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The four main stages of mitosis and their functions are...

1. G1, gap phase
2. S phase, chromosome duplication
3. G2, gap phase
4. M phase, chromosome segragation, cytokinesis


Define interphase.

G1, S phase and G2 combined.


What is G0?

A resting state.


What determines when a cell starts proceeding towards mitosis?

Passing of, in favourable conditions, the restriction point (Start) which commits the cell to DNA replication but not necessarily division.


What is the function of a Cdc gene (cell-division-cycle gene) and what does its mutation entail?

Genes encoding vital components for progression of cell towards division. Mutations may cause ex. temperature dependance (conditionalism, permissive/restrictive, cannot divide at high temperature) and arresting of cell in late G2 phase with signature look of large undivided cells.


What is the phenomenon of replicative cell senescence?

The arresting of division of a cell after a given number.


What principles makes the timer/oscillatorlike cell-cycle control system reliable?

Its independence of the events it controls and the irreversible and on/off nature of the transitions it controls.


What are the tree checkpoints in the cell cycle, what are their criterias and reults?

1. Start checkpoint, is environment favourable? Start of cell cycle.
2. G2/M checkpoint, is all DNA replicated/is environment favourable? Entering into mitosis.
3. Metaphase-to-Anaphase transition, are all chromosomes attached to the spindle? Trigger anaphase and proceeding to kinesis.


What are the effectors of the cell-cycle control system?

Cdks (cyclin-dependent kibases).


What's the function of Cdks?

They lead to "cyclical changes in the phosphorylation of intracellular proteins that initiate or regulate the major events of the cell cycle".


Cdks (cyclin-dependent protein kinases) are controlled by a multitude of other proteins and enzymes. Which one is the most essential?



Outline the four classes of cyclins in vertebrates.

1. G1/S-cyclins: activation of Cdks in late G1, help trigger progression through Start, result in commitment to entry into cell-cycle, levels fall in S phase
2. S-cyclins: binds Cdks after Start, stimulate chromosome duplication, levels elevated until first parts of mitosis
3. M-cyclins: "activate Cdks that stimulate entry into mitosis at G2/M checkpoint", destroyed in mid-mitosis
4. G1 cyclins: govern activities of G1/S cyclins


What are the different cyclin-Cdk complexes and when are they active?

G1-Cdk: "through Start in late G1"
G1/S-Cdk: mid G1 to late G1 phase
S-Cdk:late G1 to early M phase
M-Cdk: beginning of M phase to mid-M phase


What is the function of the cyclin-Cdk complexes?

To phosphorylate specific sets of substrate proteins.


What are the activation steps of Cdks?

1. Partial activation when cyclin is bound and moves T-loop
2. Full activation when CAK (Cdk-activating kinase) phosphorylates threonine on T-loop, near active site


How is the activity of the cyclin-Cdk complex regulated?

1. By diphosphorylation/dephosphorylation by kinase Wee1 and phosphatase Cdc25
2. Binding of CDIs (CDK inhibitory proteins), ex. p27


Does Cdk contain ATP?

Yes. One ATP is bound to the protein.


How does progression through the Start and G2/M checkpoint differ from profression through the metaphase-anaphase-transition?

The former are triggered by protein activation through phosphorylation, whilst the latter is triggered through proteolysis.


Describe the mechanism of progression through the metaphase-to-anaphase transition.

APC/C (anaphase-promoting complex, cyclosome) is activated through process activated by Cdc20/Cdh1. It catalyzes ubiquitylation of securin, causing release of sister chromatids from each other. It also causes ubiquitylation of cdk-bound cyclins, which together with dephosphorylation inactivates most Cdks.


Why does APC/C action persist also in the former part of F1?

It provides a stable period of Cdk inactivity.


What is the role of SCF?

It ubiquitylates certain CKIs in late G1, controlling activation of S-Cdks and DNA replication.


How does APC/C and SCF activity periods differ, and how does protein phosphorylation play a part in this?

Whilst APC/C is periodically active, SCF is constantly active but its interaction depends on the F-box recognizing target proteins based on their phosphorylation.


How big an impact on cell-cycle control does transcriptional regulation impart?

The impact is not great but still substantial, ex. for cyclin level regulation. The method is used in the more complex cell-cycles of most cell types.


Outline the initiation of replication and its regulation.

1. Assembling of pre-RC, pre-replicate complex, (ORC, origin recognition complex + 6 Mcm proteins loaded by Cdc6 and Cdt1, load the helicases) origin of replication is inhibited by Cdk activity, stimulated by APC/C.
2. Forming of preinitiation complex through activation of S-Cdk. Also attracts other proteins, forming giant preinitiation complex). Partial dismantling of pre-RC due to high S- and M-Cdk until late mitosis. Cdks phosphorylate ORC and Cdc6 resulting in inhibition.
3. Unwinding of DNA and loading of polymerases etc.
4. APC/C triggers destruction of geminin that inhibits Cdt1, thus allowing replication to start over.


How are Cdks involved in control of chromosome replication?

S-Cdk stimulates synthesis of histone momomers.


What is the function of the SMC family protrins cohesins in chromosome replication?

Non-covalent shepherding of sister chromatids, keeping them close inside the ring structure.


What is DNA catenation?

Intertwining of sister DNA molecules at replication forks. Sorter by topoisomerase IIs.


Summerize the two main steps of mitosis.

1. Abrupt increase in M-Cdk at G2/M checkpoint and subsequent early mitosis
2. At metaphase-to-anaphase transition APC/C triggers ercurin destruction, cohesin cleaving proteases liberated and subsequent separation of sister chromatids. APC/C triggered destruction leads to Cdk inactivation and dephosphorylation of target proteins, leading to late M ohase events.


In addition to M-Cdk two kinases are responsible for phosphorylation of key proteins needed for early mitosis. Ehich are these?

Polo-like kinases and Aurora kinases.


Outline the steps to activation of M-cyclin.

1. Increased M-cyclin produced during G2/M phase leads to increase in M-Cdk
2. CAK activates the M-Cdk complex
3. Wee1 inactivates it through diphosphorylation
4. Protein phosphatase Cdc25 is activated, ending M-Cdk inhibition
5. Positive feedback from M-Cdk to Cdc25 + inhibition of Wee1