MCB 2210 Lecture Notes - Lecture 17: Spindle Apparatus, Sister Chromatids, Prophase
7 Nov 2016
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Prophase, Pro-Metaphase, and Metaphase
• Prophase
o Prophase begins when condensed chromosomes can be seen by
light microscopy
o Also in prophase, interphase MTs disassemble, the two
centrosomes separate, and each centrosome nucleates a radial
array of microtubules called an aster
o The two asters move apart to opposite sides of the nucleus to begin
forming the mitotic spindle
o The formations of the mitotic spindle will involve the organizational
activity of centrosomes and chromosomes
• Pro-Metaphase
o Pro-metaphase begins with the nuclear envelope breakdown
o The spindle microtubules can center the nuclear region and interact
with some chromosomes
o The other microtubules in the spindle (not attached to the
kinetochore) are called polar microtubules
o Those outsides the spindle are astral microtubules
o One kinetochore on one sister chromatid is first captured by
microtubules emanating from one spindle pole, and then the other
kinetochore is eventually captured by microtubules from the
opposite pole
o Chromosomes move back and forth between the poles
• Search-and-capture model for centrosome organized spindles
o Pro-metaphase microtubules initially exhibit dynamic instability
o The dynamic microtubules probe the space occupied by
chromosomes (search)
o By change, some microtubules encounter a kinetochore and bind to
it via kinesins and dyneins (capture)
o If the side of a microtubule binds to a kinetochore, (+) end directed
motors will move the chromosome to the end of the microtubule
where the kinetochore will interact with the (+) end allows the
microtubule to become stabilized
o Eventually, the kinetochores on sister chromatids will be captured
by microtubules from opposite spindle poles
o This causes them to become bi-oriented allowing stronger
attachment to Ndc80
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