DNA Replication

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Chapter 6

• DNA Replication
• DNA Repair
• DNA Recombination

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DNA Replication-duplication of DNA continuous
monitoring and checking for errors Mutations in
DNA can be very harmful and can cause many
diseases (cancer) Survival of multicellular
organisms depends on preventing changes in DNA
Each DNA stand can serve as a template for
synthesis of a new strand DNA polymerase is an
enzyme responsible for synthesis of a new DNA
strand it always proceeds from 5 to 3 end
(Figure 6-2)
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The two strand of parental double helix
dissociate and each of the parental strands
serves as a template for the new DNA strand
synthesis The ability of each of the stands to
act as a template allows the cell to copy or
replicate DNA DNA copying is completed in 8
hours (3.2x109 nucleotides in human
genome) Figure 6-3
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The term semiconservative replication means that
each of the daughter cells ends up with one of
the original (old) strands and one of the new
ones In the cell, the process of unwinding DNA
before replication is catalyzed by enzymes and
requires energy Replication origin is a position
at which DNA is first opened, it is AT rich
region because A/T are easier to pull apart
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Bacteria having a very small genome have only one
origin of replication Lager genomes (human) have
multiple origins of replication (10,000) Once
the DNA is opened up, a group of many proteins is
involved in DNA replication Figure 6-4 6-5
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Replication origin (Y shaped) is also called
replication fork Replication forks move along DNA
opening up two strands of double helix
(100 nucleotides/second)
Figure 6-9
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The most important enzyme in DNA replication is
DNA polymerase It catalyzes addition of new
nucleotides to the 3 end of a growing chain, in
other words it works in the direction from the 5
to 3 end Nucleotides added to the new strand are
nucleoside triphosphates breaking the high
energy bond releases energy and this energy is
used for polymerization reaction Figure 6-10
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DNA being an antiparallel molecule runs from 5
to 3 and 3 to 5 DNA polymerase can work only
from 5 to 3 So how are the new strands
synthesized? Figure 6-11
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The leading strand is synthesized continuously in
the direction from the 5 to the 3 end. This is
the continuous strand The lagging strand is
discontinuous and uses the backstitching
method. The DNA polymerase works backwards and
adds new nucleotides from the 5 to the 3 end.
These small pieces are known as Okazaki
fragments Figure 6-12
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DNA polymerase posses self-correcting or
proofreading activity If a wrong nucleotide is
added, DNA polymerase will remove it and adds a
correct nucleotide This exonuclease activity
(nucleic acid degradation) of DNA polymerase is
from the 3 to the 5 Thus, DNA polymerase has
both, polymerase activity (from the 5 to the 3
end) and the exonuclease activity (from the 3 to
the 5 end) (Figure 6-13)
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DNA polymerase needs a primer to start polymerase
activity The enzyme primase is responsible for a
short RNA primer synthesis The RNA primer is
needed only once for the leading strand and
multiple times for the lagging strand Repair
polymerase replaces RNA primer with the DNA The
enzyme ligase seals together Okazaki fragments
(Figure 6-16 6-17)
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The enzyme telomerase (TEOS -end) counteracts
the tendency of telomers (ends of a chromosome)
to shorten with every replication There is no
space to add another RNA primer at the very end
of a chromosome Telomerase extends the parental
strand and eventually the last Okazaki fragment
is completed Figure 6-18
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Why is DNA repair important? One nucleotide
change (from A to T) causes a sickle cell
anemia The sickle cell anemia disease changes the
shape of RBC Exposure to the UV light can cause
formation of thymine dimers (Figure 6-24)
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DNA repair mechanism if only one strand is
damaged Excision of the mismatched
nucleotide DNA polymerase corrects the
mistake DNA ligase seals the nick Figure 6-26
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Examples of diseases caused by viruses
hepatitis, small pox, HIV, rabies,
mumps, polio Avian influenza, strain
H5N1 It is a RNA virus The mutations of
hemagglutinin protein (found on the surface of
the influenza viruses) is responsible for the
ability of the virus to enter host cells Figure
6-36
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A virus cannot replicate without host cell Inside
the host cell the DNA is replicated (or in the
case of RNA viruses reversly transcribed) New
viral proteins are made and new viruses are
assembled Figure 6-37
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RNA viruses (retroviruses) utilize reverse
transcriptase enzyme to produce DNA from the RNA
template (example) The DNA is integrated into the
host genome Viral proteins are assembled and many
new viral particles are released Figure 6-39