|MoBio||Mechanism of DNA Replication||Chapter 7|
DNA molecules are synthesized by DNA polymerases from deoxyribonucleoside triphosphate (dNTP). The chemical reaction is similar to the synthesis of RNA strands (Chapter 4 Section B). Both DNA and RNA polymerases can extend nucleic acid strands only in the 5' to 3' direction. However, the two strands in a DNA molecule are antiparallel. Therefore, only one strand (leading strand) can be synthesized continuously by the DNA polymerase. The other strand (lagging strand) is synthesized segment by segment (more info).
Three types of DNA polymerases exist in E. coli: I, II and III. The DNA polymerase I is used to fill the gap between DNA fragments of the lagging strand. It is also the major enzyme for gap filling during DNA repair. The DNA polymerase II is encoded by the PolB gene, which is involved in the SOS response to DNA damage. DNA replication is mainly carried out by the DNA polymerase III.
The DNA polymerase III consists of several subunits, with a total molecular weight exceeding 600kD. Among them, α, ε, and θ subunits constitute the core polymerase. The major role of other subunits is to keep the enzyme from falling off the template strand. As shown in Figure 7-B-1, two β subunits can form a donut-shaped structure to clamp a DNA molecule in its center, and slide with the core polymerase along the DNA molecule. This allows continuous polymerization of up to 5 x 105 nucleotides. In the absence of β subunits, the core polymerase would fall off the template strand after synthesizing 10-50 nucleotides.
There are five types of DNA polymerases in mammalian cells: α, β, γ, σ, and ε. The γ subunit is located in the mitochondria, responsible for the replication of mtDNA. Other subunits are located in the nucleus. Their major roles are given below: