Mechanisms of Anaphase and Telophase Lecture 41 BSCI

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. 2. 3. 4. Metaphase-to-Anaphase Transition Anaphase Mechanisms Telophase

1. 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

2 nd 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 tension ? (the anaphase inhibitor)

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 Mad 2 (Mitotic arrest defective) shown by red fluorescent anti-Mad 2. (below) Mad 2 inhibits activation of APC only when attached to k-cores. Hypothesis: Changes in kinetochore proteins under tension activate a protein kinase that phosphorylates APC so Cdc 20 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 by phosphatases 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