Mitosis

Question 1

Explain Prophase ?

Answer 1

At Prophase, the replicated chromosomes, each consisting of two closely associated sister chromatids, condense. Outside the nucleus, the mitotic spindle assembles between the two centrosomes, which have begun to move apart. For simplicity, only three chromosomes are drawn

Question 2

Explain Prometaphase ?

Answer 2

Prometaphase starts abruptly with the breakdown of the nuclear envelope. Chromosomes can now attach to spindle microtubules via their kinetochores and undergo active movement

Question 3

Explain Metaphase ?

Answer 3

At metaphase, the chromosomes are aligned at the equator of the spindle, midway between the spindle poles. The paired kinetochore microtubules on each chromosome attach to opposite poles of the spindle

Question 4

Explain Anaphase ?

Answer 4

At anaphase, the sister chromatids synchronously separate, and each is pulled slowly toward the spindle pole it is attached to. The kinetochore microtubules get shorter, and the spindle poles also move apart, both contributing to chromosomes segregating

Question 5

Explain Telophase ?

Answer 5

During telophase, the two sets of chromosomes arrive at the poles of the spindle. A new nuclear envelope reassembles around each set, completing the formation of two nuclei and marking the end of mitosis. The division of the cytoplasm begins with the assembly of the contractile ring

Question 6

Explain Cytokinesis ?

Answer 6

During cytokinesis of an animal cell, the cytoplasm is divided in two by a contractile ring of actin and myosin filaments, which pinches in the cell to create two daughter, each with one nucleus

Question 7

Mitosis Promoting Factor (MPF) controls ?

Answer 7

Entry into mitosis by phosphorylation of key targets

Question 8

Explain what MPF does?

Answer 8

• Allows entry into Mitosis (M) from the G2 phase
  1. compaction of chromosomes
  2. assembly of the mitotic spindle
  3. nuclear envelope breakdown (Lamins and nuclear pore proteins)
  4. activation of the APC (anaphase promoting complex) - targets proteins for destruction
• Cohesion between sister chromatids must be removed: must degrade “glue” for metaphase to anaphase transition
• Degradation of cyclin is essential to exit mitosis and keep cell cycle moving forward

Question 9

Why is Cohesion and Condensin complexes required ?

Answer 9

For higher order chromosome structure

Question 10

What does Kinetochores attach ?

Answer 10

Kinetochores attach chromatids to microtubules

Question 11

Explain Spindle Assembly Checkpoint ?

Answer 11

• Monitors whether cells assemble their chromosomes and spindles correctly.
  Monitors:
  1. Occupancy of kinetochores
  2. Tension

-Ensures that all chromosomes are lined up on metaphase plate and attached to microtubules to both poles before anaphase is triggered.

Question 12

What do unattached chromosomes delay ?

Answer 12

Anaphase

Question 13

What does MAD proteins at unattached kinetochores inhibit ?

Answer 13

Seperation and APC

Question 14

What does microtubules separate ?

Answer 14

Sister chromatids at anaphase

Question 15

What does APC promote?

Answer 15

The destruction of cyclin and mitotic exit

Question 16

Briefly explain what happens during Telophase ?

Answer 16

• Chromosomes arrive at poles and start to decondense

- Nuclear envelope reassembles

Question 17

Cleavage furrow formed by ?

Answer 17

Contractile ring under plasma membrane

Question 18

Segregation of chromosomes at mitosis initiated by MPF requires:

Answer 18

• Compaction of the chromosomes
• Reorganisation of the interphase
microtubules to form a spindle
• Nuclear envelope breakdown
• Attachment of the chromosomes to microtubules
• Alignment on the metaphase plate and then segregation at anaphase
• Chromosome segregation and mitotic exit is orchestrated by sequential degradation of proteins mediated by the APC
• The spindle checkpoint ensures accurate chromosome segregation.