DNA maintenance –
checkpoints and telomeres
Supplemental Instruction
Iowa State University
Leader:
Course:
Instructor:
Date:
Matt C.
Biol/Gen 313
Dr. Myers & Dr. Vollbrecht
01/22/2017
Introduction: The answers here correspond to the third set of questions on the third worksheet.
Concepts
1. DNA maintenance.
Material
1. DNA maintenance.
In this section, we’ll look at licensing, cell cycle checkpoints, and telomeres.
1. Why is it important for licensing factors to attach before replication in eukaryotic
cells?
Licensing factors bind during the G1 phase of the cell cycle. They mark all of the
origins of replication in the eukaryotic genome. Eukaryotic replication proteins
then recognize transcription factors during the beginning of the S phase and only
open origins that have transcription factors. Transcription factors fall away and
degrade after they’ve served their purpose.
This prevents origins from being opened and replicated multiple times during S
phase. If you didn’t have transcription factors, DNA replication would consist of
make a bunch of semi-complete DNA strands one on top of another.
2. If a cell skips past the S-phase checkpoint and immediately replicates, what might
happen to the resulting cells?
One or both of the resulting cells may have incomplete chromosomes. The Sphase checks for completion of DNA replication. If all the DNA hasn’t been
synthesized, the cell remains in S phase. If you skip that checkpoint then the cell
may continue regardless of how much DNA it’s succeeded in replicating.
3. Describe the end-replication problem in eukaryotes.
The end replication problem arises when DNA replication reaches the ends of
chromosomes. The leading strand can continue synthesis all the way up until the
end, but the lagging strand can’t. Instead, the final gap where a DNA Okazaki
fragment should be is too small to allow the DNA synthesis proteins to finish
making that new strand. If allowed to stay this way, chromosomes would
continually shorten and we would age much faster.
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4. What would happen if the RNA sequence in telomerase wasn’t a repeat? E.g. a
normal sequence may be UAACCCUAACCC, but the non-repeat is
UAACCCUAACGC.
Telomerase is an enzyme that is made up of an RNA component and a protein
component. The RNA component is a repeated pattern that will match up with the
gap on the lagging strand at the end of the chromosome. One part of the RNA
repeat pattern will stick to that gap, while the rest of the RNA repeat will stick off
the end of the chromosome. Telomerase then synthesizes new DNA to match the
pattern on its RNA, extending the old strand of DNA. Since the telomerase RNA
is a repeated pattern, telomerase can fall off and attach again one pattern repeat
down the line. It again adds new nucleotides to the old DNA strand to match the
telomerase pattern. Telomerase continues like this until the old, parent strand has
become long enough again to synthesize a new Okazaki fragment. Now, making
this fragment will again leave a gap at the end of the new DNA strand, but since
that gap is pushed further away than the old gap, no DNA ends up being lost and
some can even be gained.
In this question, the telomerase RNA no longer has a repeating pattern. This
means that the telomerase will stick to the gap the first time and add one set of
additional nucleotides, but it won’t be able to reattach again because the pattern
doesn’t match.
5. Why don’t prokaryotes have licensing factors or express telomerase?
Prokaryotes have circular chromosomes, so they don’t have chromosome ends.
This means they won’t leave any gaps that need to be filled, so they don’t have an
end replication problem and thereby don’t need telomerase.
Since prokaryotes only have one origin of replication, using a licensing factor is
just an extra step. Prokaryotes could instead prevent multiple syntheses by
limiting expression of the initiation complex which is energetically and
organizationally simpler than using licensing factors.
Once again, these images aren’t mine, but they should be helpful.