W5 The eukaryotic cell cycle and interphase/Lec 9 Flashcards
Why is the cell cycle important?
For the repair/renew process in cells
Do all cells divide?
No and division rates are different between cells
What is the resting stage of the cell cycle called?
When does this occur?
G0 (quiescent). This occurs when cells reach a certain size, growth either stops its cycle or the cell must divide.
What are the 4 stages of the cell cycle in eukaryotic cells?
M phase, comprising mitosis and cytokinesis
G1 phase
S phase
G2 phase
Why must cell division be tightly controlled?
Uncontrolled cell division can result in cancer
What is the cell-cycle control system?
An internal control to ensure the proper progression of the cell cycle and that key steps occur in the right sequence
(Checkpoints)
How does the cell-cycle control system work?
- Using a set of checkpoints (biochemical switches),to pause the cycle at 3 main transition points (G1/S,G2/M & metaphase/anaphase transition phases)
- It checks that all the required events for progression have occurred and it does not trigger the next step in
the cycle unless the cell is properly prepared
What does progression of every checkpoint of the cell-cycle control system depends on?
Some cyclically activated Cyclin-dependent protein kinases (Cdks)
How do Cdk’s work?
Cdks must bind to a specific regulatory
protein called cyclin to become active
(cyclin-Cdk complex).
* Cdks must also be in a particular
phosphorylation state
➢ Cyclins do not have enzymatic activity Activated Cdks phosphorylate some proteins crucial for the checkpoint
transitions, which allows the cell to proceed along the cell cycle
➢ E.g. proteins to initiate DNA replication at G1/S transition (e.g. helicases)
What occurs during the G1 phase?
Period of metabolic activity, cell growth, and
general repair. The cell grows in mass to
prepare the cell for division
* G1 - decision-making for the cell (to pass the
G1/S checkpoint), according to:
➢ Cell size
➢ Presence of nutrients, grow factors
➢ DNA integrity
What occurs after the G1 phase?
Cell can proceed to S phase; extracellular signals(mitogens) induces progression
▪ Delay the entrance in S phase (to further grow or if DNA is damaged)
▪ Exit the cell cycle to G0 (temporarily or permanently)
▪ Induce a programmed cell death (apoptosis), if there is a severe DNA damage
What occurs during the S phase?
The phase in which DNA is replicated, therefore
this phase is highly regulated
* Once entered into the S phase, S-Cdk activates
helicases and other proteins to form the replication
forks and initiate the DNA replication
Sister chromatids are then connected by cohesins (for chromosome segregation)
In S phase, centrosome is also duplicated
What is a centrosome
Centrosome is a cellular structure that controls
location, number, and orientation of the microtubules
* Composed of 2 centrioles (9 triplets of microtubules)
Functions of a centrosome
- It serves to organise microtubules to make cytoskeleton
- It organises microtubules during cell division and forms the mitotic spindle
(microtubules machinery that controls chromosome movement during mitosis)
What occurs during the G2 phase?
- Rapid cell growth and protein synthesis
(accumulation of enzymes) to prepare for mitosis - To check for unreplicated or damaged DNA
