Replication

The steps in DNA replication in eukaryotic cells are very much the same as the steps in bacterial replication discussed above. The differences in bacterial and eukaryotic replication relate to the details of the proteins that function in each step. Although amino acid sequences of eukaryotic and prokaryotic replication proteins have diverged through evolution, their structures and functions are highly conserved. However, the eukaryotic systems are often somewhat more complicated.

For example, bacteria require only a single DNA polymerase, using DNA polymerase III for both leading and lagging strand synthesis, and are able to survive without DNA polymerase I. In contrast, eukaryotes require at least four DNA polymerases, DNA polymerases α, δ, ε, and σ. DNA polymerases δ and ε both interact with the sliding clamp, and some evidence suggests that one of these polymerases is used for the leading strand and the other for the lagging strand. One required function of DNA polymerase σ is the synthesis of the RNA primers for DNA synthesis. The precise role of DNA polymerase is not yet known. A second example is removal of the RNA primers on Okazaki fragments. In eukaryotes, primer removal is carried out by RNase H and two other proteins, Fen1 and Dna2, which replace the 5′-to-3′ exonuclease provided by the bacterial DNA polymerase I in bacteria.

Replication continues until two approaching forks meet. The tips of linear eukaryotic chromosomes, called telomeres, require special replication events. Bacterial chromosomes, which contain circular DNA molecules, do not require these special events.