Cell-Division Cycle

Question 1

Define cell cycle.

Answer 1

= sequenceof events in which cell duplicates its contents and divides into two.

Question 2

How does the whole cycle looks like - what phases do we have, what happens during it?

Answer 2

The main phase = M phase, includes mitosis (nucleus divides) and cytokinesis (cell splits into 2)
-> followed by Interphase
- S phase (=synthesis), when cell replicated its DNA
- is in between G1 and G2 (gap phases) - cell keeps growing, monitoring its internal and external conditions to decide whether to proceed into the next phase
- together the whole phase - transcription of genes, synthesis of genes and replication of organneles

NOTE: if only S was done (i.e. only DNA replication without increase in mass) cells would shrink with each division

Question 3

What is meant by “cycle-cycle control system”? At what points and what kind of checks does it monitor?

Answer 3

= complex network of regulatory proteins that makes sure that the cell cycle will proceed in the correct sequence
- E.g. if DNA replication gets delayed (S) -> mitosis also has to wait
- Does so by monitoring molecular checkpoint along the way, being informed by each phase where in this cycle cell lies
- 3 main transition points:
- From G1 to S - is the environment favourable for proliferation e.g. sufficient nutrients
- if not, may even enter G0 = resting state
- G2 to M - DNA not damaged and fully replicated
- During M - are the chromosomes properly attached to the mitotic spindle so that we can initiate “pulling apart”

Question 4

What does the molecular switch of cycle-cycle control system rely on?

Answer 4

• On the process of phosphorylation and dephosphorylation
  - One type of protein kinases becomes active at the end of G1 (drives cell into S) while another at the end of G2 (-> M)
  - Then becomes immediately deactivated by protein phosphatases
• The process of activation is governed by Cyclins
  - called that way because their concentration varies periodically across the cycle
  - no enzymatic activity themselves, bind to cell-cycle specific kinases -> cyclin dependent protein kinases (Cdks)

Question 4

There are apparently different kinds of CdkS - What arethe main ones?

Answer 4

• In G2 to trigger M = M cyclin
  -Cyclins S and G1/S bind to sdk proteins later in G1 phase ro form S-Cdk and G1/S-Cdk => launch S
• G1 cyclins bind and form G1-Cdks -> drive G1 towards S

Each phosphorylates different set of proteins

Question 5

What happens to cyclins during these phases?

Answer 5

When needed -> transcription of more cyclins -> concentration rises ->act on the cell

• At the end of a phase (e.g.M) -> activation of large enzyme complex = anaphase-promoting comple APC -> tags cyclins with ubiquitin -> mark for proteasomes -> their degradation and return of Cdk into inactive state
  => allows to switch to a different phase

Question 6

If cyclins increase progressively, how come do we get only abrupt changes?

Answer 6

That’s because cyclic-Cdk complex also contained inhibitory phosphotates -> to become active Cdk needs to be dephosphorylated by a specific protein phosphate

=> protein kinases and phosphotases regulate the cycle

Question 7

What else modulates activity of Cdk inhibitor proteins?

Answer 7

• Cdk inhibitor proteins -> help maintain Cdks in an inactive state e.g. during G1 phases, letting the cell increase in mass before division

Question 8

Reminder - what is the G1 phase?

Answer 8

• one of the interphases that prepare the cell for mitosis, cytokinesis
• involves monitoring of extracellular signals about the environment as well as internal signals regarding e.g. cell size
• opportunity to grow
• often, if decided to continue into S it usually goes through the entire cycle

Question 9

What needs to happen after M in order to enter G1 again?

Answer 9

• The synthesis of cyclins (S-cyclin and M-cyclin) must be stopped
• Cdk inhibitor proteins inactivate any remaining cyclin-Cdk complexes
  => shut down of Cdk activity, robust stop

NOTE: in some early embryos the cell might skip Gs -> becomes smaller with each division (doesn’t allow itself to grow)

Question 10

What extracellular signals does G1 need to proceed? What if their not there - can it still enter cell cycle afterwards?

Answer 10

• Need for mitogens -> act on signaling pathways ultimatelly producing G1, G1/S cyclins and other proteins relevant for DNA synthesis and chromosome duplication
• If NOT supplied for longer period of time => cell enters non-proliferating state G0
  - to get from such a state there would need to be a greater build up of G1/S-Cdk activity -> gets relieved of the negative control that blocks progression to S

Question 11

What acts as a negative break in G1 phase? How dowe remove it?

Answer 11

• Retinoblastoma (Rb) protein -> binds to particular transcription regulators and prevents them from turning on genes related to cell proliferation
  
  • Mitogens -> activate G1-Cdk and G1/S-Cdk -> phosphorylates Rb -> changes its conformation -> releases control over transcription regulators -> genes free to activate

Question 12

How can G1 -> S be halted due to DNA?

Answer 12

• When there is aDNA damage the cell shouldn’t continue with replication and so on (-> would lead to high number of mutations)
• So if that happens -> increase in protein p53, transcription regulator that turns on genes for producing Cdk inhibitor protein p21 -> increase in p21 -> binds and inhibits G1/S-Cdk and S-Cdk -> prevents transition to S
  - gives cells time to repair the DNA
• If damage too great -> p53 can induce apoptosis

Question 13

How often the cells divide (no need for specific numbers)?

Answer 13

That depends on the type of cells and the conditions
- some cells may enter a permanent non-proliferating state in which the genes for cyclins and Cdks are completely shut down e.g. nerve cells
- but some can shut down only temporarily (G0) and be activated every onow and then or when situation calls for it
- e.g. epithelial cells of the gut divide more than twice a day X liver cells can divide if the organ gets damaged

Question 14

How does DNA become replication-ready?

Answer 14

During G1 -> recruitment of proteins to nucleotide sequences (“origins of replication”) sites along chromosomes
- e.g. protein complex “origin recognition complex” (ORC) recruits protein Cdc6 (increase of its concentration early in G1 -> together load DNA helicases -> that opens the double helix => this “prereplication complex” is in place

Question 15

How is replication officially initiated? How do we ensure thatit doesn’t happen again right away?

Answer 15

At the end of G1 cyclin complex S-Cdk gets activated -> triggers DNA helicases prereplication complex + assemples all other corresponding proteins into the replication fork

It also prevents rereplication: helps phosphorylate Cdc6 which marks it for degradation -> its elimination gets rid of the initial signal