Enzymology of DNA Replication in Bacteria

As already discussed, the replication of a replicon can be divided into three stages initiation, elongation and termination. During the elongation phase, DNA chain growth takes place at 

Question Which enzyme is responsible for the synthesis of new DNA chains at the replication fork  

The enzyme, which uses unwound, single-stranded DNA as a template, is called a DNA polymerase. There are three distinct DNA polymerases in E. coli DNA polymerase I, II, and III. DNA polymerase I is the most abundant, and DNA polymerase III the least abundant. These two enzymes have important roles in the overall process of DNA replication. The role of DNA polymerase II has not yet been clearly established. 

However, this is only one of three types of reactions catalyzed by DNA polymerase I. Its two other activities involve the hydrolysis of phosphodiester bonds. One is a 5 — 3 exonuclease acting on double-stranded DNA, and the other is a 3 - 5 exonuclease acting on a frayed or mismatched terminus of double-stranded DNA. DNA polymerase I is thus a multifunctional enzyme. It consists of a single 

DNA polymerase III is also a multifunctional enzyme. It resembles DNA polymerase I in catalytic properties however, there are slight differences with respect to the type of template primer preferred for DNA synthesis as well as the preferred substrates for the two exonuclease activities. It contains many polypeptide subunits (a, e, 6, t, y, 8. y, ip and B)- The complex containing all subunits (Mr 900,000) is called the DNA polymerase III holoenzyme, while that comprising just a, e and 8 exhibits the polymerase activity and is referred to as the core enzyme. The holoenzyme carries out most of the DNA synthesis at the replication fork in vivo. 

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