Download Chapter 16.2 - DNA Replication Details 2 - kyoussef-mci

DNA Replication
Lesson 2
• Label the ends of DNA strand as either 5’ or 3’.
5’
Leading Strand
DNA polymerase III
SSBPs
3’
5’
primase
gyrase
helicase
5’
3’
3’
The leading strand is synthesized
continuously during DNA
replication
5’
Lagging Strand
5’
3’
3’
5’
The laggin strand is synthesized
discontinuously during DNAreplication
3’
Okazaki fragment
5’
Connecting Lagging Strands
• DNA polymerase I – removes the RNA
primer
• DNA ligase – connects the sugarphosphate backbone of Okazaki
fragments
Mistakes in DNA Replication
• less than 1 error in
107 (10 million) NTs
• exonuclease –
enzymes which can
cut out sections of
the DNA strand
• DNA polymerase I
and III and
exonuclease work
together to fix
mistakes
1
A distorted section of DNA.
2 A nuclease enzyme cuts
the damaged DNA strand
at two points and the
damaged section is
removed.
Nuclease
DNA
polymerase
3 Repair synthesis by
a DNA polymerase
fills in the missing
nucleotides.
DNA
ligase
4 DNA ligase seals the
Free end of the new DNA
To the old DNA, making the
strand complete.
Replicating the Ends of DNA Molecules
5
End of parental
DNA strands
Leading strand
Lagging strand
3
Last fragment
Previous fragment
RNA primer
Lagging strand
5
3
Primer removed but
cannot be replaced
with DNA because
no 3 end available
for DNA polymerase
Removal of primers and
replacement with DNA
where a 3 end is available
5
3
Second round
of replication
5
New leading strand3
New lagging strand 5
3
Further rounds
of replication
Figure 16.18
Shorter and shorter
daughter molecules
• The ends of eukaryotic
chromosomal DNA
– Get shorter with each
round of replication
• Eukaryotic chromosomals
– Have repeated short nucleotide sequences, called
telomeres at their ends that do not code for amino
acids.
– postpone the erosion of genes near the ends of
DNA molecules (e.g. in humans  100-1,000
TTAGGG)
Figure 16.19
1 µm
• Some cells (like cervical cancer cells) have
an enzyme called telomerase
– Lengthens the telomeres, thus restoring original
length and compensating for shortening that
occurs during DNA replication.
– Contributes to the “indestructible” nature of
cancer cells
Explain the function of each of the following
enzymes in DNA replication:
DNA gryase
DNA helicase
DNA
polymerase I
DNA
polymerase III
DNA ligase
RNA primase
Relieves the tension produced by the unwinding
DNA during replication
Unwinds the DNA double helix by disrupting hydrogen
bonds between base pairs
Removes RNA primers and replaces them with the
appropriate deoxyribonucleotides (DNA bases) during
DNA replication
Makes complementary strands of DNA (adds
deoxyribonucleoside triphosphates to the 3’ end of
the elongating strand
Joins DNA fragments together by catalyzing the
formation of a bond between the 3’ hydroxyl group
and a 5’ phosphate group on the sugar-phosphate
backbones
Makes RNA primers
• Review from previous lessons
– Pg. 198 Section Review - #2, 3, 7 (info from
previous lessons)
• Material from today’s lesson
– Pg. 204 Section Review - #2
– Pg. 205 Understanding Concepts - #8, 9, 10,
15, 18, 25, 33, 42