checkpoints

Question 1

what mediates Rb hypophosphorylation?

Answer 1

cyclin D/cdk4/6 complex

Question 2

what mediates Rb hyperphosphorylation?

Answer 2

cyclin E/cdk2 complex

Question 3

describe the role of E2Fs in mediating cell cycle progression?

Answer 3

during early-mid g1 - unphosphorylated or hypophosphorylated Rb binds E2Fs + blocks its transcriptional activity
during r point - hyperphosphorylated Rb releases E2Fs from complex + E2Fs transcribe target genes mediating g1/s transition
E2F target gene = cyclin E -> E2Fs drive their own activation

Question 4

how is E2F activity short lived?

Answer 4

as cell undergoes g1/s transition cyclin A/cdk2 complex inhibits E2Fs - targets E2Fs for degradation via ubiquitination

Question 5

describe how cells progress to anaphase when chromosomes are aligned

Answer 5

APC/C activated
cohesin holds 2 sister chromatids together
APC/C ubiquitinates securin -> targets securin for proteosomal degradation
separase is held in a complex with securin = inactivates separase
when securin is degraded, separase = active
separase degrades cohesin so 2 sister chromatids can separate

Question 6

what is MPF = maturation promoting factor also known as?

Answer 6

M-phase promoting factor

cyclin B/CDC2 complex

Question 7

what is cell cycle arrest mediated by?

Answer 7

ATM

ATR

Question 8

p53

Answer 8

tumour suppressor
transcription factor promoting expression of genes promoting growth suppression, apoptosis and DNA repair
target gene = p21cip1 = cdk inhibitor –> prevents cell cycle progression

Question 9

DEFINE: anoikis

Answer 9

cell detachment induced apoptosis

Question 10

myc-max complex

Answer 10

promotes proliferation

inhibits differentiation

Question 11

mad-max complex

Answer 11

promotes differentiation

inhibits proliferation

Question 12

what are the actions of the myc/max complex on the cell cycle clock?

Answer 12

• promotes cyclin D2/cdk4 complexes
• promotes E2F transcription factors
• promotes myc/miz1

Question 13

what is the role of myc/miz1

Answer 13

• suppresses transcription of ckis so liberates cyclin D2/cdk4 & cyclin E/cdk2 complexes from inhibition
• promotes degradation of p27kip1
  allows progression through r point

Question 14

describe an experiment showing that myc activation alone is sufficient to relieve all constraints on proliferation

Answer 14

ectopically expressed myc as a fusion protein by attaching myc to an oestrogen receptor
no ligand? -> myc is sequestered in cytoplasm = inhibited
oestrogen or tamoxifen added? -> change in conformation of complex driving myc translocation to nucleus as a TF = active
when oestrogen or tamoxifen was added during G0 in absence of growth factors, myc was active enough to drive entry into g1 and s phase

Question 15

what are some cancer cell strategies used to oppose TGFβ signalling?

Answer 15

• mutant Smad = inactivates smad
• mutant TGFβ receptor = inactivated
• deregulate pRb pathway by:
• mutate pRb = inactivates pRb
• very high expression levels of cyclin D1 - hypophosphorylates Rb
• downregulate INK4 proteins
• mutate CDK4 so unable to bind CKIs

Question 16

where do the vast majority of point mutations in p53 occur in?

Answer 16

dna binding domain

p53 unable to bind dna + promote transcription of p53 target genes

Question 17

what are some signals inducing p53?

Answer 17

damage
stress
* hypoxia 
* UV radiation
* lack of nutrients

Question 18

what regulates p53 levels and how does it do so?

Answer 18

mdm2
cell in normal conditions: mdm2 binds p53 and promotes ubiquitination then proteosomal degradation
stress or damage:
1. atm/atr pathways activated
2. leads to activation of chk2
3. phosphorylation of p53
* phosphorylated p53 cannot bind to mdm2 so cannot be ubiquitinated by mdm2
* ATM phosphorylates mdm2 –> inactivating mdm2
mitogenic or cell survival signals present:
1. more mdm2 transcription induced
mdm2 phosphorylated at a different residue - activates mdm2 + facilitates mdm2 binding to p53
2. more p53 ubiquitination and degradation

Question 19

which subunit activates APC/C

Answer 19

Cdc20

Question 20

what is the synaptonemal complex

Answer 20

protein structure that forms between homologous chromosomes

Question 21

DEFINE: non-disjunction

Answer 21

2 homologues not properly separated resulting in gametes with an extra chromosome or lacking a chromosome