Lecture 24: Microtubules in Mitosis

What happens during interphase?

• chromosome duplication and cohesion

• centrosome duplication

What happens during prophase?

• breakdown of interphase MT array and replacement of miotic asters

• mitotic aster separation

• chromosome condensation

• kinetochore assembly

• duplicate centrioles

What happens during prometaphase?

• nuclear envelope breaks down

• chromosomes are captured and brought to spindle equator

What happens during metaphase?

• chromosomes align at the metaphase plate

True or False: Mitosis is the only time that chromosomes can be seen under a microscope.

true

What chromatic structures are seen in interphase/prophase?

• short region of DNA double helix

• “beads on a string”/10 nm structure

• 30-nm fiber

• 300 nm structure

What chromatic structures are seen in mitosis?

• 700 nm structure

• fully packaged chromosomes

What is the size of the average nucleus?

10 um

What is the average DNA diameter?

2 nm

What is the average DNA length?

2 meters

What happens during anaphase?

• APC/C is activated and cohesins are degraded

• Anaphase A: chromosomes move to poles

• Anaphase B: spindle pole separation

What happens during telophase?

• nuclear envelope assembly

• assembly of contractile ring

• chromosomes decondense

What happens during cytokinesis?

• reformation of interphase MT array

• contractile ring forms cleavage furrow

True or False: There are more MTs in mitosis than in interphase.

true

What leads to dynamic instability in mitosis?

MAPs and catastrophe factors

What is increased in centrosomes in mitosis?

gamma-TURC

What are the 3 classes of MTs?

• kinetochore

• polar

• astral

What is the zone of interdigitation?

the region where polar MTs overlap

Where do kinetochore MTs bind?

to centromeres

True or False: All 3 types of mitotic MTs are involved with moving chromosomes

true

Where are the plus ends of polar MTs?

towards the middle

What are the functions of astral MTs?

project away from the cell center and towards the cell membrane

What part of the kinetochore do MTs embed in?

the outer layer

What are the 2 types of chromosome movements?

• chromosomes move to the middle

• chromosomes move to the poles

What happens when there’s proper attachment to spindles?

tension and MTs are stabilized at kinetochore

What happens when there’s improper attachment?

no tension and MTs are destabilized and released

How can the cell determine if the MTs are properly attached?

MTs begin pulling the sister chromatids right away—generating tension between sister chromatids

What happens when there’s no tension on the kinetochore?

Auora B phosphorylates proteins in kinetochore

What happens when kinetochores are under tension?

no phosphorylation by Auroa B kinase

What forces act on chromosomes during congression?

• kinetochores pull towards the spindle fibers

• kinesins pull toward middle of the cell

True or False: Kinesins are turned off in anaphase, but kinetochores are highly active.

true

True or Falses: Motors bind chromosomes at sites other than kinetochores.

true

What helps position spindle poles?

the metaphase plate/zone of interdigitation

Is there spindle assembly in the absence of centrosomes?

yes, the action of motor proteins on MTs is what arranges MTs properly

What happens if there’s no spindle poles in a cell?

MTs will arrange into a spindle-like structure due to the motor proteins

How is the nucleo-cytoplasmic trafficking regulated?

through Ran

What does Ran-GTP do?

• signals MAPs locally

• stabilizes MTs

• promotes assembly of MTs

• facilitates search and capture

• breaks down MTs not heading towards chromosomes

What is there a high concentration of when Ran-GEF is bound to chromosomes?

high conc. of Ran-GTP around chromosomes

Where is Ran-GAP and Ran-GEF found?

cytoplasm and chromosomes, respectively

What defines the metaphase plate?

the concentration of Ran-GTP near the chromosomes

When do kinetochore MTs treadmill?

during prophase, prometaphase, and metaphase

What do MTs do during anaphase?

will de-polymerize from both ends

What does the energy of de-polymerization in anaphase contribute to?

the movement of chromosomes in anaphase A

What did Speckle Microscopy show?

kinetochore MTs treadmill

Why is it puzzling that kinetochore MTs treadmill towards the minus end?

both the plus and minus ends are capped

What occurs during anaphase A?

• shortening of kinetochore MTs (depolymerization)

• movement of daughter chromosomes to poles

• forces generated mainly at kinetochores

Where is there a net movement to prior to anaphase?

to the center (plus end)

What do dyneins do before anaphase?

pulls on chromosomes

What end shrinks during anaphase?

the plus end

What harnesses the energy of depolymerization?

DAM1

True or False: DAM1 hovers around the MTs and does not make direct contact.

true

What happens when MTs depolymerize?

the DAM1 rings will slide together

True or False: Chromosomes will segregate normally if only depolymerization (and not DAM1) is present.

true

How can the kinetochore hold onto the end and also allow treadmilling to occur?

the DAM1 ring holds onto near the depolymerization region

Where is the end of MTs between?

the inner and outer layers of the kinetochore

What occurs during anaphase B?

• sliding force is generated between interpolar MTs from opposite poles to push the poles apart

• interpolar MTs also elongate

• pulling force acts directly on poles to move them apart

• MT grow at plus end of polar MTs

What are the motors acting in anaphase B?

• dyneins anchor to membrane and pull astral MTs to the cell’s outer edge

• kinesins push spindles farther apart