PAUSE - CELL CYCLE 3

Question 1

what happens when an APC inhibitor is added to cells undergoing mitosis, and what does that indicate?

Answer 1

the cell stops at metaphase
proteolysis is required for the separation of sister chromatids and to advance to anaphase

Question 2

what happens when a non-degradable form of M-cyclin is added to cells undergoing mitosis, and what does that mean?

Answer 2

the cell stops at anaphase
M cyclin is not required for sister chromatic separation but is needed for the exit from mitosis

Question 3

what is the structure of condensin or cohesin dimer?

Answer 3

DNA and ATP binding domains at one end
a hinge region joined by two long coiled coil region

Question 4

what is the function of cohesin and what is the function of condensin?

Answer 4

cohesin cross links two adjacent sister chromatids
condensin mediates intramolecular cross linking to coil DNA in chromosome condensation

Question 5

how is sister chromatid separation triggered by APC?

Answer 5

1. inactive APC is converted to active APC by the binding of Cdc20
2. securin is destroyed, which holds separase in an inactive state
3. separase is now active and will cleave a subunit of the cohesin complex holding the chromatids together
4. chromatids are pulled apart

Question 6

what is something that happens in budding yeast during the separation of sister chromatids?

Answer 6

the cohesin cleavage by separate is facilitated by the phosphorylation of the cohesin complex next to the cleavage site, mediated by Polo kinase
gives extra control of the timing

Question 7

what happens when a chromatid is not attached to the spindle fire?

Answer 7

the Mad2 will bind the kinetochore
delays anaphase till all the chromatids are bound by spindle fibers

Question 8

how is the cell brought back to G1 after mitosis?

Answer 8

1. drop in M-cdk activity leads to activation of Hct1-APC
2. ensures continued suppression of Cdk activity after mitosis, required for G1

Question 9

how does the cell cycle end and start again in G1?

Answer 9

1. cells exit mitosis and inactivate M-cdk
2. increase in Hct1 and Sic1 activity
3. inactivation of Cdk
4. when conditions are right, G1-Cdk and G1/S-Cdk activity increase
5. that inhibits Sic1 and Hct1 by phosphorylation
6. S-Cdk activity can increase

Question 10

how is the S-phase initiated with positive feedback in animal cells?

Answer 10

1. G1-Cdk activity phosphorylates Rb protein
2. Rb protein releases E2F, and now E2F is active
3. this leads to the activation of transcription of S phase genes
4. more G1/S cyclin (cyclin E and cyclin A)
5. this further enhances Rb phosphorylation
6. E2F stimulates the transcription of its own gene, positive feedback loop
7. activate S-Cdk leads to DNA synthesis