Lecture 32 - Cell Cycle

Question 1

Overview of the cell cycle

Answer 1

• in G1 phase, the cell synthesizes all of the proteins, ribosomes, and other machinery it will need
• in S phase DNA is replicated
• G2 is a gap phase
• in M2, the cell divides

Question 2

S and M phase

Answer 2

• chromosomes duplicate in S phase
• duplicated chromosomes are segregated into a pair of daughter nuclei in M phase
  
  • nuclear division (mitosis) and cell division (cytokinesis) are collectively called M phase

Question 3

Why were temperature sensitive mutants in yeast the key to understanding the cell cycle?

Answer 3

• showed that certain proteins are important to control transition from 1 stage to another
• cdc mutants arrest before cytokinesis => cdc is important for cytokinesis

Question 4

What was the evidence to show that cell-cycle control is similar in all eukaryotes?

Answer 4

• if we complement the yeast cdc mutant with the human gene, cell cycle functions as normal
• 1st human/ yeast complementation
• also called rescue experiments (cell recovers to normal size

Question 5

The cell-cycle control system is based on what?

Answer 5

• cyclically activated protein kinases
• when cyclin forms a complex with Cdk (cyclin-dependent kinase), the protein kinase is activated to trigger specific cell-cycle events
• without cyclin, Cdk is inactive

Question 6

How are different events/ stages of the cell cycle triggered?

Answer 6

by Cdk associating successively with different cyclins

Question 7

How is Cdk activity terminated?

Answer 7

by cyclin degradation

Question 8

What are the 3 main classes of cyclin that are required in eukaryotic cells?

Answer 8

1. G1/S cyclins: activate Cdks at the end of G1 and commit the cell to DNA replication
2. S cyclins: activate Cdks during S phase and are required for the initiation of DNA replication
3. M cyclins: promote the events of mitosis

Question 9

What is the fourth cyclin?

Answer 9

G1-cyclin => controls start of G1
- not always there

Question 10

Do cyclin-Cdk complexes phosphorylate different sets of substrate proteins?

Answer 10

yes
- M-cyclin/Cdk phosphorylates different proteins than S -cyclin/Cdk

Question 11

What does phosphorylation of the cyclin/Cdk complex do?

Answer 11

adds another layer of control
- Wee1 kinase adds a phosphate => inactivates complex
- Cdc25 phosphatase removes a phosphate => activates complex

• a wee mutant (i.e., no wee) results in small cells that transition into mitosis quick
• a Cdc25 mutant results in elongated cells because cell never knows its time to undergo mitosis (i.e., keeps growing but doesn’t divide)

Question 12

What triggers the final stages of cell division?

Answer 12

protein degradation; anaphase promoting complex (APC) promotes degradation of M-cyclin

Question 13

Anaphase promoting complex (APC)

Answer 13

• activated during metaphase and promotes degradation M-cyclin
• APC polyubiquitinates M-cyclin to mark it for protein degradation

Question 14

What is the central problem for a cell in M phase?

Answer 14

to segregate and separate its chromosomes

Question 15

What does each replicated chromosome consist of at the end of S phase?

Answer 15

• a pair of identical sister chromatids glued together by a centromere

Question 16

What molecule acts like a ring to hold the sister chromatids together?

Answer 16

• cohesin
• cohesin is loaded into unreplicated chromosomes and then encircles the new sister chromatids

Question 17

What happens during prophase?

Answer 17

• the replicated chromosomes condense
• mitotic spindle starts assembling outside the nucleus
• centrosomes start separating to break the nuclear envelope

Question 18

Prometaphase

Answer 18

• nuclear envelope breaks
• without envelope, microtubules attach to the chromosomes at kinetochore
• one kinetochore is attached to one of the chromatids (kinetochore is like the glue between the microtubules and chromosomes)

Question 19

Where are kinetochore microtubules attached?

Answer 19

• to sister chromatids by their plus ends
• kinetochore proteins interact with centromeric chromatin

Question 20

Metaphase

Answer 20

chromosomes align to center of spindle (i.e., cell)

Question 21

What is the mitotic spindle made of?

Answer 21

• microtubules and their associated proteins (astral and kinetochore microtubules)
• non-kinetochore microtubules do not connect to chromatids, but connect entire structure together

Question 22

What do microtubules do?

Answer 22

pull the daughter chromosomes towards the poles of the spindle (i.e., to opposite ends of the cell)

Question 23

How do motor proteins contribute to the assembly and function of the mitotic spindle?

Answer 23

• kinesin-5 slides microtubules oriented in opposite directions
• kinesin-14 crosslinks antiparallel microtubules at the center and moves one of them
• kinesin-4/10 are plus directed and push chromosomes to center
• dynein pulls microtubules back to centrosome (i.e., pulls from edge of cell towards center)

Question 24

What happens during anaphase?

Answer 24

• sister chromatids are separated into 2 chromosomes
• shortening of microtubules (i.e., catastrophe) brings the chromosome to poles
• spindles also move apart

Question 25

Metaphase to anaphase transition (how does the cell know the chromosomes are properly aligned and ready to be separated?)

Answer 25

• Aurora B is a kinase force sensor that tells the cells the chromosomes are oriented properly
  
  • low tension => aurora B is active
  • high tension => aurora B is inactive

Question 26

What does Aurora B do?

Answer 26

• it prevents the stable attachment of microtubules to chromosomes
• when there is tension , Aurora B is inactive
• when there is high tension (from both sides), Aurora B cannot reach target to phosphorylate

Question 27

Separase

Answer 27

• a cysteine protease responsible for triggering anaphase by hydrolyzing cohesin
• indirectly activated by anaphase promoting complex (APC)

Question 28

Telophase

Answer 28

• chromosomes arrive to poles
• new nuclear envelope assembles
• mitosis ends and cytokinesis starts

Question 29

How does cytokinesis separate the two cells?

Answer 29

by using a contractile ring made of actin and mysosin (microtubules have NOTHING to do with cytokinesis)

Question 30

Centrosome vs centriole vs centromere

Answer 30

centrosome: an organelle that serves as the organizing centre of all microtubules in an animal cell => regulates cell cycle

centriole: paired barrel-shaped organelles located in the cytoplasm of animal cells near the nuclear envelope => microtubules arranged in a certain way

centromere: the central region of the chromosome which consists of highly constricted DNA