DNA replication discontinuous

Yes, but in small quantities and only transiently. The nascent (or Okazaki) fragments formed through discontinuous DNA replication contain a short stretch of RNA which serves as a primer for DNA chain growth (Chap. 16). 

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. 

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... 

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. 

DNA replication in E. coli starts at a unique origin (oriC) and proceeds sequentially in opposite directions. More than 20 proteins are required for replication. An ATP-driven helicase unwinds the oriC region to create a replication fork. At this fork, both strands of parental DNA serve as templates for the synthesis of new DNA. A short stretch of RNA formed by primase, an RNA polymerase, primes DNA synthesis. One strand of DNA (the leading strand) is synthesized continuously, whereas the other strand (the lagging strand) is synthesized discontinuously, in the form of 1-kb fragments (Okazaki fragments). Both new strands are formed simultaneously by the concerted actions of the highly processive... 

DNA replication is semiconservative and proceeds bidirectionally from an origin of replication. DNA synthesis is always in the 50 to 30 direction relative to the template strand. A replication fork consists of a leading strand in which DNA synthesis is continuous, and a lagging strand characterized by the discontinuous formation of short Okazaki fragments. 

DNA replication is discontinuous—synthesis of one strand (called the lagging strand) lags behind the other (called the leading strand) and occurs in pieces called Okazaki fragments (Figure 24.4). Replication of the leading strand is continuous (Figure 24.3). 

Mechanisms - The basic mechanisms of DNA replication are quite similar in eukaryotes and prokaryotes. DNA replication is semiconservative and is continuous on one strand and discontinuous on the other. As in prokaryotes, eukaryotic replication entails the assembly of short RNA primer molecules, elongation from the primers by a DNA polymerase, and (on the discontinuous strand) ligation of Okazaki fragments. A significant difference in eukaryotic and prokaryotic DNA replication is in the smaller size of the Okazaki fragments in eukaryotic cells - about 135 bases long, or about the size of the DNA on a nucleosome. 

The general features of DNA replication in eukaryotes are similar to those in prokaryotes. Table 10.5 summarizes the differences. As with prokaryotes, DNA replication in eukaryotes is semiconservative. There is a leading strand with continuous synthesis in the 5 3 direction and a lagging strand with discontinuous synthesis in the 5 3 direction. An RNA primer is formed hy a specihc enzyme in eukaryotic DNA replication, as is the case with prokaryotes, hut in this case the primase activity is associated with Pol a. The structures... 

Recall Describe the discontinuous synthesis of the lagging strand in DNA replication. 

In conclusion, an inhibitor of thymidylate synthesis and uptake and thus of DNA synthesis has been found. It consists of two components, methotrexate and uridine. It inhibits DNA synthesis to the extent and in ways which lead to blockage of the next cell division. This would be expected if DNA replication itself or events coupled to progression in the S phase are required to take the cell on to division. Inhibition by methotrexate + uridine can be discontinued by addition of excess thymidine, or just by washing of the cells with fresh growth medium, with or without added folate. These agents shall be used as tools in attempts to control DNA replication independently of cell division in populations with temperature-synchronized cell division. 

This report has indicated that truly synchronous cell division is limited to the major fraction of cells that initiates replication prior to a time point midway between the two last heat shocks. At this time the population lacks the information that the heat treatment will be discontinued after the next heat shock, and cells continue to engage asynchronously in DNA synthesis. Experiments indicate that this occurs until around the time (EH + 40 minutes) when the next heat shock would have occurred. We have argued that cells that replicate late in the program of heat shocks perturb the division synchrony and subsequently perturb the DNA replication synchrony. It is therefore suggested that further work on temperature synchronization of Tetrahymem cell division should be directed toward the goal of preventing new engagement in DNA replication after a critical time in advance of the synchronous division. 

Okazaki fragment One of the short, discontinuous DNA segments formed on the lagging strand during DNA replication. 

Okazaki fragments The discontinuous stretches in which the lagging strand is initially synthesized during DNA replication these fragments are later joined to form a continuous strand. 

Theoretically, discontinuous synthesis is not necessary for the progress of replication along the 3 ->5 replicating parent strand, but there is evidence that this may also be discontinuous. In addition to normal DNA-dependent DNA synthesis, RNA-dependent synthesis can occur in some cases (see RNA-depen-dent DNA polymerase). [A.Komberg DNA Replication, W.H.Freeman Co., 1980 A.Komberg 1982 Supplement to DNA Replication, W.H.Freeman Co., 1982]... 

S and S/G2 phases of the cell cycle may be related to the discontinuous nature of DNA replication. 

By 1969 there were several reasons to believe that E. coli DNA polymerase could not by itself account for the polymerization events in replication, (a) Genetic analysis of the T4 bacteriophage disclosed that at least six proteins were required for replication of the phage DNA requirements for the E. coli chromosome were at least as complex, (b) Although replication of the DNA duplex was shown to be discontinuous, there was no mechanism known for initiation of the nascent replication fragments no DNA polymerase was observed to have the capacity to start a DNA chain, (c) An E. coli mutant, which appeared to lack DNA polymerase, maintained normal rates and levels of DNA replication but was deficient in repairing DNA lesions.< > Thus many scientists dismissed the relevancy of this enzyme in replication and relegated it to a subsidiary role in repair. 

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