>
>
> >ZOOHCC - 501: Molecular Biology (Theory)
>
>Unit 2: DNA Replication >
>
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>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. >
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>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. > >
>
>
> >
>
> >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
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> >
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>
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width="320"
/> >
>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. >
style="margin-left: 1em; margin-right: 1em;"
>
data-original-height="433"
data-original-width="643"
height="269"
src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh8tMWbwAtQA-TZKwQGP2zyHZs5PL3lULvqhxgeLvCPvP0OmhtP7fvGgJzYsII009pHf0SdoBvtuhZM2TE5s5pZ8Iu9aNr0fHRU1BFWkBUjtW85JA45Zjfr1_-e2JdkKjXenPnz7IjhMtzhr1CrIZ9SRKWRikZzr2hslLMQV1eIjpvWWEX9rr-J_qL59w/w400-h269/article-Semidiscontinuous-DN-YZv.jpg"
width="400"
/>
>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. > >
>
>
> >
>
> >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
> >