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Lecture 3

BIO130H1 Lecture 3: Week 4 - DNA Replication Part 2

Course Code
Melody Neumann

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Week 4 - DNA Replication Part II...
1) Issues in DNA replication
2) DNA repair
Readings: 505-509, 397-399, 549-550, 556-558, 564-568
A quick keyword review
1) Leading strand is synthesized continuously from single RNA primer(s)
2) Lagging strand is synthesized discontinuously from multiple primer(s)
3) Okazaki fragments: RNA primer+DNA
4) DNA synthesis proceeds in which direction?5’ --3’
5) Primosome: helicase + primase
6) The predominant helicase is on which strand? lagging
Issues in DNA Replication
1) What happens at the ends of eukaryotic linear
chromosomes during replication
2) How is DNA unwound?
3) How are mistakes found and corrected?
What happens at the ends of Chromosomes?
leading strand: RNA primers made, continue
replication process all the way to the end
lagging strand: needs more RNA primers but
no where to put them at the end
The first problem at ends of chromosomes
Why is the shortening of the 5’ end of the daughter DNA a potential problem?
Loss of sequence information
shortening of the daughter DNA
For which strand is this a major problem?
especially important for stem cells and germ cells
The second problem at ends of chromosomes - shortening
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further processing of the 5’end of the newly replicated DNA
cell needs to be able to make a sequence to tuck around
cells see single strnaded and long end as damaged DNA
What happens at the ends?
In the majority of eukaryotes, this problem is solved by having repetitive DNA
sequences at the ends of the chromosomes. The ends are called: telomeres
Telomerase to the rescue
Telomerase: a reverse transcriptase
able to add DNA nucleotides on RNA templates
has its own built-in template
highly conserve, repeates of TTGGG added onto the 3’ end of the parental
lagging strand
The repetitive sequence that is added to the 3’ end of the parental strand (i.e. the
lagging strand template) is determined by the RNA template in telomerase.
RNA templat --- > make DNA complimentary copyeeeeeeeeeeeee e
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telomeric sequences tend to be G rich
Telomere Replication
1 RNA template (insiede the telomerase)
telomerase base pair the sequence and add on TGs on
the 3’ end
2) Resembles: Reverse Transcriptase
3) Generates: G-rich ends
4) Adds nucleotides to: 3’ ends of parental strand
elongate the DNA template, so the rest of the
sequence can be replicated by DNA polymerase
Telomere Dynamics
telomere length is negatively related to the number of time a cell divide
longest telomere length longest in a zygote or embryo
germ cell, stem cell: telomere length stay stable; continuous telemetric
somatic: decrease in telomerase activity as time passes
a protective mechanism at the dip
too short: replicative senescence
most cell stop replicating
but some cell continue to divide...
if very very short: apoptosis - programmed cell death
sometimes the damaged cell can start replicating out of control
eg. cancer
Telomeres and Cancer
Most cancer cells produce high levels of telomerase
Modification of the telomerase RNA template interferes with cancer cell growth
antisense RNA
disrupt the telomerase activity, preventing it from making more
Prognoses of some cancers (eg. neuroblastoma) can be ascertained by telomerase
high telomerase activity -- malignant tumor
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