Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

Replication
Eukaryotic chromosome replication

Several DNA polymerases have been discovered in eukaryotes: DNA polymerase α; DNA polymerase β; DNA polymerase δ; DNA polymerase ε; DNA polymerase γ; and DNA polymerase ζ. DNA polymerases α, β, δ, and ε participate in METABOLISM/36.html">DNA Replication.

DNA polymerase α forms a tight complex with primase. This complex is capable of initiating DNA Synthesis de novo. First, the primase component synthesizes RNA primers approximately 10 NUCLEOTIDES in length, after which the DNA polymerase takes over to synthesize DNA. DNA polymerase α typically lacks 3'→5' exonuclease activity.

DNA polymerase β participates in Okazaki fragment Processing and nuclear DNA Repair.

DNA polymerase δ exhibits both polymerase and 3' → 5' exonuclease activities.

DNA polymerase ε exhibits both polymerase and 3' → 5' exonuclease activities.

DNA polymerase γ is responsible for the Replication and Repair of Mitochondrial DNA and is encoded by the nuclear genome.

DNA polymerase ζ possesses polymerase and 3' → 5' exonuclease activities, and carries out DNA synthesis during SOS repair.

The rate of DNA synthesis in eukaryotes is approximately an order of magnitude lower than in prokaryotes, running at about 50 nucleotides per second.

A schematic Overview of eukaryotic DNA replication is shown in Fig. 2.12.

Class="center">

Fig. 2.12. Eukaryotic DNA replication

Okazaki fragments are shorter in eukaryotes than in prokaryotes, measuring about 100 nucleotides. Replication initiation in eukaryotes occurs at numerous replication origins. For example, in mammals, replication origins are spaced approximately ~ 100 kb apart. DNA synthesis within replicons proceeds in both directions (Fig. 2.13). The presence of multiple replicons in eukaryotes is due to the large size of their DNA and the lower activity of their DNA polymerases. This polyreplicon Organization of DNA requires that each replicon fires only once during each Cell Division cycle. Replication ensures the faithful transmission of genotypes across generations.

Fig. 2.13. DNA synthesis within replicons

The progression of the Replication fork halts when it collides with another fork or reaches the end of the chromosome. Thus, replication yields two daughter DNA molecules that are exact copies of the parental molecule. Furthermore, each daughter DNA molecule consists of one parental strand and one newly synthesized daughter strand. This mode of replication is semiconservative. Following mitosis, the resulting DNA molecules are distributed evenly between the daughter Cells. Replication ensures the faithful reproduction of genotypes across successive generations.