Cell Cycle Flashcards
What are the three checkpoints?
- G1 checkpoint:
- Checks if cell has reached critical size
- Checks if DNA is free from errors
- Checks whether enough nutrients and growth factors are present - G2 checkpoint:
- Check errors in DNA
- Checks whether cell is appropriate size to enter mitosis
3.
- Checks if chromosomes are attached to spindle and accurately aligned
What causes a cell to go into G0?
If cell does not receive necessary signals
Stays until all conditions are met
Cdk form complexes with what protein?
Cyclins
How is Cdk activity regulated? What proteins are involved?
- Cyclin degradation
- Cdks only active when bound to cyclin
- cyclin degradation leaves Cdks inactive and unable to promote transition to next stage - Inhibitory phosphorylation
- Wee1 - phosphorylates active site in Cdk (inhibits activity) - Inhibitory binding
- p16 - when DNA damage occurs during G1
- p21:
- DNA damage in G1 phase
- activated by p53 (tumor suppressor)
- binds and inhibits G1/S Cdk and S Cdk (arrests cells in G1)
- p27:
- G1/S phase transition
- when cell lacks resources for DNA replication
- binds and inhibits G1/S Cdk and S Cdk
S phase
- DNA replication begins at origins of replication
- Origin recognition complexes (ORCs) bind to DNA and serve as docking sites for proteins
- Early G1 phase: Cdc6 and Cdt1 bind to ORCs and aid in assembly of MCM proteins
- MCM proteins - function as DNA
helices - S phase: S-Cdk phosphorylates initiator proteins -> promotes recruitment of helicase activators
- Helicase activators activate DNA helicases and recruit DNA polymerase
- S-Cdk phosphorylate Cdc6 and Cdt1 proteins -> release from ORCs
What components make up the pre-replicative complex (pre-RC)?
- ORC
- Cdc6
- Cdt1
- MCM
What is the role of S-Cdk in S phase?
- involved in cell cycle control during S phase
- prevents re-initiation of replication
Transition into mitosis is characterized by activation of which Cdk?
M-Cdk
Describe M-Cdk.
- Cdk1 bound to M cyclin
- M cyclin levels peak during G2
- Phosphorylated by CAK
- Inhibited by protein kinase Wee1 (two inhibitory sites)
- Activated by protein phosphatase Cdc25 (removes phosphates that inhibit M-Cdk and suppresses Wee1 activity)
How does M-Cdk contribute to each phase?
Prophase:
- Stimulates shortening and compaction of chromosomes (chromosomes condensation)
- Initiates formation of mitotic spindle (separates chromosomes into two daughter cells)
Prometaphase:
- Helps degrade nuclear envelope
Metaphase:
- Mediates attachment of sister chromatids to opposite poles of spindle
- Promotes Golgi reorganization
- Involved in actin cytoskeleton reorganization
What is the mitotic spindle?
- Separates sister chromatids and moves them to opposite poles during anaphase
- Two sets of microtubules arranged on opposite ends/poles
- Minus end: center of spindle poles
- Plus end: extend outward from poles
What are the different types of microtubules within the mitotic spindle?
- Kinetochore
- Bind chromosomes to spindle pole by attaching at their plus ends to kinetochores - Interpolar
- Resembles a pair of clasped hands
- Plus end overlaps with plus end of another interpolar MT
- Motor proteins associated - Astral
- anchor spindle
- resemble a starburst (positive end projecting from spindle pole to cell cortex)
Centrosome
- Present at each spindle pole
- Two centrioles surrounded by pericentriolar matrix
- Produces, organizes, and anchors microtubules
Cohesin
Hold sister chromatids together at centromere
APC/C
- Ubiquitin ligase enzyme
- Triggers progression from metaphase to anaphase
- Phosphorylates cyclin-Cdk
What is the relationship between securin and separase?
Securin inhibits separase
Describe the separation of sister chromatids
- Phosphorylated APC/C binds to Cdc20 (forms active complex)
- Complex recognizes securin (inhibitory protein) bound to separase (protease enzyme)
- Complex tags securin with ubiquitin (targets from proteasomal degradation)
- Destruction of securin releases separase
- Separase cleaves cohesin -> loss of chromatid cohesion (triggers transition from metaphase to anaphase)
Chromosome segregation takes place during which phase in mitosis?
Anaphase
What happens during anaphase?
Sister chromatids separate and move towards opposite poles
Anaphase A
- Two poleward forces:
1. Microtubule plus-end depolymerization
2. Microtubule flex (minus-end depolymerization) - Sister chromatids move toward poles
Anaphase B
- Spindle poles move apart (elongates spindle)
- Motor proteins
1. Kinesin-5 - crosslink plus ends of interpolar microtubules
- Dynein - link astral microtubule plus ends with cell cortex (minus-end directed); pulls spindle toward cell cortex
What are kinetochores?
Protein complexes assembled at chromatid centromere (specialized sequence that links sister chromatids)
How does cycling-Cdk regulate separase? How does APC take care of this?
Negative regulation (inhibitory phosphorylation)
APC/C causes ubiquitination of cyclins (destruction of cycling removes enzymatic activity of Cdk)
