BIOLOGY 285 Study Guide - Quiz Guide: Chromosome, Dna Replication, Restriction Point

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Published on 16 Nov 2020
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Bio285 Sp19 Problem Set 4 Francis
Question 1
A. Explain how CDK activation in G1 leads to commitment to the cell cycle (the
restriction point aka “R” in vertebrates aka START in yeast). Be sure to include G1-
cyclin/CDK, Rb and E2F in your explanation. Include as much mechanistic explanation
as possible.
Once growth factors are received by the cell, the Ras/Map pathway is activated, and this
will produce cyclin-D. CDK4-cyclin D complexes will activate E2F-Rb complex through
phosphorylation. The phosphorylation of the protein Rb will cause it to release the
transcription factor E2F and allow it to perform its function. E2F is a family of
transcription factors involved in the transcription of proteins required for S phase. E2F
activation is also the “point of no return” and a place where the cell commits to the cell
cycle.
B. List two specific sequences/structures found in the DNA/chromosome, and their
functions, that act to ensure that the number and morphology of chromosomes are
constant from one generation of the cell to the next. Explain.
(Hint: we’ve discussed them in class, and they are required for DNA replication and
chromosome segregation)
The Origin of Replication Complex It is an ATPase on DNA that recruits another
protein called cdc6 (ATPase). This activates the recruitment of cdt1 and mcm-helicase.
Cdt1 helps to move MCM-helicase to the DNA so it can unwind the DNA and allow for
replication to occur.
Kinetochores are found attached to centromeres on chromosomes. Spindle microtubules
will attach to the kinetochores and pull the sister chromatids apart so there is an even
number of chromosomes in the daughter cells.
C. Undergoing two rounds of DNA replication in a single S-phase is lethal to the cell.
One of the ways that cells help prevent “re-replication” is by ensuring that pre-RCs do
not re-form on origins of replication more than once per cell cycle, thus they suppress
pre-RC formation after G1-phase. What are the mechanisms by which the cells ensure
that pre-RCs are not re-assembled during S-phase or beyond? Please “bullet-point” or
number your response for ease of grading purposes.
CDK phosphorylation of cdc6 leads to ubiquitin-mediated proteolysis of cdc6.
This prevents the recruitment of cdt1 and the binding of mcm-helicase to this
complex. Also, without cdt1, mcm-helicase will not be able to get to the DNA.
CDK phosphorylation of mcm (ones not being actively used for DNA replication)
causes them to be exported out of the nucleus.
CDK phosphorylation of ORC prevents it from binding to Cdc6. This prevents the
rest of the pathway detailed in the first bullet point.
Question 2
The figure to the right is depicting the events required to activate M-phase cyclin-CDK:
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Bio285 Sp19 Problem Set 4 Francis
A. Identify the proteins labeled as
A. Mitotic CDK
B. M-cyclin
C. Wee1 (an inhibitory kinase)
D. Cak (an activating kinase)
E. Cdc25 (an activating phosphatase in
its inactive state)
B. Explain what is happening in the processes labeled as:
‘X’ -- An active Cdc25 phosphatase removes the inhibitory phosphate to activate M-
CDK.
‘Y’ – When M-CDK is active, it phosphorylates Wee1 and Cdc25. Then, M-CDK
comes back and turns off the Wee1 so it “inhibits the inhibitor.” Here at Y, M-CDK finds
inactive Cdc25s and activates them so you can activate more CDKs resulting in a positive
feedback loop.
C. If the G2 checkpoint were activated, the cell would want to stop the activation of M-
phase cyclin/CDK. Name two ways (that involve proteins in the above pathway) that
cells could keep M-phase cyclin/CDK from being activated.
One way is to inactivate the Cdc25 phosphatase once the G2 checkpoint is activated, so it
doesn’t remove the inhibitory phosphate from M-CDKs. With the phosphate still on the
M-CDK, it will not be active to turn off Wee1, the inhibitor.
Another method is to stop the formation of the M-CDK complex. Without the complex
present, the cell will have nothing to activate in order to continue growing.
Question 3
A. What is the difference between the kinetochore and the centromere?
The centromere (DNA) is the region of a chromosome where the chromatids attach to one
another. The kinetochore is a complex of proteins that is attached to the centromere, but
not part of the chromosome. The kinetochore is also where kinetochore microtubules
bind.
B. Nocodazole reversibly inhibits microtubule polymerization, which is essential for
formation of the mitotic spindle. By treating a population of mammalian cells with
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