T6: Cell cycle and checkpoints

Question 1

Reasons for the occurrence of cell division (4)

Answer 1

1. growth (of tissues and entire organism)
2. repair damaged tissues
3. reproduce
4. neoplastic proliferation (cancer)

!! cells need to divide because every cell’s size is limited by the need of granting fluxes of nutrients –> important to maintain a certain SA:V ratio

Question 2

4 phases in cell division and the events within each one

Answer 2

G1 PHASE: organelle duplication and synthesis of molecules needed for DNA replication before S phase

S PHASE: DNA and centrosome duplication (creates two sister chromatids but maintains cell ploidy)

G2 PHASE: finishes organelle duplication and synthesis of proteins needed in M phase (eg. tubulin for mitotic spindle formation)

MITOTIC PHASE: nuclear and cytoplasmic division.

Question 3

Checkpoints (3) and their purpose

Answer 3

1. LATE G1: asseses DNA integrity and checks environment for whether level of nutrient availability would allow DNA replication in S phase
2. G2-M: assesses effectiveness of DNA replication, checks for environmental favorability HENCE if the cell can pass from interphase to mitosis
3. METAPHASE TO ANAPHASE: checks correct attachment of chromosomes on their spindle fibers (to ensure proper segregation of sister chromatids in anaphase)

Question 4

General pathway triggered from checkpoint failure during cell cycle

Answer 4

1. DNA damage or replication stress is detected
2. signals sensed from DNA damage receptor molecules
3. Signal sent to transducers
4. Signal sent to effectors

EFFECTORS EITHER:
Repair DNA/modulate transcription to repair the DNA, send signals to stop cell cycle transition
!! if repair is not possible apoptosis is triggered

Question 5

2 classes of molecules present in cell cycle control system

Answer 5

1. CYCLINS; A,B,D,E
2. CDKs (cyclin dependent protein kinases): 1,2,4,6

Question 6

Changes in concentration of each type of cyclin along cell cycle

Answer 6

CYCLIN A: starts increasing in late G1 and peaks in the middle of G2, drops to 0 at the end of M

CYCLIN B: increases in S phase, peaks in transition of G2-M, reaches 0 at the end of M

CYCLIN D: increases in G1 reaches peak in S and 0 at the end of M –> smooth inverse parabola

CYCLIN E: increases middle of G1, peak in transition between G1-S, drops to 0 in middle of S

Question 7

Ways that the concentration of cyclins is modified and maintained across the cycle (3)

Answer 7

1. transcription
2. translation
3. Protein stability (ratio between synthesized and degraded proteins)

REASON: cyclins are proteins so they are regulated by extent of protein synthesis from their genes

Question 8

Cdks general description

Answer 8

-Cdks: 1,2,4,6
-serine-threonine protein kinases
-activity is modulated (positively) by the interaction with cyclins
-concentration stays roughly the same across cell cycle but their activation is variable (depending on cyclin conc)

Question 9

Cdk-cyclin complexes formed along the cell cycle (4)

Answer 9

1. G1-Cdk complex: formed in the middle of G1 with cyclin D and Cdk 4/6
2. G1/S-Cdk complex: formed in transition between G1/S with cyclin E and Cdk 2
3. S-Cdk complex: formed in S with cyclin A and Cdk 1/2
4. M-Cdk complex: formed in M with cyclin B and Cdk1

DONT EAT ANY BREAD: cycling D/E/A/B in order

46 2121 –> FS TOTO –> cyclins used in order of complexes

Question 10

activation and inactivation mechanism of the cyclin-cdk complex

Answer 10

-association of cyclin with the appropriate CDk
-phosphorylation of complex due to kinase activity which can either:
1. activate it (position 160)
2. inactivate it (position 14/15)

-presence of CDKi (inhibitor) prevents activation of complex

!! the complex is in its active form if the Pi group is only found in its 160 position AND the Cdki has detached and Pi at 14/15 have been detached via phosphatase enzymes

Question 11

Types of Cdkis and their role in the cycle (in budding yeast and mammals)

Answer 11

1. YEAST: overexpression of Sic1 protein suppresses CDK1 activity during G1
2. MAMMALS:
   -p27: suppresses G1/S complex
   -p21: suppresses G1-S complex
   -p16: suppresses G1 complex

Question 12

result of malfunction of a cdki

Answer 12

malfunction leads to the alteration of cell cycle which leads to a cancer pathogenesis

Question 13

Substrates affected by the S and M cdk complexes and general CDK substrates (10)

Answer 13

S-CDK: catalyzes the phosphorylation of proteins initiating DNA replication

M-CDK: phosphorylates condensin (responsible for condensation of chromosomes) and Lamin (trigger nuclear membrane disassembling)

GENERAL CDK SUBSTRATES:
-androgen receptor
-beta tubulin
-DNA pol lambda
-elongation factor
-dyne light intermediate chain
-Histone kinesins
-connexin
-dystrophin
-GM130

-Lamins A/B

a boy doesn’t enjoy dancing, he can’t do glissades

Question 14

Process of cyclin final degradation once a checkpoint has been passed

Answer 14

Cyclins are degraded by the action of ubiquitin-proteasome system. Signals are sent to detach the Pi 160 activating group. Degradation of cyclin also inactivate CDks

Question 15

what is the importance of final cyclin degradation

Answer 15

If final degradation fails to happen, cell arrest can occur since it is important for the progression of the cell through the cycle phases, and so arrest of the cycle can trigger apoptosis

Question 16

Role of the extracellular environment in activating and inhibiting cell cycle

Answer 16

1. activation: occurs if there is a surplus of nutrients, or of GFs and hormones
2. inhibition: lack of nutrients (starvation)

Question 17

GF and FN stimulating pathway

Answer 17

GF = growth factors
FN = fibronectin

-GF/FN interaction with receptors (GF with TRKS and FN with integrins)
-activation of signaling pathways involving P13 kinase, ERK (MAP kinase) and RAC.
-these molecules are able to signal an induction of expression for mRNA encoding for cyclin D1 (hence increase in cyclin D1 protein concentration)
-D1 protein signals the progression to G1 phase and enter S phase

Question 18

Factors that affect the size of an organ/organism (3)

Answer 18

1. cell growth
2. cell division
3. cell survival

Question 19

extracellular signals that control the size of an organism (3)

Answer 19

1. mitogens: substances that induce or stimulate mitosis (like PDGF, EGF, NGF, FGF)
2. GFs: promote cell growth and increase in cell mass by increasing protein synthesis
3. Survival factor: reduce apoptotic rate and promote survival

Question 20

GF signaling through RAS pathway to allow ACTIVATION OF E2F PROTEIN

Answer 20

1. Binding of mitogenic compound to TRK receptor
2. activation of kinase domain via phosphorylation
3. anchorage of adaptor proteins to switch inactive GDP RAS to active GTP RAS
4. active RAS activates series of MAP kinases to trigger cascade
5. Cascade induces nuclear TFs including immediate early gene expression: able to promote mRNA coding for other TFs like the yamanaka factor Myc
6. Myc activation modulates the expression of genes coding for G1/S cyclins
7. increase in intercellular cyclin levels increase activation of CDks
8. active Cyclin-CDk complexes activate RB (retinoblastoma) protein cause the RB to inactivate
9. inactive RB releases E2F that translocates to nucleus and modulates expression of genes including those for S phase cyclins

! release of E2F proteins promote protein synthesis for G1-S transition molecules

Question 21

E2F Proteins def

Answer 21

TF activated by RAS cascade which activates expression of proteins required for S phase such as: G1/S and S cyclins

Question 22

Ways that molecules of the extracellular matrix modulate cell proliferation

Answer 22

1. Anchorage dependency: positive ECM signals continue cell proliferation which verify that the cell is properly adhered to its substrate
2. Contact Inhibition: proliferation of cells continues until there is cell to cell contact which triggers it to stop. (this is lost in cancerous cells which are able to grow on top of eachother)

Question 23

Definition and structure of a centrosome

Answer 23

-The MTOC (microtubule organizing center) of the cytoskeleton.
-composed of 2 centrioles (mother and daughter) perpendicular to eachother made of hollow microtubule cylinders
-held together by interconnecting fibers and surrounded by amorphous tissue
-mother centriole has a distal and a subdistal apendage

Question 24

Describe the stages of the centrosome cycle (7)

Answer 24

1. disengagement: tight link between mother and daughter is severed and they are only held by loose fibrous connections
2. duplication: during S PHASE
3. engagement: new centrioles formed reach full length, daughter becomes mother
4. maturation: collection of pericentriolar material
5. separation: during PROPHASE, generation of spindle fibres and centrioles move away from eachother
6. bipolar spindle formation: METAPHASE, centrioles at opposite poles of cell
7. cell division: each daughter cell receives one centrosome during division (ie, a pair of centrioles)

DDE, MSFD

Question 25

events during anaphase

Answer 25

-sister chromatids are separated to different poles due to kinetochore contraction
-the enzyme separase digests cohesin permitting the separation of the chromosomes
-

Question 26

3 different types of microtubules found in anaphase + function

Answer 26

1. kinetochore MT: connects spindle pole with kinetochore of dividing chromosomes
2. interpolar MT: connect 2 spindle poles
3. astral MTs: in charge of mediating association of spindle pole with membrane of the 2 cell poles

Question 27

notation of the orientation of microtubules in anaphase

Answer 27

-ve end faces spindle poles
+ve end faces center of cell

Question 28

Factors affecting cell elongation of cell in anaphase

Answer 28

1. Microtubule dynamics (addition of subunits in positive end)
2. Motor proteins (generate movement)

Question 29

changes in length of the 3 types of Microtubules in anaphase

Answer 29

Kinetochore: SHORTER
Interpolar: LONGER
Astral: SHORTER