Cel Biology - Section 4: Stages of the Cell Cycle

!A!: stages of the cell cycle

G1, S, G2, M (s3)

stages of m phase

1. prophase
2. prometaphase
3. metaphase
4. anaphase
5. telophase
6. cytokinesis

G1 phase

preparation of cell for DNA replication

S phase

DNA replication

G2 phase

cell growth (size, biomass, materials)

M phase

- sorting of materials (chromosomes, organelles) in prep. for separation
- separation of cell into two daughter cells

M phase: prophase

- chromosomes begin to condense
- mitotic spindle begins to assemble

M phase: prometaphase

- breakdown of nuclear envelope to allow chromosomes to position accordingly
- attachment of the chromosomes to the spindles (microtubules)

M phase: metaphase

alignment of chromosomes at equator (site at which cells will eventually separate)

M phase: anaphase

- sister chromatids (copies of same chromosomes) separate
- chromatids are pulled apart towards spindle poles

M phase: telophase

- full sets of daughter chromosomes arrive @ spindle poles
- a contractile ring begins to form that starts to "pinch" the cytosol @ site that cells will eventually separate
- nuclear membrane begins to re-form on either pole of cell

M phase: cytokinesis

- a contractile ring "squeezes" the cell to form two daughters
- once fully contracted, this generates cleavage furrow, which reps final stages of separation of two daughter cells

!B!: 3 distinct checkpoints/regulation points of cell cycle

- G1/G0 cell cycle start
- G2/M checkpoint
- metaphase to anaphase transition

G1/G0 cell cycle start (G1-->S)

to enter cell cycle and proceed to S phase:
- is the environment favourable?
- replicating chromosomes costs a lot of E so are there enough nutrients (glucose, AAs, etc.)?
- has it received growth factors and hormones?

G2/M checkpoint

to enter mitosis:
- is all DNA replicated?
- is environment favourable?
- condensing DNA is extremely dangerous for cells
- is every chromosome anchored and ready to be pulled apart?

metaphase to anaphase transition

to trigger anaphase and proceed to cytokinesis:
- are all chromosomes attached to the spindle?

cell cycle is controlled by..

cyclin-dependent kinases (CDKs)
- can phosphorylate several diff. substrate proteins that are critical to cell cycle
- phosphorylation of proteins by CDKs acts as "switch" to progress through specific stages of cell cycle

what is the switch mechanism to turn the CDK on?

binding to one of several specific cyclin proteins

there are several diff cyclin proteins and their levels..

.. rise and fall at different stages of the cell cycle

cyclins not only "activate" CDK but also...

.. target activate CDKs (active when cyclin-bound) to specific substrates

levels of specific cyclins rise and fall at

different stages of the cell cycle
CDK: levels do not change during stages of cell cycle
cyclin: levels rise and fall with specific stages of the cell cycle (e.g. G1/S-, S-, M-, G2M-cyclins), appear and disappear when needed

how does M-CDK control G2/M transition? (s7)

- M-CDK activation triggers condensin (protein complex), which causes concensation of chromosomes (orange loops in diagram)
- M-CDK activation causes formation of the mitotic spindle

M-CDK activation starts mitosis but ... (s8)

... removal of M-CDK needed to finish mitosis
APC/C is a protein complex
- required to allow metaphase to anaphase transition
- leads to degradation (removal of M-CDK)

cyclin-CDK complexes act like a "timer"

- in some cases, causes degradation or removal of cyclin of previous stage of cell cycle
- in some cases, will eventually cause up-regulation of cyclin of next stage of cell cycle
OVERALL, allows cyclin-CDK complexes to make sure that stages of cell cycle don't all happen at the same time

cyclin-CDK complexes allow for cell cycle checkpoints

- cyclin-CDK complexes can be controlled (i.e. turned off) if cell cycle conditions are not met: damage, insufficient nutrients, insufficient growth factor simulation, failure to complete prev. stage)
OVERALL, allow cyclin-CDK complexes to monitor to make sure moving to next stage of cell cycle is SAFE, if DNA didn't replicate, cycle stops, repair occurs, cycle resumes

triggers for checkpoints that arrest cell cycle s10

3 checkpoints:
1. favourable extracellular environment (arrest in G phase if not)
2. unreplicated/damaged DNA (arrest at G2/M phase if so)
3. chromosome unattached to spindle (arrest in M phase)

- BUT cancer cells can pass these checkpoints and continue the cell cycle over and over again

some CDKs are very effective

cancer drug targets
- palbociclip, ribocuclib, abemaciclib
- used for treatment of metastatic breast cancer, pancreatic cancer, other cancers

!C!: mitotic spindle begins to assemble in...

.. early mitosis
- begins in prophase
- involves microtubules, centrosomes to anchor microtubules, and many other proteins
- way for cells to ensure that chromosomes are counted for ad divided equally

mitotic spindle s4

- bipolar array of microtubules involved in proper separation of chromosomes and other materials during mitosis

spindle pole

made up of centrosome and other components, all 3 types of microtubules emanate from here

astral microtubules

from spindle pole to cell cortex, helps to "anchor" spindle poles

kinetochore microtubules

from spindle pole to chromosome (attaches @ kinetochore region of chromosome)

interpolar microtubules

from spindle pole to interpolar microtubule coming from other spindle pole to ensure spindle poles stay at opposite poles of cell

kinetochore s5

region of chromosome made up of proteins that allow for anchoring of spindle (kinetochore) microtubules

sister chromatids

identical copies of chromosomes that are held together during early stages of mitosis (i.e. through metaphase)

positioning chromatids for correct separation into daughter cells

at transition from metaphase to anaphase, the "glue" that holds sister chromatids together is removed and each sister chromatid is "pulled" towards one spindle pole

how does a cell know when it is time to proceed from metaphase to anaphase

- in metaphase, all chromosomes need to be attached to kinetochore microtubules to prevent improper sorting of chromosomes during cell division
- this can be sensed by making sure all kinetochores are attached to microtubule (that no "free" kinetochores remain)
- this is the metaphase to anaphase transition checkpoint (sensing that there are no "free" kinetochores remaining)

mitotic assembly is a major checkpoint, sensed by..

.. APC/C and controlling metaphase to anaphase transition
- when APC/C blocked, cell stops @ anaphase

drugs targeting the mitotic spindle assembly are..

.. effective cancer therapies

paclitaxel, docetaxel

- chemotherapies used in many types of cancer treatment
- these work by binding and stabilizing microtubules, prevents assembly of mitotic spindle
- trigger metaphase-anaphase checkpoint arrest