Mechanisms of Anaphase and Telophase

Lecture 41 BSCI 420,421,620 Dec 9, 2002

“They paved paradise and put up a parking lot”- Joni Mitchell

1. Metaphase 2. Metaphase-to-Anaphase Transition3. Anaphase Mechanisms4. Telophase

1. Metaphase

Metaphase may look static, but chromosomes are held at the

metaphase plate by a dynamic balance of forces on kinetochores.

Also, MTs are treadmilling, not tied up at ends, but held by motor

proteins. Ev. for treadmilling: 1. Movement of tubulin speckles

2nd evidence for treadmilling in metaphase MTs:Photoactivation of “caged” fluorescence on tubulin moves toward poles.

2. Metaphase-to-Anaphase Transition

Mitotic cells wait in metaphase until they pass the spindle-attachment checkpoint, which asks: Are all

kinetochores attached to the spindle and under tension? Evidence:a. Colchicine-treated mitotic cells – Chromosomes

condense but no spindle forms, so sister chromatids never separate.

b. When one chromosome is detached from the mitotic spindle,

all the other chromosomes wait until it re-attaches & moves to the midpoint of the spindle.

How is M -> A controlled?

Central is the Anaphase-Promoting Complex (APC), a ubiquitin

ligase that targets certain metaphase proteins for destruction,

Especially securin and cyclin B.

Anaphase trigger mechanism

Chromosomes wo (the anaphase inhibitor)tension

?

How do cells detect that all chromosomes are under equal tension? Nicklas et al at Duke discovered that only unattached kinetochores bind certain proteins. Eg Mad2 (Mitotic arrest defective) shown by red fluorescent anti-Mad2. (below) Mad2 inhibits activation of APC only when attached to k-cores.

Hypothesis: Changes in kinetochore proteins under tensionactivate a protein kinase that phosphorylates APC so Cdc20 binds and activates the anaphase promoting complex.

3. Anaphase Mechanisms. Two components of Anaphase:Movement of chroms to poles Movement of poles apart

Kinetochores move to poles by following depolymerizing (+)Ends of kinetochore MTs. Evidence:

Photobleached

(stays the same) (shortens)

Anaphase B – separation of the poles by MT sliding, Powered by kinesin like motors (+) end directed.

Degradation of M-cyclin and removal of phosphates byphosphatases allows TELOPHASE (reversal of the events of prophase) and CYTOKINESIS.

Destruction of M-cyclin is not the anaphase trigger, and is not required for anaphase.

But inactivation of M-Cdk is required for the events of telophase :

4. Telophase events

a. Decondensation of chromatin

b. Reformation of nuclear envelope

c. Breakdown of MA & formation of interphase MT array

d. Formation of the contractile ring for cytokinesis