Final answer:
During DNA replication, the nitrogen bases are added at the replication forks by the enzyme DNA polymerase. This enzyme adds nucleotides in a specific direction and requires a primer to initiate the process. Replication begins at origins of replication and involves unwinding the DNA and elongating the new strand in a step-by-step manner.
Step-by-step explanation:
The nitrogen bases are added to the split DNA molecule during replication at the replication fork by an enzyme called DNA polymerase. The replication process begins at specific nucleotide sequences called origins of replication, where certain proteins recognize and bind. DNA helicase unwinds the DNA and opens up the helix, forming Y-shaped structures known as replication forks that extend bi-directionally. DNA polymerase then extends an RNA primer synthesized by RNA primase, as it can only add nucleotides in the 5' to 3' direction and requires a free 3'-OH group to form a phosphodiester bond with the incoming nucleotide.
Therefore, replication involves several key steps: initiation, unwinding by helicase, primer synthesis by RNA primase, and the elongation of the new strand by DNA polymerase, which sequentially adds nucleotides that are complementary to the template strand.