ZOOHCC - 501: Molecular Biology (Theory)
Unit 2: DNA Replication
Unit 2: DNA Replication
Semi-discontinuous mode of DNA replication:
Semi-discontinuous mode of DNA replication was discovered by Okazaki and Okazaki
DNA replication is 'semi-discontinuous' because one strand is synthesized continuously and the other strand is discontinuously synthesized by the formation of Okazaki fragments. It was discovered in 1968 by two Japanese scientists, Reiji Okazaki and his wife Tsuneko Okazaki. The term "Okazaki fragment" was given after them.
Explanation:
A. Kornberg was the first to characterize the enzyme now known as DNA polymerase I and believed it to be responsible for DNA replication. However, it was later shown that this enzyme is primarily involved in DNA repair and not in DNA replication. Another enzyme, DNA polymerase III, is now known to be responsible for DNA replication, synthesizing DNA in the 5'→3' direction. The two DNA strands have opposite polarities, so the same enzyme cannot be used to synthesize her DNA on both strands unless the synthesis occurs disjointly. Such fragments, called Okazaki fragments (after their discoverer's name), have indeed been observed in which DNA synthesis is discontinuous in one of the two strands and the segment is later enlisted with the help of a ligase enzyme. Known to borrow and fuse
It was believed that there was good evidence that DNA synthesis was discontinuous on both strands.However, it is now known that DNA synthesis is continuous on one strand and discontinuous on the other. (Figure 26.6). Once the DNA double helix is unwound, DNA replication readily proceeds continuously in either strand in the 5' to 3' direction (3' to 5' strand). This is the main chain. If synthesis on the other strand (5'-3') is done in the 5'-3' direction, it must be synthesized in the opposite direction to the leading strand. This strand is the lagging strand, in which synthesis occurs discontinuously in segments and these segments fuse to produce an intact lagging strand. This behavior in which the leading strand is synthesized continuously and the lagging strand is synthesized discontinuously is called semi-discontinuous replication.
In certain viruses like adenovirus or Φ29, linear DNA replicates from the two ends by strand . displacement, so that both strands can be copied in 5' to 3' direction simultaneously without any need for discontinuous replication. Therefore, it is obvious that semi-discontinuous replication is the result of a need to synthesize both strands simultaneously from the same origin.
Why is DNA replication said to be semiconservative?
DNA replication is said to be semi-conservative because of the replication process in which the resulting double helix is composed of both the old and new strands. serves as a template for synthesizing The DNA replication process takes place during the synthetic stage (S) of the eukaryotic cell cycle. Since each strand of DNA carries the same genetic information, both strands of the duplex serve as templates for replicating complementary new strands. The two double helices, each containing an "old" DNA strand and a "new" DNA strand, are identical to the original double helix.
Polyoma virus DNA replication is semi-discontinuous:
In sharp contrast to Simian Virus 40 (SV40), Polyomavirus (PyV) has been reported to replicate discontinuously on both arms of the replication fork. To clarify the relationship between the mechanisms of DNA replication in these closely related viruses, the distribution of DNA strands simultaneously RNA-primed at the replication forks of PyV and SV40 replicating DNA purified from virus-infected cells was investigated. It was investigated. Approximately one-third of the PyV DNA strands contained 7–9 ribonucleotides covalently attached to their 5' ends. A similar fraction of DNA strands from replicated SV40 DNA contained oligoribonucleotides 6–9 residues in length and beginning with (p)ppA or (p)ppG. Over 80% of PyV or SV40 RNA-primed DNA strands specifically hybridized to the retrograde template. Moreover, at least 95% of the RNA-primed DNA strands from either PyV or SV40 were able to assign unique nucleotide positions for their start sites and were derived from retrograde templates. Therefore, the DNA replication forks of PyV and SV40 are essentially the same. DNA synthesis occurs predominantly, if not exclusively, discontinuously on the retrograde template span >
