Cell cycle + DNA repair Flashcards
What are the 4 phases of the cell cycle?
cell enters cell cycle to duplicate its DNA content for production of 2 daughter cells
- G1: cell prepares for replication of DNA
- S: replication of DNA
- G2: cell prepares for cell division
- M: mitosis
induced in response to growth signal
![](https://s3.amazonaws.com/brainscape-prod/system/cm/151/315/223/a_image_thumb.png?1659468993)
How do you call the phase when cells are not dividing?
Why does it happen?
quiescent, G0 phase
Explain the terms ploidy, haploid, diploid, tri- and tetraploid.
ploidy = number of sets of chromosomes in a cell
- haploid (n): 1 set of chromosomes present (in zygotes)
- diploid (2n): 1 pair of homolog chromomosomes present (in somatic cells)
- triploid (3n): third chromosome present (i.e. in Down syndrome)
- tetraploid (4n): DNA content of somatic cell after S-phase
So what are euploidy and anploidy?
- euploidy: “good ploidy”, so only exact multiple of the haploid number (n) should be present
- anploidy: chromosomal number of a cell is not a multiple of the haploid number (n) (often in tumors) → causes chromosomal instability
Give an example for a synthetic nucleotide analog.
Medical application?
5-fluorodeoxyuridine
- inhibit enzymes for DNA replication
- incorporation into nucleic acids causing disruption of base pairing
⇒ chemotherapy, sometimes also used for organ transplantation to suppress immunologic rejection
What do colchine or taxol do?
How are they used in medical practice?
both disrupt mitotic spindle
- colchicine used for karyotype analysis
- taxol used as cytostaticum
What does FACS do?
fluorescence-activated cell sorter, special type of flow cytometry
sorts a mixture of biological cells into 2+ containers, one cell at a time, based upon the specific light scattering/fluorescent characteristics
→ dependent on chromatin layout present in cell, shows DNA content of diff. cell phases
How was the concept of permissive and restrictive temperatures discovered?
yeast strains were incubated at different temperatures
- permissive (low) temperature → only those strains could enter cell cycle and proliferate
- restrictive (high) temperature T → caused denaturation of enzymes, no proliferation
→ then used mutated yeast strains, observed that they could adapt to restrictive temperatures and still proliferated
Which proteins are directly involved the regulation of the cell cycle?
- cyclins
- cyclin-dependent kinases (Cdks)
What are cyclins?
Features.
group of proteins controlling progression through cell cycle past restriction points by binding and activating Cdks
- don’t have enzyme activity
- can interact w/ different Cdks
NOTE: have changing conc. during cell cycle due to regulated synthesis/degradation
What are Cdks?
What would be another name?
During which phases can they be found?
cyclin-dependent kinases,
also called cdcs (_cell division cycle strain_s)
are protein kinases w/ 2 domains:
- kinase domain for phosphorylation of targets
- regulatory domain for cyclin binding
NOTE: unlike cyclins their concentration does NOT fluctuate during cell cycle
The concentration of cyclins fluctuates during the cell cycle.
What is the function of the different cyclins?
Which Cdks do they bind?
-
cyclin D causes progression from G1 to S phase
→ Cdk4, 6 -
cyclin E initiates DNA replication in early S phase
→ Cdk2 -
cyclin A governs transition from S to G2 phase, also remains until early M phase
→ Cdk2, 1 -
cyclin B appears during G2 phase, causes transition to M phase
→ Cdk1
REMEMBER: DEAB 6,4 / 2 / 2,1 / 1
![](https://s3.amazonaws.com/brainscape-prod/system/cm/189/039/211/a_image_thumb.png?1463303800)
What are SPF and MPF?
cyclin/Cdk complexes
- SPF (S-phase promoting factor): cyclin E + Cdk2 → initiates DNA replication
- MPF (maturation promoting factor): cyclin B + Cdk1 → initiates mitosis
List the mechanisms that regulate the presence cyclin/Cdk complexes in the nucleus.
- de-/phosphorylation of Cdks
- ubquitination + proteasomal degradation of cyclins and Cdks
- Cdk inhibitor proteins (CKIs)
- translocation of Cdks into nucleus
- regulation on level of transcription by p53, pRB
How are the cyclin/Cdk complexes generally activated?
have low activity when phosphorylated
→ need to be dephosphorylated and rephosphorylated at a different AA to be active
What is the function of MPF?
How is it activated?
mitosis-promoting factor, induces mitosis
- when inactive, active center of Cdk1 phosphorylated by Wee and Myt kinase
- for activation, Cdk1 phosphorlyated by Cak
- now cdc25 phosphatase dephosphorylates active center
→ active MPF now causing all its effects, incl. further activation of cdc25 phosphatase (feedforward)
(cyclin B/Cdk1 complex)
When and how is MPF inactivated?
inactivated to leave mitosis
- Kap phosphatase dephosphorylates earlier phosphorylation site of Cak
- cyclin B is ubiquinated + degraded by proteasome
→ cdk1 can reassociate w/ new cyclins, becomes phosphorylated in active site again
Which AA of Cdk1 are exactly phosphorylated/dephosphorylated to activate/inactivate MPF?
-
Wee, Myt kinase phosphorylate Thr14, Tyr15
→ inactivating b/c in active center -
Cak phosphorylates Thr160
→ required for full activation of MPF -
Kap dephosphorylates Thr160
→ inactivates MPF again
What is the functions of the activated MPF complex?
causes multiple events to initiate mitosis
- breaks down nuclear envelope
- condensates chromatin
- leads to cytoskeletal rearrangements, formation of centrosome
- activates APC
- activates SCF
Which signals induce a transition from G1 to S phase?
GFs bind to mitogen-Rs on cell surface, activate gene regulatory proteins like transcription factor c-myc that induces the synthesis of cyclin D
→ forms synthesis-promoting factor (SPF) w/ Cdk4 or Cdk6
What does SPF do?
phosphorylates tumor suppressor protein pRB (retinoblastoma protein)
unphopshorylated pRB binds transcription factor E2F, but releases it when phosphorylated
What does E2F do?
Give examples for induced proteins. Outcome?
stimulates synthesis of S-phase proteins following a cascade induced by GF binding
- cyclin E: forms complex w/ Cdk2, further phosphorylates pRB → feedforward
- DNA polymerase: starts DNA replication
Which CKIs do you know?
What is their effect?
2 families of CKIs
-
INK Cdk inhibitors:
p15, p16, p18, p19 → basically inhibit all exc. SPF
⇒ causing senescence -
KIP/CIP Cdk inhibitors:
p21, p27, p57 → inhibit SPF (cyclin A/Cdk2)
⇒ causing cell cycle arrest
![](https://s3.amazonaws.com/brainscape-prod/system/cm/189/040/584/a_image_thumb.png?1463307785)
What is cellular senescence?
cells that stopped dividing, “grow old”
What is contact inhibition?
regulatory mechanism that keeps cells from growing if in too close contact
→ no progression from G1 to S-phase
Explain the process of ubiquination.
cytosolic proteins receive ubiquitin as a mark for degradation by proteasomes
-
E1 = ubiquitin-activating enzyme
ubiquitin binds to E1 via thioester, ATP hydrolyzed to AMP -
E2 = ubiquitin-conjugating enzyme
ubiquitin transferred from E1 to E2 -
E3 = ubiquitin ligase
ubiquitin transferred from E2 to substrate protein, forming an iso__peptide bond
The concentration of p53 is normally held low.
How?
forms tetramer, each p53 binds the ubiquitin ligase Mdm2 → marked for ubiquitination + proteasomal degradation
What is the function of SCF?
Skp1/cullin/F-box protein, E3 ubiquitin ligase
activated by MPF
→ ubiquinates p27 (inhibitor of cyclin D+E) → disinhibition of of G1-S phase + S-phase cdks
What is the function of APC?
anaphase-promoting complex, E3 ubiquitin ligase
activated by MPF
- ubiquinates anaphase inhibitors → transition from meta- to anaphase
- ubiquinates cyclin B → cell returns to G1 phase after finishing mitosis