Lecture 12: Mitosis

Question 1

What are the two parts of the M phase

Answer 1

1) Mitosis - the precise and equal segregation of the duplicated chromosomes by the microtubule-based mitotic spindle, resulting in two nuclei

2) Cytokinesis - the division of the cytoplasm in two halves to generate two cells4

Question 2

events that set the stage (during interphase) for M-phase

Answer 2

1. genome is replicated (duplicated) by DNA polymerase
2. as part of the replication process, cohesin proteins bind to the genome and form rings that hold the duplicated “sister chromatids” together
3. sister chromatids are two exact copies of the same chromosome. they each consist of DNA and histone proteins (chromatin)

Question 3

what is the centrosome?

Answer 3

location of the gamma-tubulin complexes that nucleate microtubules; during S & G2 phases, it is replicated

Question 4

what are the 5 stages of mitosis

Answer 4

1. prophase
2. prometaphase
3. metaphase
4. anaphase
5. telophase

Question 5

what happens during prophase

Answer 5

mitotic spindle begins to form outside the nucleus and chromosomes condense

kinetochore also assembles

Question 6

condensin purpose

Answer 6

condensin: form rings that condense chromosomes to facilitate segregation during mitosis; enables the chromatin to form loops, resulting in compaction

cohesin: bind to the genome and form rings that hold the duplicated “sister chromatids” together

Question 7

what is the function of the mitotic spindle

Answer 7

the mitotic spindle is the protein machinery that segregates sister chromatids into separate nuclei

made of microtubules, motors, and two centrosomes

has a built-in quality control system (the spindle-assembly checkpoint)

Question 8

microtubules review

Answer 8

plus end = favored for growth
minus end = favored for disassembly

Question 9

what are the three classes of micotubules contained within the mitotic spindle?

Answer 9

1. astral microtubules
2. kinetochore microtubules
3. interpolar microtubules

each centrosome in the mitotic spindle is called a “spindle pole”

each spindle pole with its collection of microtubules is called an “aster”

Question 10

what happens during prometaphase

Answer 10

the nuclear envelope breaks down and microtubules begin attaching to chromosomes

phosphorylation of nuclear lamins (intermediate filament proteins) and nuclear pore proteins, causes the nuclear envelope (double membrane) to break down into vesicles

microtubules begin attaching to kinetochores

Question 11

what is a kinetochore

Answer 11

a protein complex that links microtubules to the centromere of each chromatid

kinetochore links the centromere to kinetochore microtubules

Question 12

what happens during metaphase?

Answer 12

chromosomes are arranged on the “metaphase plate” (midway between spindle poles)

Question 13

what does tension create during metaphase?

Answer 13

a stable bi-polar attachment

only the correct bipolar (“bi-oriented”) attachment has tension across the kinetochores

Question 14

how does the spindle-assembly checkpoint function during metaphase?

Answer 14

1. when properly connected to both spindle poles, the kinetochore microtubules pull on the kinetochores microtubules pull on the kinetochores - cohesins hold the chromosomes together
2. any unattached kinteochore sends out a signal preventing exit from metaphase by preventing activation of APC/C
3. when all kinteochores are attached to appropriate icrotubules, this satisfies the checkpoint, which leads to the activation of the APC/C (Anaphase Promoting Complex)
4. Activation of APC/C leads to the degradation of M cyclin, thereby inactivating M-Cdk + degradation of securin, allowing separase to degrade the cohesins holding the chromosomes together, initiating the start of anaphase

Question 15

what hapens during anaphase?

Answer 15

anaphase A: chromosomes are pulled poleward

anaphase B: poles are pushed and pulled apart

Question 16

anaphase A

Answer 16

shortening of kinetochore microtubules: forces are generated at kinetochores to move chromosomes towarf their spindle pole

microtubule depolymerization at the kinetochore pulls the chromosome to the pole

Question 17

anaphase B

Answer 17

a sliding force (1) is generated between interpolar microtubules from opposite poles to push the poles apart; a pulling force (2) acts directly on the poles to move them apart

Question 18

what happens during telophase?

Answer 18

nuclear envelopes reassmble and cytokinesis begins

nuclear envelopes form around each set of chromosmes to produce two nuclei; cytokinesis is intiated (in animal cells the contractile ring begins to form)

Question 19

what happens during cytokinesis?

Answer 19

the cell completes the division process by separating the cytoplasm

in animals: contraction of the actin based contractile ring

in plant cells: secretory trafficking to the “phragmoplast” - same as animals except the cell wall must also separate