Cell Cycle, Meiosis + Mitosis

Question 1

Describe different phases of cell cycle and what happens in each phase

Answer 1

G1 Phase:
Cell growth
- cells have maternal / paternal chromosomes
- differentiated cells may exist to G0

S Phase:
- DNA replication

G2: Phase:
Cell growth
- prepares for division

M Phase:
- mitosis (nuclear division)
- cytokinesis (cytoplasmic division)

Question 2

define ‘cyclin-dependent kinases’ and cyclins

Answer 2

Cyclin-dependent Kinases:
- phosphorylate and active substrates critical for movement of cell through specific phases of cell cycle
- expression is regulated in phase-specific manner

Cyclins:
- interact + activate the Cdks in phase-specific / phase transtion- specific manner
- expression + destruction are regulated

(movement through cell cycle depend on phase-specific cyclin-cdk complexes)
(cyclin + Cdk levels + activity are coordinated + regulated in individual phases of cell cycle)

Question 3

Describe mechanisms by which levels and activity of cyclins + Cdk’s are regulated

Answer 3

1. transcriptional regulation:
   - gradual increase / decrease in protein levels
2. ubiquitylation (not reversible)

i. ubiquitin ligases add chains of short polpetides of ubiquitin to cyclin:
ii. Proteosome recognizes the ubiquitylation + rapidly degrades cyclin
iii. cyclcin-Cdk comple activty drops

(ubiquitin ligases such as APC drive destruction of S and M phase cyclins)

3. phosphorylation + dephosphorylation

inhibitaroy kinases and phosphatases trigger conformational changes that inhibits activity without interfering with Cdk/ cyclinc complex interactions

4. inhibitory protein binding (reversible)
   - inhibitaory proteins bind to Cdk/ cyclcin complexes and prevent substrate phosphorylation –> prevents substrate molecule from binding to active site
   (reversible –> inhibitory proteins can be degraded)

Question 4

Describe cell cycle checkpoints

Answer 4

G1 / S Checkpoint
(Cdk inhibitors block entry to S phase)
Impact of Oncogenic mutations:
- mitogenes –> local paracrine growth factors that bind to cell surface receptrs and initate signal transduction cascades to active G1 and G1/S Cdk/cyclin complexes (pushes cell into S phase)
- oncogenic mutations constituively activate these receptors + transduction pathways (initates unwanted cell proliferation)

Role of Rb
- tumour suppressor: bind to + inactivate transcription factors
- Rb phosphorylated by G1-Cdk and G1/S Cdks –> no longer suppress
- loss of function Rb mutations –> tumour formation

Role of p53:
- tumour supressor:
(in presence of DNA damage, protein kinases phosphrylates, stabilize and activates p53 –> binds to regulatory region of p21 gene to code for it –> makes p21 gene (inhibitor protein and inactivates G1/S CDk complexes
- mutations –> leads to 50% of all tumours

G2 / M Checkpoint
(regulated by desphosprylation of M-Cdk)
- active Cdc25 removes phosphates from M-Cdk (activates M-Cdk)
- M-Cdk phosphroylates more Cdc25 00> reinforces more M-Cdk activation (positive feedback)
–> triggers movement from Gd2/Interphase to M phase
M phase Checkpoint

Question 5

Describe structures + processes involved in cell cycle
(keep in mind cyclin-Cdk complexes, critical protein regulators, and mediators)

Answer 5

S phase:
S-Cdk/ cyclin complex triggers centrosome replication
- aster formation: 2 centrosomes move to opposite ends of cell + begin to readiate dynamically unstable microtubules

M Phase:
M-Cdk/ cyclin complex initates condensation of duplicated chromosomes
- plus ends of microtubules from asters bind to kinetochore complex on each condensed and dubplicated chromosmes to form a mitotic spindle

Mitotic Spindle:
1. kinetochore microtubules
(bind to each sister chromatic via central kinetochore complex)
2. Aster microtubules: pull spindle poles away from each other to opp ends of cell. (generate spulling tension on sister chromatids by intereactiing with cytoskeleton associated with plasmamembrane via dynein.
3. Interpolar microtubles: bind to each other, help direct formation _ orientation of contractile ring during cytokinesis

Contractile Ring:
(made up of actin + myosin)
- formed under plasmamembrane in mid body of cell perpendicualr to interpolar microtubles

• protein anillin interactis with microtuble binding proteins on interpolar microtubules + scaffold proteins associated with plasma membranes (influence ring position +orientation.)
• Rho GTPase initates ATP-dependent actin-myosin contraction (elongates to form intercellular bridge)
• spastin (ATPase that severse microtubules) removes interpolar microtubules from bridge –> bridge plasma membrane fuse to complete cytokinesis)

Question 6

Describe difference between somatic + germ line cells

Answer 6

Germ-line cells
(cells that undergoe meiosis to produce haploid reproductive cells)
- develop in early embryo + migrate to ovaries / testes
- DNA changes inherited by progeny

(diploid –> 2N = 2 sets of chromosomes)
(haploid –> 1N = 1 set of chromosomes)

Somatic
(later becomes cells of the body)

(what is the concept + location of germ-line cells)
define haploid, diploid, and aneuploid genomes

Question 7

Explain steps involved in meiosis

Answer 7

Somatic cell undergoes 1 round of cell division –> produce 2 identical diploid cells w/ 1 maternal and 1/ paternal set of chromosomes

Meiosis 1:
- maternal and paternal chromatids of same chromosome exhange DNE (cross over) via homologus recombination
- chromosmes move to 2 daughter cells randomly

Meiosis 2:
- no DNA replication –> sister chromatids from each maternal / paternal chromosome segregate to generate 4 non-identical haploid cells

(how do meiosis + mitosis differ) /
relationship between meiosis + genetic variation )
Meiosis –> 4 unique haploid cells
Mitosis –> 2 identical diploid cells

(what is the outcome of crossing 0ver events during meiosis 1?)
- increased genetic variation
- gives 2^n possibilities of indepenent assortment of maternal and paternal homologyous chromosomes