact), and each of the two separated single strands then acts independently as a template for building one …