Mitosis from Rich Notes

Question 1

What is the cell division phase where the nucleus and cytoplasm are divided into two daughter cells?

Answer 1

Mitosis

Question 2

What is the last checkpoint that the cell must pass before it can go into mitosis? What does it confirm?

Answer 2

G2; DNA is completely replicated and that there’s no DNA damage

Question 3

Entry into mitosis is regulated by what factor?

Answer 3

MPF

Question 4

MPF is a complex of what?

Answer 4

Cyclin B and Cdk1

Question 5

How is MPF activated?

Answer 5

During interphase, cyclin B made and complexes w/ Cdk1, CAK primes Cdk1 w/ P on Thr 161, activates Cdk1, at same time - Wee1 kinase adds P to Tyr 15 on Cdk1, this inhibits Cdk1 until cell ready for division; once G2 cleared, cdc25 phosphatase takes off P of Cdk1, MPF now active and adds P to substrates in mitosis

Question 6

What are the key targets for MPF phosphorylation?

Hint: 3 processes and 5 types of proteins

Answer 6

chromosome condensation - condensins and cohesins

spindle formation - microtubule associated and motor proteins

nuclear envelope breakdown - lamins

Question 7

During prophase, what do the centrosomes and microtubules do?

Answer 7

Centrosomes move to opposite sides of nucleus.

Microtubules grows out from centrosomes to form spindles.

Question 8

What two protein complexes are responsible for maintaining chromosome stability during mitosis?

Answer 8

cohesins and condensins

Question 9

During S phase, the cohesin complex binds to what?

Answer 9

newly made sister chromatids

Question 10

What phosphorylates cohesins during prophase along the arms of the chromosomes except at the centromere? What does this allow the chromosome arms to do?

Answer 10

MPF; separate and be compacted by condensins

Question 11

What triggers condensin import into the cell? What energy source does condensin use to compact the chromatin? What does the compaction prevent?

Answer 11

phosphorylation of accessory proteins by MPF; ATP hydrolysis; chromosomes from being tangled and broken, RNA synthesis

Question 12

Phosphorylation of the nuclear lamins by MPF causes the nuclear lamina to disassemble and the membrane to fragment into vesicles. What lamin remains with the vesicles? What lamins disperse into the cytoplasm along with nuclear pore complexes?

Answer 12

B; A and C

Question 13

Generally speaking, what happens during prometaphase?

Answer 13

Spindle formation continues and chromosomes attach to the spindle through the kinetochore.

Question 14

MPF phosphorylates what proteins during prometaphase to regulate the balance between assembly and disassembly of the microtubules?

Answer 14

MAPs and motor proteins

Question 15

What are the four types of microtubules in the mitotic spindle? What do they each do?

Answer 15

> astral - orient spindle and separate it during anaphase
kinetochore - move chr. from metaphase plate to spindle pole through attachment at kinetochore
chromosomal - help position chr. at metaphase plate
polar - help separate the two poles

Question 16

What drug stabilizes microtubules, preventing disassembly? (It interferes w/ the ability of the microtubules to align the chr. on the metaphase plate and to move the chr. to the spindle poles.)

Answer 16

paclitaxil (taxol); used as chemotherapy agent to prevent proliferation of cancer cells

Question 17

What is a specific region of DNA that contains repetitive sequences but also binds to CENP that in turn binds kinetochore proteins?

Answer 17

centromere

Question 18

What binds to either side of the centromere and forms attachment site for the mitotic spindle?

Answer 18

kinetochore; some of its proteins are involved in the spindle checkpoint

Question 19

Chromosomes are aligned on the metaphase plate. To ensure that each daughter cell gets one copy of each chromosome, the cell has to pass the spindle checkpoint. What happens during the spindle checkpoint?

Answer 19

CENP-E attaches to both mt’s and BubR1 on the kinetochore, CENP-E senses the tension; BubR1 regulates Mad1p –> act. Mad2p –> inhibits APC through prevention of Cdc20 binding –> prevents mitosis; bipolar orientation achieved, CENP-E inact. BubR1–> Cdc20 binds APC, mitosis proceeds to anaphase

Question 20

What happens during anaphase A? What happens during anaphase B?

Answer 20

A: chromosomes separate through disassembly of kinetochore microtubules and move to opposite poles; motor proteins also involved
B: spindle poles move apart: polar microtubules elongate and kinesins push the poles apart, astral mt’s via dyneins at the cell cortex pull the poles toward the periphery

Question 21

What enzyme does the anaphase promoting complex contain? When the APC is activated, what two proteins does it target for degradation by the proteosome?

Answer 21

E3 ubiquitin ligase

securin and cyclin B

Question 22

When securin is destroyed, what happens?

Answer 22

separase is activated and degrades cohesins; chromosomes are free to move to opposite poles in anaphase

Question 23

When Cyclin B is destroyed from targeting by APC, what happens?

Answer 23

Cdk1 is inactivated and exit from mitosis is triggered

Question 24

Does it matter if securin or cyclin B is degraded first?

Answer 24

yes; securin has to be degraded first; cyclin B is then destroyed, which reverses all of the phosphorylation from prophase and prepares cell to divide

Question 25

During telophase, what initiates the reassembly of the nuclear membrane and decondensation of the chromosomes by dephosphorylating MPF substrates?

Answer 25

phosphatases

Question 26

What happens when the nuclear membrane reforms during telophase?

Answer 26

Vesicles form around the chromosomes, the vesicles fuse to form the nuclear membrane, the nuclear pores reassemble, and lamins A and C are imported from the cytoplasm to complete the nuclear lamina

Question 27

Once chromosomes are surrounded by the nuclear membrane, the chromosomes decondense. Then what happens?

Answer 27

DNA returns to its interphase conformation and transcription resumes

Question 28

What is the contractile ring in cytokinesis made of? How is the ring regulated?

Answer 28

actin, myosin, and some linker proteins that attach the ring to the membrane; phosphorylation of myosin

Question 29

What is kept in an activated state by MPF to prevent contraction of the ring in cytokinesis before the proper time?

Answer 29

myosin light chain phosphatase (keeps myosin dephosphorylated)

Question 30

What is needed to activate myosin light chain kinase which phosphorylates myosin which makes the ring contract in cytokinesis?

Answer 30

increase in Ca2+

Question 31

What is the part of the cell cycle that is NOT mitosis? This phase is subdivided into G1, S, and G2 phases

Answer 31

Interphase