Download Seminar 5 : Cell Cycle A

Seminar 5 : Cell Cycle A
1. During the G1 phase of cell cycle a pre-replication complex consisting of MCM2-7 helicase,
ORC and initiation factors Cdt1 and Cdc6 is assembled at the origin of DNA replication. What
does CycE-CDK2 complex (SPF) do to initiate DNA replication?
2. Yeast genome has several origins of replication. What are the biochemical mechanisms that
ensure that each origin “fires” once and only once during the cell cycle? What are the additional
mechanism that control the Ori firing in mammals?
3. Replisome in eukaryotes consists of MCM2-7 helicase, Primase, two DNA polymerases (
and ), RFC clamp loader, PCNA clamp and some other additional proteins. How and where
does RFC load the PCNA clamp? Why is the presence of PCNA behind DNA Pol important for
DNA Pol processivity? Why the accuracy of DNA Pol is so high? Why is the lagging strand
replicated in a discontinuous manner? Describe the mechanism of this discontinuous
polymerization.
4. Mitosis Promoting Factor (MPF) phosphorylates lamins. How does lamin phosphorylation by
MPF promote nucleus disassembly? Which proteins are phosphorylated by MPF to condense
chromatin? Which are the common targets (proteins) of SPF and MPF phosphorylation?
5. Active APC-Cdc20 Ub-ligase is instrumental for the entry into Anaphase. Describe how APCCdc20 induces the cleavage of Kleisin by Separase. What are the roles of Securin and Cohesin in
the cell cycle?
6. Describe the SAC checkpoint that regulate the activity of APC in mitosis. What is the role of
Kinetochore capture in Cdc20 activation? What is the role of tension in checking for a proper bipolar Kinetochore attachment?
7. (a) Ubiquitin ligase APC activates two major transitions during mitosis when it forms
complexes with its Cdc20 or Cdh1 specificity subunits. Describe in details the mechanisms of the
cyclin B destruction. What is the role of Cdc14 phosphotase in the cyclin B destruction and exit
from mitosis? How and when Cdc14 is activated?
(b)The ubiquitin ligase complex, called SCF is involved in at least two important steps in the cell
cycle. Which are those steps? Describe the function of each step and explain the specific role of
the respective ubiquitin ligase complex. What is the main difference between SCF and APC in
the way they select their substrates?
8. (a) Describe the overall structure of telomeres of mammalian chromosomes. How does their
structure maintain by telomerase?
(b) Which role is considered to be the main, the most important role of telomeres in eukaryotic
cells?
9. (a) Cyclin-dependent kinases (CDKs) are among the most important factors governing cell
proliferation. Thus, their activity must be dynamically regulated during cell cycle progression.
Describe three different mechanisms used by eukaryotic cells to inactivate CDKs during various
phases of the cell cycle.
(b) Explain the roles of Wee1 kinase, Cdc25 phosphotase and the CAK complex in the regulation
of CDK1 activity by phosphorylation. How are the activities of Wee1 and Cdc25 regulated?
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(c) p27KIP and Sic1 proteins have analogues functions in high and low eukaryotes. Which are
those?
10. It is of vital importance for any organism that their DNA is replicated accurately. Quality
control can happen before, during, and after the DNA replication.
a) Give an example of pre-replicative DNA damage and explain the repair mechanism.
b) Explain the mechanism (Pol 3’-exonuclease activity) which prevents and repairs errors during
the process of DNA replication.
c) Explain a mechanism by which mistakes can be repaired right after DNA replication.
11. One of the most important cell cycle checkpoints senses DNA damage in the cells. This
checkpoint consists of a rapid and a slow response.
(a) Describe how the fast, p53-independent response to the single stranded DNA damage
operates through the ATR/Chk1 pathway. Which are the main substrates of Chk1?
(b) Why is there normally no active p53 in the cell? Describe in details how p53 is activated by
CHK2 and ATM kinases in response to DNA damage in the cell. What is the role of Mdm2
ubiquitine ligase in p53 regulation ?
12. An important step at the G1-S-transition in the budding yeast cell cycle is called START. In
mammalian cells, the analogous step is called the restriction point.
(a) Describe the critical components, changes and events at the molecular level that control cell
cycle machinery and allow the cells to transit from the G1 to S phase, i.e. to pass
START/restriction point? Give the description for both yeast and mammalian cells.
(b) pRB protein in mammal and Whi5 in yeast have analogous functions. Which are those?
(c) mTOR kinase is the main kinase regulating the cell cycle entry in yeast. Describe its role in
the initiation of cell cycle.
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