Replication forks discontinuous growth

Growth during Replication Is Bidirectional Growth at the Replication Forks Is Discontinuous Proteins Involved in DNA Replication Characterization of DNA Polymerase I in Vitro Crystallography Combined with Genetics to Produce a Detailed Picture of DNA Poll Function... 

Models for synthesis at the replication fork, (a) Continuous synthesis on both strands. Note that both growth arrows are pointing in the same direction, which would require growth in the 5 — 3 direction on one strand and in the 3 — 5 direction on the other. If growth occurs only in the 5 — 3 direction, synthesis would have to be discontinuous on one strand, as in (b). Alternatively, it could be discontinuous on both strands (c). 

The structure of replication forks in eukaryotes is essentially the same as in bacteria. Chain growth is continuous on the leading strand and discontinuous on the lagging strand. There are equivalents of the polymerases, helicase, primase, SSB, etc., but there are clearly some differences. For example, two different polymerases, DNA polymerase 8 and DNA polymerase a, function on the leading and lagging strand, respectively. Also, the mitochondrion has its own DNA polymerase. 

DNA chain growth occurs on both daughter arms at a replication fork. On one arm, chain growth occurs continuously (5 —>3 ), in the same direction as fork movement. On the other arm, chain growth occurs in separate short pieces (5 — 3 ) and in the direction opposite to fork movement. The short pieces (nascent or Okazaki fragments) subsequently join. Replication in the latter fashion is known as discontinuous DNA replication. 

Unlike the DNA polymerases, RNA polymerase is able to initiate a new RNA chain, using DNA as a template (Chap. 17). The DNA polymerases are able to extend the DNA from an RNA primer. In discontinuous DNA chain growth, a particular type of RNA polymerase, called primase in E. coli, lays down short RNA primers at fairly regular base intervals, as unwinding of the helix at the replication fork proceeds. These primers are involved in the initiation of synthesis of nascent DNA chains by DNA polymerase. 

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