slide set 18

Question 1

active Cdk1/cyclin B drives…

Answer 1

drastic changes in mitosis to segregate the replication genome and produce two daughter cells

Question 2

As DNA is replicated in S phase, copies are held together with…

Answer 2

cohesion proteins
hold sister chromatids together in S phase to M phase

Question 3

Errors in mitosis or meiosis

Answer 3

have profound consequences
several successful chemotherapies block mitosis

Question 4

Taxol (paclitaxel)

Answer 4

Stabilizes microtubule filaments by binding along them and inhibits mitosis

Question 5

Prophase

Answer 5

1. replicated chromosomes condense (pack together tightly)
2. mitotic spindle assembles between 2 centrosomes (outside the nucleus)
   
   1. 2 centrosomes have replicated and moved apart

nucleus and cytoplasm are still separate

Question 6

In prophase, the chromosomes condense

Answer 6

• condensin protein complexes catalyze restructuring and compaction of chromosomal DNA
• condensin complexes resemble cohesion, which holds sister chromatids together
• It is not clear how it functions, but the complex is through to encase loops of DNA

Question 7

replicated chromosome is made up of

Answer 7

2 closely associated sister chromatids

Question 8

Prometaphase

Answer 8

1. abrupt breakdown of nuclear envelope
2. chromosomes can attach to spindle microtubules via their kinetochores and undergo active movement

there is no longer a distinction between nucleus and cytoplasm

Question 9

mitotic spindle

Answer 9

2 centrosomes formed at opposite sides of the cell

composed of 2 spindle poles

Question 10

Metaphase

Answer 10

Brief!

• chromosomes are aligned at equator of spindle, midway between spindle poles
• kinetochore microtubules attach sister chromatids to opposite poles of the spindle

Question 11

Anaphase

Answer 11

• sister chromatids synchronously separate to form 2 daughter chromosomes
  
  • each is pulled slowly toward the spindle pole it faces
• Kinetochore microtubules gets shorter, spindle poles move apart

Both processes contribute to chromosome segregation

Question 12

Telophase

Answer 12

• 2 sets of daughter chromosomes arrive at poles of spindle and decondense
• New nuclear envelope reassembles around each set, completing formation of 2 nucleu and ending mitosis
• division of cytoplasm begins with contraction of contractile ring
• contractile ring: actin myosin ring is just under cell membrane
  
  • starts to pinch cells together

Question 13

contractile ring

Answer 13

contractile ring: actin myosin ring is just under cell membrane

starts to pinch cells together

Question 14

Cytokinesis

Answer 14

• cytoplasm is divided in 2 by a contractile ring of actin and myosin filaments
  
  • contractile ring pinches the cell in 2 to create 2 daughter cells, each with 1 nucleus

Question 15

spindle structure at metaphase

Answer 15

sister chromatids are held together and connected to spindle poles on microtubules

centrosomes have gamma tubulin nucleating complexes

centrosome nucleates a lot of microtubules: plus ends face out

plus ends project away from spindle pole

minus ends anchored at spindle poles

Question 16

different types of microtubules

Answer 16

differ in where they are and what they bind to

structure is always the same! (we learned this last exam)

Question 17

Centrosomes

Answer 17

nucleate MTs

the minus MT ends are focused at the spindle pole

Question 18

kinetochore

Answer 18

protein complex that links together microtubules and chromatid

Question 19

Centrioles replicate in ___ phase.

Answer 19

centrioles replicate in S phase (at same time that DNA is replicated)

• As cell moves from S phase, through G2 and into M phase, the pericentriolar material (including gamma tubulin ring complexes) expands
• More gamma tubulin ring complexes at the centrosome lead to greater ability to nucleate microtubules

Question 20

kinetochores are…

Answer 20

kinetochores are protein complexes that link the sister chromatids to microtubules

Question 21

kinetochore structure

Answer 21

kinetochore layers are composed of many proteins

Question 22

kinetochores attach to…

Answer 22

microtubule plus ends

• proteins extend out of the kinetochore that hold on to the plus ends of the microtubule
• there is a gap in space between the chromatid and microtubule, which are linked by the kinetochore
• end-on attachment: the microtubule end is attached to the kinetochore

Question 23

MT dynamic instability

Answer 23

switches at mitotic entry to promote more rapid MT turnover

• MT are shorter during mitosis and more frequently switch between growing and shortening
• centrosomes mature and nucleate ~5x more MTs from each spindle pole
• These changes are driven by active Cdk1/cyclin B complexes phosphorylating target proteins

Question 24

Cdk1/cyclinB (directly or indirectly) targets…

Answer 24

microtubule-associated proteins and motors

• Cdk1/cyclin B, when active, can phosphorylate target proteins to turn them on or off
• The net effect is a dramatic shift in microtubule dynamics during mitosis

Question 25

Rapid MT dynamics from oppostie centrosomes allows:

Answer 25

MTs to connect to chromosomes

Question 26

MT dynamic stability to search space

Answer 26

kinetochores capture dynamic MTs at plus ends and MTs become stable

(A) In late prophase of most animal cells, the mitotic spindle poles have moved to opposite sides of the nuclear envelope, with an array of overlapping microtubules between them. (B) Following nuclear envelope breakdown, the sister-chromatid pairs are exposed to the large number of dynamic plus ends of microtubules radiating from the spindle poles. In most cases, the kinetochores are first attached to the sides of these microtubules, while at the same time the arms of the chromosomes are pushed outward from the spindle interior, preventing the arms from blocking microtubule access to the kinetochores. (C) Eventually, the laterally-attached sister chromatids are arranged in a ring around the outside of the spindle. Most of the microtubules are concentrated in this ring, so that the spindle is relatively hollow inside. (D) Dynamic microtubule plus ends eventually encounter the kinetochores in an end-on orientation and are captured and stabilized. (E) Stable end-on attachment to both poles results in bi-orientation.