DNA polymerase is known to perform all of the following functions, except
Removal of RNA primer
Synthesis of Okazaki fragments
DNA synthesis in 5' → 3' direction
Joining of discontinuously synthesised DNA fragments
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Answer:
As discussed in Chapter 3, DNA replication is a semiconservative process in which each parental strand serves as a template for the synthesis of a new complementary daughter strand. The central enzyme involved is DNA polymerase, which catalyzes the joining of deoxyribonucleoside 5′-triphosphates (dNTPs) to form the growing DNA chain. However, DNA replication is much more complex than a single enzymatic reaction. Other proteins are involved, and proofreading mechanisms are required to ensure that the accuracy of replication is compatible with the low frequency of errors that is needed for cell reproduction. Additional proteins and specific DNA sequences are also needed both to initiate replication and to copy the ends of eukaryotic chromosomes.
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DNA Polymerases
DNA polymerase was first identified in lysates of E. coli by Arthur Kornberg in 1956. The ability of this enzyme to accurately copy a DNA template provided a biochemical basis for the mode of DNA replication that was initially proposed by Watson and Crick, so its isolation represented a landmark discovery in molecular biology. Ironically, however, this first DNA polymerase to be identified (now called DNA polymerase I) is not the major enzyme responsible for E. coli DNA replication. Instead, it is now clear that both prokaryotic and eukaryotic cells contain several different DNA polymerases that play distinct roles in the replication and repair of DNA.
The multiplicity of DNA polymerases was first revealed by the isolation of a mutant strain of E. coli that was deficient in polymerase I (Figure 5.1). Cultures of E. coli were treated with a chemical (a mutagen) that induces a high frequency of mutations, and individual bacterial colonies were isolated and screened to identify a mutant strain lacking polymerase I. Analysis of a few thousand colonies led to the isolation of the desired mutant, which was almost totally defective in polymerase I activity. Surprisingly, the mutant bacteria grew normally, leading to the conclusion that polymerase I is not required for DNA replication. On the other hand, the mutant bacteria were extremely sensitive to agents that damage DNA (e.g., ultraviolet light), suggesting that polymerase I is involved primarily in the repair of DNA damage rather than in DNA replication per se