SM14 Flashcards
(20 cards)
which aspect of cyclin-cdk has the kinase ability?
CDK
cyclin levels stay the same throughout the cell cycle. T/F
F. They fluctuate.
which cyclin-cdk complexes are involved in G1?
cdk4/Cyclin D
cd6/cyclin D
Which cyclin-cdk is involved in G1/S phase transition?
cdk2/cyclin E
which cyclin-cdk complex is involved in G2/M phase?
Cdk1/cyclin B
Which cyclin-cdk complex is involved in S phase, G2 phase?
Cdk2/cyclinA
What do E3 ligases do?
They recognize cyclins targeted with ubiq for degradation.
What does p27 do?
It binds cyclin-cdk complex and holds it so it is inactive.
Give two examples of CKIs.
Cip/Kip - directly inhibit the kinase by binding cyclin-cdk complex throughout the cell cycle
Ink 4 - blocks cdk 4- blocks interaction with the G1-type cyclin (aka blocks cyclin D-cdk4)
which checkpoint is called the restriction point?
G1 check point
How does mitogen stimulate cell proliferation?
Receptor tyrosine kinase recognizes mitten which activates signal transduction pathway that leads to expression of Myc (a TF) which will increase Cyclin D synthesis, increase Kip 1 (p27) degradation and increase E2F synthesis.
How does mitogen decrease activity of KIP 1?
Normally when mitogen is low Kip1 is busy inhibiting Cyclin D and E. When mitogen is high, more cyclin D is made and the Kip1 gets busy with the cyclin D so it releases Cyclin E. Cyclin E will bind CDK2 which will inhibit Kip1 and progress through the cell cycle.
How is Rb involved in positive feedback controlling S phase initiation.
In normal G0 - Rb tighly bound to E2F on chromatin, recruit histone deacetylases, which inhibit acetylation of nucleosomes repressing trasncription.
Once more Cyclin D, cell gets signals to proliferate, it phos Rb, the more Cyclin D, the more cyclin E the more phosphorylated Rb. Hyperphosphorylated Rb will let go of E2F which will transcribe the genes downstream including itself, cdk 2, cyclin E, and repication factors and S-phase factors.
Cdk4/6-cyclin D also binds up excess p21 and p27, which are inhibitory for the G1/S-phase Cdk2-cyclin A and E complexes, allowing them to continue phosphorylating pRb. This increased phosphorylation of pRb establishes a feedback loop with more active Cdk2-cyclin E and Cdk2-cyclin A complexes become active and less inhibition by pRb. Cdk2-cyclin E also phosphorylates p27Kip1 targeting it for degradation by the ubiquitin-proteasome system, further relieving inhibition of S-phase cyclin activity.
Mutations in Rb will result in unregulated growth.
How do we avoid replicating our DNA more than once in the cell cycle?
Licensing limits activation of origins of replication to once per cell cycle to avoid re-replicating DNA. cdc 6 and cut 1 bind to/inhibit the ORC and recruit a complex of Mcm proteins forming the pre-replication complex. Mcm proteins only load in the absence of a protein kinase activity called S-phase promoting factor (SPF) composed of Cdks 1 and 2 complexed with cyclins A, E or B (the exact composition is unclear). Since S-phase cyclins are degraded in the previous mitosis, their levels are very low in G1 cells. Once the restriction point is passed, SPF prevents formation of new pre-replication complexes.
A second factor in vertebrates called geminin blocks Cdc6p/Cdt1 from loading Mcm proteins. Accumulation of geminin in S phase also prevents the formation of new pre-replication complexes.
When S-cdk (Cdk2-cyclin E and then Cdk2-cyclin A) come they phosphorylate the cdc 6 releasing it from ORC and cdt 1. Allows Mcm (helicase) activity to start and phos ORC so it can’t re-recruit these proteins. M-cdk maintains phos of ORC.
How do we stop S phase when we detect DNA damage at the G1/S phase checkpoint.
In G1, DNA damage is signaled by a kinase called ataxia telangiectasia mutated (ATM) and DNA-dependent protein kinase (DNA-PK), which activate Chk1 and Chk2 Kinase which phosphorylates p53 and stabilizes/activates it.
p53 is a transcription factor that cause increase in the transcription of the CKI protein p21Cip1, thereby inhibiting the G1/S-cdk and S-phase Cdk2-cyclin E activity.
Un-repairable DNA damage triggers cell death (apoptosis) or senescence through p53. Because of this protective activity of p53, it is sometimes called “the guardian of the genome”.
How does DNA replication suppress mitotic progression
ATR senses ss DNA. ATR phos CHK1 which phos the phosphatase which is degraded and keeps the phos present to maintain inactivation of cdk2-cyclin to avoid progression into mitosis.
How is quiescence reinforced?
It is reinforced by CKIs. Ink4A binds to cyclin D(which has already low levels bc no mitogen-cdk4/6, releasing KIP1, which inhibits cyclin E-cdk 2. Cyclin D levels are low and Rb remains hypophos and therefore bound to E2F.
How to we maintain cell senescence?
Telomere shortening. The cell activates p53 and CKI p16Ink4 to permanently arrest the cells. Chromatin changes are driven by pRb to enforce the permanent arrest.
Give three examples of cellular signals that regulate cell cycle progression?
- Binding of transforming growth factor β (TGF-β) to a cell surface receptor serine-threonine kinase sets off a signal transduction cascade resulting in the increased expression of the CKI’s p16Ink4a and p15Ink4b, thus inhibiting Cdk4-cyclin D.
- In muscle, the transcription factor myoD, the master regulator of muscle development, activates the transcription of p21Cip1, inhibiting several Cdk-cyclins leading to differentiation. p21 also binds to the DNA replication factor PCNA blocking DNA replication.
- Contact inhibition: Cells that become crowded and make contacts with their neighbors enter G0. The Cdk inhibitor p27Kip1 is upregulated in contact inhibited cells. p27Kip1 is important for regulating organ size during development.
How do cells get aroundtelomere shortening?
- Breakage fusion bridge - they have ss ends, which cells don’t like so they link two chromatin.
- reactivate telomerase or by Alt pathway.
