Cell Cycle Flashcards
name the different phases of the cell cycle
G1 or preparatory phase, longest phase
S phase, where DNA replication happens
G2 phase, where the cell is preparing for mitosis, checking for the right order
M phase - mitosis
Cytokinesis, cells split into two
endoreduplication
cells with more than 2n copies of DNA
cells go through cell cycle stopping before mitosis
replication without splitting over and over again
interphase
the time when the cell grows and replicates its DNA, divded into G1, S, and G2
cyclin-dependent kinases (Cdks)
family of 11 related protein kinases, crucial for cell cycle progression and other activities
binds to cyclins, and Cdk-cyclins only have partial kinase activity
phosphorylation of specific amino acids on the Cdks can either activate or inhibit the kinase
cyclins
bind to Cdks, at least 16 differeny cyclins, have varying levels during the cell cycle to dictate what happens at different points in time during the cell cycle
four mechanisms that regulate Cdk activity
- Association with cyclins
- activating phosphorylation of threonine around position 160
- inhibitory phosphrylation of threonine 14 and 15
- Association with Cdk inhibitors (CKIs)
CKI
cyclin-dependnt kinase inhibitor, along with inhibitor of Cdk4 (INK4 families bind to and activate the Cdk-cyclin complexes
some members of the CKI family also can act as positive cofactors for Cdk-cyclin assembly and activation
Describe the four classes of cyclins that affect Cdk activity throughout the cell cycle.
Cyclin D synthesis beings in G1 and stays relatively constant until it is degraded and disappears through mitosis
Cyclin E is the G1/S cyclin, rises at the end of G1 and rapidly decreases at the beginning of S phase
Cyclin A is important for S phase and transition through G2
Cyclin B increases throughout G2 and then drops during mitosis after it peaks
cyclical proteolysis of CKIs
SCF phosphorylates CKI and makes it a target for E1 and E2 ubiquitin ligase, which targets the CKI for degradation
cyclical proteolysis of cyclins
done by APC after an activating subunit Cdc 20
ubiquitinylates cyclin/Cdk complex
APC = Anaphase promoting complex
M-Cdk activates APC, negative feedback loop
three major cellc ycle checkpoints
G1/S, G2/M, and M/cytokinesis
cell growth signals
GSK3beta and Ras
GSK3beta inhibits formation of Cyclin D1/Cdk4, Cyclin D1/Cdk6, and Cyclin E/Cdk2
Ras promotes Cyclin D1/Cdk4 and Cyclin D1/Cdk6
TGFbeta
binds to a surface receptor serine-threonine kinase
signaling cascade promotes Smad3 and Smad4, which indirectly inhibits cdc25A and subsequently Cyclin D1/CDK4
Smad3/4 also indirectly inhibits p15 thorugh 19
differentiation signals
p15 through 19, which inhibit Cyclin D1/Dck4 formation
cell-cell-ECM contacts
activate GSK3beta and RhoA
GSK3beta inhibits beta-catenin/TCF, which indirectly activates Cyclin D1/Cdk6/4
RhoA indirectly inhibitis p21 and p27, which inhibit Cyclin D1/Cdk6/4 as well as Cyclin E/Cdk2
DNA damage
activates ATM, which activates p53, which indirectly activates p21 and p27
p16
indirectly inhibits Cyclin E/Cdk2
retinoplastoma (Rb)
a protein that binds to transcription factor of E2F
can be mildly phosphrorylated by Cyclin D/Cdk4/6
hyperphosphorylated by cyclin E/Cdk2
E2F
transcription factor regulated by Rb, when bound to Rb, recruits histone deacetylases to chromatin
when not bound to Rb, activates transcription of several hundred genes important for DNA synthesis and cell cycle progression
E2F is one of the genes activated, so there is a positive feedback loop
cyclin E, Cdk2, cyclin A, and other DNA replication factors are also activated
CIP/KIP and INK4
two families of cyclin/Cdk inhibitors that help regulate cell cycle checkpoints
Describe the process of Rb phosphorylation that leads to the activation of DNA replication
p27 helps promote the formation of Cyclin D/Cdk4 complexes, which gets transported to the nucleus
Cyclin D/Cdk4 mildly phosphorylates Rb, which allows the build up of cyclinE/Cdk2 through E2F release, can also soak up free-floating p27 in the nucleus
p27 in the nucleus initially inhibits Cyclin E/Cdk2 complexes, but as more Cyclin E/Cdk2 are formed and more Cyclin D/Cdk4 enters the nucleus, p27 gets phosphorylated and degraded
free Cyclin E/Cdk2 buildup can then hyperphosphorylate Rb and launch the cell into S phase
Descrive the two positive feedback loops related to Cyclin E/Cdk2 formation
CKI is both phosphorylated and degraded as well as sequested onto Cyclin D/Cdk4 complexes
myoD
a transcription factor that is the master regulator of muscle development, activates the transcription of p21, which inhibits several Cdk-cyclins
p21 also binds to the DNA replication factor PCNA, blocking the DNA replication
contact inhibition
a process in cells that become growded, where Cdk inhibitor p27 is upregulated in cells that are touching neighboring cells
control of DNA replication by S-phase cyclins
to ensure DNA replicatino only happens once per cycle, Cdc6 and Cdt1 bind to the origin recognition complex (ORC)
MCM is able to assemble in the presence of Cdc6
cyclin complexes for the S-phase promoting factor (SPF)
a build-up of Cyclin B/Cdk1 and Cyclin E/Cdk2 during phase leads to phosphorylation of Cdc6, leading to its degradation
the assembled MCM complexes drive replication, but the ORCs are phosphorylated by the Cyclin/Cdk complxes and can no longer bind Cdc6/Cdt1 until the cycle is completed
geminin
prevents Cdc6/Cdt1 from loading Mcm proteins, accumulates in S phase
replication origin clusters
all clusters on the same chromosome fire at about the same time
about 1000 of these clusters in human cells
can be divided into “early” or “late” replicating origins
R-bands are gene-rich and early replicating
G-bands are gene poor and late-replicating
later replicating DNA is concentrated around nucleoli and near the nucelar periphery where heterochromatin is abundant
histone genes
transcription rises dramatically during S phase
processing increases about 10 fold
histone mRNAs are not polyadenylated, increased staability in cytoplasm during S phase
stability decreases 40-fold in G2
centrosome duplication
happens in S phase in preparation for the next mitosis
cyclin E substrates
initiation of transcription from histone gene clusters initated by Cyclin E/Cdk4
also regulates centrosome replication
Cyclin E/Cdk2 also phosphorylates NPM/B23, which releases it from the centrosome
cyclin H
phosphorylates the cyclin D1/Cdk/pRB complex and is necessary for full cyclin D1 activity
Describe the activities of the regulatory network that results from DNA damage.
ATM and ATR are the major sensing kinases that lead to phosphorylation of downstream factors, which lead to control of the cell cycle when DNA damage is detected
How is mitosis suppressed until DNA replication is completed?
ATR is associated with the replisome, and it actively phosphorylates CHK1, which then phosphorylates CDC25A
CDC25A gets degraded when phosphorylated by SCF, preventing the activation (dephosphorylation) of the Cyclin/Cdk2 complex
Describe how the HPV oncoproteins deregulate the cell cycle checkpoints
E7 binds to Rb and related E2F suppression proteins to prevent Rb control of the G1/S checkpoint
E6 binds p53 and targets it for degradation, which prevents the activation of p21, which workds to inhibit Cyclin E/A/B - Cdk2/1 complexes
Describe how cells enter replicative senescence and what happens when telomerase becomes involved at various points of the cell’s replicative lifespan.
replicative senescence after 50-70 divisions
crisis after 80 divisions if checkpoints are lost, then telomeric crisis and death
if telomerase is activated before replicative senescence, cells become immortal
if cells are undergoing crisis and then telomerase is activated, they will become immortal with many genetic defects, often leading to cancer
