Lecture 28 Flashcards
describe separase and the role it plays in the cell cycle
separase is a protease enzyme
separase cleaves cohesin, allowing sister chromatids to separate and become daughter chromosomes
True or False: during the course of mitosis, there is only 1 set of PRE-RC’s synthesized, to ensure there are no duplicate strands of DNA created. explain your answer
False
NO NEW PRE-RC’s are made during mitosis
PRE-RC’s are made only once in the G1 phase
describe cohesin and the role it plays in the cell cycle
cohesins are members of the SMC (structural maintenance of chromosomes) proteins
large cohesin protein complexes are what glue sister chromatids together along their length
(cohesin forms rings around sister chromatids “rungs of a ladder”)
describe the role of securin
Securin protects cohesin by inhibiting separase
explain the levels S-Cdk and M-Cdk during the S and M phase. What effect does this have on the amount of PRE-RC’s that are formed?
The levels of S-Cdk and M-Cdk are high during both the S and M phase of the cell cycle
This has no effect on the amount of new PRE-RCs formed during these phases because PRE-RCs are only formed during G1 phase
describe the order of events involving APC/C, Cdc20, securin, separase and cohesin in sister chromatid separation.
APC/C, once activated by Cdc20, ubiquitinates securin
securin was bound to and inhibiting separase, however it is degraded, which activates separase
separase then cleaves cohesin, which allows sister chromatids to separate during anaphase
List and describe the 4 regulating methods of cyclin-Cdk complexes (be sure to mention when the levels drop for the first 3 complexes)
G1/S cyclin-Cdk complexes: start the cell cycle in late G1 phase by commiting to the cycle
(levels drop in S phase)
S cyclin-Cdk complex: stimulates chromosome duplication
(these levels remain high until mitosis)
M-Cyclin-Cdk complex: stimulate the entry into mitosis at G2/M checkpoint
(these levels drop at mid mitosis)
4-G1 Cyclin-Cdk complex: governs the activity of G1/S cyclins
explain the levels S-Cdk and M-Cdk during the S and M phase. What effect does this have on the amount of PRE-RC’s that are formed?
The levels of S-Cdk and M-Cdk are high during both the S and M phase of the cell cycle
This has no effect on the amount of new PRE-RCs formed during these phases because PRE-RCs are only formed during G1 phase
when do APC/C levels rise?
mid mitosis
preparing to trigger anaphase
What is the role of PRE-RC when it comes to ensuring that chromosomes only duplicate one time?
PRE-RC: (prereplicative complex) is a CRITICAL structure for controlling mitosis
at G1 phase in the cell cycle, PRE-RC’s assemble are origins of replication and ensure that there is only 1 copy of DNA synthesized
State the stages and names of the cell cycle in their chronological order
S Phase: DNA synthesis occurs here
G2 phase: growth phase that occurs between S and M phase
M phase: chromosomes are separated from one another and pulled to opposite ends of the cell as cytokinesis occurs, yielding 2 identical daughter cells
G1 phase: growth phase that occurs between M and S phase
(GAP Phases include G1 and G2 (the 2 other phases)
state and describe the checkpoints of the cell cycle
Checkpoint 1: known as the “start checkpoint” or “restriction point” that occurs at the end of the G1 phase
Basically evaluates the environment (nutrient levels) and decides if the cell will enter into the cell cycle (beginning with S phase) or not
Checkpoint 2: occurs between G2 and M phase
evaluates the quality of DNA replication (wants 1 perfect round of DNA replication)
Checkpoint 3: occurs at the metaphase-anaphase transition and triggers the separation of sister chromatids
evaluates if all chromosomes are properly attached to the spindle before anaphase pulls them to opposite ends of the cell
Describe the role of CAK’s in terms of it’s control mechanism of the cell cycle.
the phosphorylation of Cdk is caused by CAK (Cdk Activating Kinases)
Before a cyclin and Cdk come together (in order to activate the Cdk) the T loop of the Cdk is blocking the active site of the Cdk
the binding of cyclin to the Cdk moves the T loop out of the active site, and into a “cave site”
it is in this cave site that CAK phosphorylates, specifically at the T loop, the Cdk in the “cave site” and fully activates the Cdk
Describe the role of SCF in terms of it’s control mechanism of the cell cycle. Include what is necessary for SCF activity to occur.
SCF- ubiquitin ligase is used to add ubiquitin to phosphorylated CKI’s, to target them for destruction
SCF protein MUST have the F-box subunit (helps SCF recognize target proteins) or else it will be inactive
SCF adds ubiquitin to CKI proteins in G1, which inadvertently restores the activity of S-Cdk (starts DNA replication)
Describe the role of CKI’s in terms of it’s control mechanism of the cell cycle. State a general example of a CKI
CKI’s (Cdk Inhibitory proteins) cause cyclin-Cdk complexes to be inhibited by binding to both of them
P27 is a CKI that binds to and inactivates cyclin and Cdk
When ubiquitination is used to destroy proteins as a control mechanism for the cell-cycle, what is another term for this process?
proteolysis (just the act of tagging and destroying proteins)
Describe the role of ubiquitination in terms of it’s control mechanism of the cell cycle.
Proteolysis of CKI’s can turn on S-Cdk’s
SCF-ubiquitin ligase adds ubiquitin to CKI’s so they are destroyed.
once the CKI’s are degraded, S-Cdk’s stimulate DNA replication (entry into the cell cycle)
(any other protein that needs degraded can be ubiquitinated but doesn’t necessarily regulate the cell cycle)
Describe the role of Wee1 Kinase in terms of it’s control mechanism of the cell cycle.
Wee1 kinase inhibits M-Cdk activity by phosphorylating the “roof site” of it, while it is bound to cyclin in a Cdk-cyclin complex
With the M-Cdk-cyclin complex inhibited, the cell cycle cannot progress
(part of the Wee1 kinase and Cdc25 phosphatase activation and inactivation cycle)
True or False: during the course of mitosis, there is only 1 set of PRE-RC’s synthesized, to ensure there are no duplicate strands of DNA created. explain your answer
False
NO NEW PRE-RC’s are made during mitosis
PRE-RC’s are made only once in the G1 phase
Describe the role of cdc25 phosphatase in terms of it’s control mechanism of the cell cycle.
Cdc25 phoshatase removes inhibitory phosphates from M-Cdk
M-Cdk is a kinase that directly triggers/controls mitosis
(part of the Wee1 kinase and Cdc25 phosphatase activation and inactivation cycle)
Describe APC/C and the role it plays in the cell cycle
APC/C is a member of the ubiquitin ligase family of enzymes
APC/c catalyzes the addition of ubiquitin to proteins so that they will be degraded
In the cell cycle, APC/C catalyzes the ubiquitination of securin, which binding to and inactivating separase
after securing is ubiquitinated, it is destroyed and separase is activated (destroys cohesin)
Describe Cdk’s and cyclins. Be sure to mention how they are related and the levels at which they occur in the cell.
Cdk’s (Cycling dependent kinases) main function is to phosphorylate proteins downstream to activate them and regulate cell cycle events.
The quantity of Cdk’s does not change, however the ACTIVITIES of Cdk’s rises and falls throughout the cell cycle (the cycling of their activity is dependent upon the presents/absence of cyclins)
Cyclins: proteins that regulate Cdk’s and serve to activate Cdk’s (without cyclins present, Cdk’s are inactivated)
The quantity levels of cyclins changes though out the cell cycle (this is where they get their name)
State the 2 steps in DNA replication that ensure that DNA is only copied once per cell cycle. include what phase of the cell cycle both of these steps occur in.
prereplicative complexes assemble at the origins of replication during G1 phase
Replication forks are created at the sites of DNA replication during S phase
What is INK4A and what phase of the cell cycle is it associated with? if it mutated to the point that is loses its normal function, what condition occurs?
INK4A is a CKI associated with the G1 phase of the cell cycle
Hereditary melanoma occurs if INK4A has a loss of function mutation
