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What are the phases of the cell cycle?

1. G0 (quescent)
2. G1 (gap between M and S)
3. S (DNA replication)
4. G2 (gap between S and M)
5. M (mitosis - nuclear division and cytokinesis - cytoplasmic division)


Approximately how long is interphase? M-phase?

22-24 hours; 1 hour


What happens during G1?

Nutrients and cytoplasmic organelles are produced to increase the cell volume.


Before G1 ends, what question must be answered?

Is the environment favorable?


What happens during S?

DNA is replicated and chromosomes are duplicated


What happens during G2?

Cell volume increases


Before G2 ends, what questions must be answered?

Is DNA properly replicated?
Is the environment favorable?


What happens during M?

Metaphase, Anaphase, Telophase, Cytokinesis


Before metaphase ends, what question must be answered?

Are chromosomes attached to spindles?


The cell cycle ___ triggers essential processes of the cycle.

Control system


What are the two key components of the cell-cycle control system and how do they interact?

Cyclin (regulatory protein) and cyclin-dependent kinase (CDK); cyclin binds CDK to activate it


Levels of ___ regulate the cell cycle; ___ levels stay constant.

Cyclin; CDK


There are specific cyclins and CDKs for each phase of the cell cycle - what are the phases?

G1, G1/S, S, M


Describe the level of G1/S-cyclin across the course of the cell cycle, beginning with G1.

Begins to increase in G1, peaks during G1, begins to decreases early in S, remains low until the cycle repeats


Describe the level of S-cyclin across the course of the cell cycle, beginning with G1.

Begins to increase in the transition from G1 to S, peaks during S, remains high until it decreases shortly before the metaphase/anaphase transition, at which point it remains low until the cycle repeats.


Describe the level of M-cyclin across the course of the cell cycle, beginning with G1.

Begins to increase in G2, peaks during M, begins to decrease at the metaphase/anaphase transition, remains low until the cycle repeats.


Describe the process of activating/inactivating CDKs.

CDK and cyclin bind to form an inactive dimer. CAK (CDK-activating kinase) phosphorylates CDK to activate it. Wee 1 kinase phosphorylates it again to inactivate it. Cdc25 phosphatase dephosphorylates it to activate it.


___ bind to both cyclin and CDK in a complex, distorting the active site. It also inserts into the ___-pocket, further inhibiting it.

CKIs (p27, p21, p16); ATP


What is the result of CKIs binding to CDK and cyclin?

Cell cycle arrest in the G1/S-phase (typically leads to long-term inhibition, which leads to cell differentiation)


Describe control of proteolysis by APC/C.

Inactive APC/C dimerizes with Cdc20 (activating subunit), forming activate APC/C. This ubiquitinates M-cyclin (using ubiquitin, E1, and E2). This leads to degradation of M-cyclin in the proteosome. This inactivates M-CDK. Cytokinesis can proceed.


What are APC/C and SCF?

E3 ligases


Describe control of proteolysis by SCF.

SCF binds F-box protein to form an activate SCF complex. CKI is phosphorylated. This is recognized by the SCF complex, which polyubiquitinates CKI (using ubiquitin, E1, and E2). CKI is degraded in the proteasome. This increases CDK/cycling activity. The cell cycle proceeds.


What are the protein kinases and phosphatases that modify CDKs?

CAK (on), Wee1 (off), Cdc25 (on)


What are the CKIs?

p27, p21, p16 (all off)


What are the ubiquitin ligases and their activators?

APC/C, Cdc20, Cdh1, SCF (all on)


Describe control of chromosome duplication.

1. In G1, prereplicative complexes (DNA helicases) assemble at replication origins. S-CDK is activated and phosphorylates proteins of complexes to initiate the replication fork. It also prevents multiple origin activations. Initiation begins.
2. The replication forks form. Elongation occurs. The entire chromosome is duplciated.
3. M-Cdk is activated, chromosomes segregate.
4. APC/C is activated, CDK is inactivated.
5. New Pre-RCs assemble at origins.


Describe the activation of M-CDK, including its positive feedback.

CDK and M-cyclin bind to form inactivate M-CDK. CAK and Wee1 both phosphorylate to keep M-CDK inactive. Cdc25 dephosphorylates M-CDK to form active M-CDK. Active M-CDK helps activate Cdc25 and inactivate Wee1.


A decrease in M-CDK in late mitosis leads to...

...an increase in Cdh1-APC/C. This ensures continued suppression of CDK activity after mitosis.


Describe the process of mitogen stimulation of cell cycle entry.

Mitogen binds to a receptor (like an RTK). This activates Ras, which activates MAP kinase (Mitogen Activated Protein Kinase). This leads to activation of transcription regulatory proteins. This leads to immediate early gene expression, which includes creation of Myc. This leads to expression of other genes, leading to active G1-Gdk. This phosphorylates Rb. Rb releases a now-active E2F. This leads to S-phase gene transcription. This leads to G1/S-cyclin and S-cyclin, which leads to active S-CDK, which leads to DNA synthesis.


What are mitogens?

Growth factors such as EGF, insulin, and HRG.