PolymerasesDNA polymerases are enzy

PolymerasesDNA polymerases are enzymes that synthesize a new strand of DNA complementary toan existing DNA or RNA template (Figure 4.5a). Most polymerases can function onlyif the template possesses a double-stranded region that acts as a primer for the initiationof polymerization.Four types of DNA polymerase are used routinely in genetic engineering. The first isDNA polymerase I, which is usually prepared from E. coli. This enzyme attaches to ashort, single-stranded region (or nick) in a mainly double-stranded DNA molecule, andthen synthesizes a completely new strand, degrading the existing strand as it proceeds(Figure 4.5b). DNA polymerase I is therefore an example of an enzyme with a dualactivity, namely DNA polymerization and DNA degradation.The polymerase and nuclease activities of DNA polymerase I are controlled by dif￾ferent parts of the enzyme molecule. The nuclease activity is contained in the first 323amino acids of the polypeptide, so removal of this segment leaves a modified enzymethat retains the polymerase function but is unable to degrade DNA. This modifiedenzyme, called the Klenow fragment, can still synthesize a complementary DNA strandon a single-stranded template, but as it has no nuclease activity it cannot continue thesynthesis once the nick is filled in (Figure 4.5c). Several other enzymes – natural poly￾merases and modified versions – have similar properties to the Klenow fragment. Formany years, these polymerases were used in DNA sequencing but have now been largely