The semi-conservative process by which one DNA replicates itself to form two identical daughter DNA molecules.
Explanation:
DNA replication occurs in the synthesis (S) phase of the cell cycle, in preparation for mitosis. It is said to be a semi-conservative process, meaning one original strand of DNA from the original molecule is always conserved, as suggested by the Meselson and Stahl experiment using N-14 and the N-15 isotope. It involves the enzymes DNA helicase, which unwinds the original DNA double helix, and DNA polymerase, which forms the phosphodiester bond between the deoxyribose sugar of one nucleotide and the phosphate molecule of another to form the sugar-phosphate backbone of the new DNA strand.
The process is as follows:
Stage 1: The original (parent) DNA molecule is a double helix.
Stage 2: The DNA molecule unwinds as hydrogen bonds between the complementary base pairs are broken. This forms two separate DNA (daughter) strands. Each of these is known as a template. This requires the enzyme DNA helicase.
Stage 3: From the pool of free DNA nucleotides in the nucleus, those with the complementary bases to the exposed bases on the two open template DNA daughter strands pair up.
Stage 4: Hydrogen bonds reform between the complementary base pairs, i.e. between A and T and G and C.
Stage 5: Phosphodiester bonds form between the pentose sugar molecule of one nucleotide and the phosphate group of the next nucleotide in line. This linking together of the nucleotides is called polymerisation and is catalysed by the enzyme DNA polymerase.
Stage 6: The result is two new daughter DNA molecules. These are identical to each other and the original parent DNA molecule.
