The Cell Cycle #lec6 Flashcards
1
Q
Bipolar Spindle Assembly in Most Animal
A
2
Q
Cells Begins with Centrosome Duplication
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3
Q
A
4
Q
The self-organization mechanisms:
A
5
Q
A
6
Q
The other mechanisms depend on the ability of mitotic
A
7
Q
chromosomes to nucleate and stabilize microtubules and on
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8
Q
the ability of motor proteins to organize microtubules into a
A
9
Q
bipolar array.
A
10
Q
A
11
Q
These “self organization” mechanisms can produce a bipolar
A
12
Q
spindle even in cells lacking centrosomes.
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13
Q
A
14
Q
The centrosome cycle
A
15
Q
A
16
Q
There are interesting parallels between centrosome
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17
Q
duplication and chromosome duplication.
A
18
Q
A
19
Q
Both use a semiconservative mechanism of duplication
A
in
20
Q
which the two halves separate and serve as templates for
A
21
Q
construction of a new half.
A
22
Q
A
23
Q
Centrosomes
A
like chromosomes
24
Q
only once per cell cycle
A
to ensure that the cell enters mitosis
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with precisely two copies.
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An incorrect number of centrosomes can lead to defects in
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spindle assembly and thus errors in chromosome
29
segregation.
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The Complex Mechanisms that limit
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centrosome duplication to once and only
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once per cell cycle
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These mechanisms are reminiscent of those that
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restrict DNA replication to once per cell cycle.
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After duplication has occurred in S phase
passage
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through M phase is required for the “licensing” of
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centrosome duplication in the next cell cycle.
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Spindle Assembly in Animal Cells
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Requires Nuclear-Envelope Breakdown
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In centrosome containing cells
spindle assembly begins when
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the two centrosomes move apart along the nuclear envelope in
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prophase
pulled by dynein motor proteins that link astral
49
microtubules to the cell cortex.
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The plus ends of the microtubules between the centrosomes
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interdigitate to form an array of antiparallel microtubules
and
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kinesin-5 motor proteins cross-link these microtubules and
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push the centrosomes apart.
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Spindle Assembly in Animal Cells
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Requires Nuclear-Envelope Breakdown
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Centrosomes and microtubules are located in the cytoplasm
where
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they have no access to the sister-chromatid pairs inside the nucleus.
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In addition
many of the motor proteins and microtubule regulators that
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promote spindle assembly are associated with the chromosomes inside
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the nucleus.
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Thus
breakdown of the nuclear envelop is required.
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Nuclear-envelope breakdown is a complex
multistep process
70
thought to begin when M-Cdk phosphorylates several subunits of the
71
nuclear pore complexes in the nuclear envelope.
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Spindle Assembly in Animal Cells
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Requires Nuclear-Envelope Breakdown
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This phosphorylation initiates the disassembly of nuclear pore
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complexes and their dissociation from the envelope.
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M-Cdk also phosphorylates components of the nuclear lamina
the
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structural framework beneath the envelope
resulting in nuclear lamina
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disassembly.
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In parallel
phosphorylation of several inner-nuclear-envelope proteins
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leads to detachment of lamin proteins and chromosomes from the
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nuclear envelope
which is then incorporated into the membranes of the
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endoplasmic reticulum.
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Mitotic Chromosomes Promote Bipolar
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Spindle Assembly
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When they are present
centrosomes drive spindle assembly. However
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chromosomes are not just passive passengers in the process of spindle
94
assembly.
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By creating a local environment that favors both microtubule nucleation
97
and microtubule stabilization
chromosomes play an active part in
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spindle formation.
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A guanine nucleotide exchange factor (GEF) that is bound to chromatin
101
stimulates a small GTPase in the cytosol called Ran to bind GTP in
102
place of GDP.
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The activated Ran-GTP releases microtubule-regulatory proteins from
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protein complexes in the cytosol
thereby stimulating the local
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nucleation and stabilization of microtubules around chromosomes.
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Activation of the GTPase Ran around mitotic
109
chromosomes
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111
Mitotic Chromosomes Promote Bipolar
112
Spindle Assembly
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The best-understood mechanism depends on the release of a protein
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called TPX2
which then binds and activates the protein kinase Aurora-
117
A.
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Aurora-A phosphorylates regulatory proteins that activate γ-TuRCs
120
leading to an increase in local formation of new microtubules.
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TPX2 might also activate augmin to promote the formation of new
123
microtubule branches on the sides of existing microtubules.
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125
Local microtubule stabilization is also promoted by the protein kinase
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Aurora-B
which associates with mitotic chromosomes.
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Mitotic Chromosomes Promote Bipolar
129
Spindle Assembly
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131
In the absence of centrosomes
spindle assembly is thought
132
to begin with the formation of microtubules around the
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chromosomes.
134
135
Various microtubule-associated proteins then organize the
136
microtubules into a bipolar spindle
137
138
Spindle self-organization by motor proteins
139
140
Cells that normally lack centrosomes
such as those of
141
142
higher plants and any animal oocytes
use chromosome-
143
based self-organization mechanisms to form spindles.
144
145
These mechanisms also assemble spindles in experimental
146
systems where centrosomes are removed.
147
148
For example
in certain animal embryos that have been
149
induced to develop from eggs without fertilization (called
150
parthenogenesis).
151
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Mitotic Chromosomes Promote Bipolar
153
Spindle Assembly
154
155
Bipolar spindle assembly without centrosomes in
156
parthenogenetic embryos of the insect Sciara
157
158
Kinetochores Attach Sister Chromatids to
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160
the Spindle
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162
After the assembly of a bipolar microtubule array
the second major step in
163
spindle formation is the attachment of the array to the sister-chromatid
164
pairs.
165
166
Spindle microtubules become attached to each chromatid at its
167
kinetochore.
168
169
Kinetochores Attach Sister Chromatids to
170
171
the Spindle
172
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Microtubule attachment sites in the kinetochore
174
175
Kinetochore attachment to the spindle occurs
176
by a complex sequence of events.
177
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Chromosome attachment to the mitotic spindle in animal cells
179
180
Another attachment mechanism also plays a
181
part
particularly in the absence of
182
centrosomes.
183
184
Short microtubules in the vicinity of the chromosomes
185
become embedded in the plus-end binding sites of the
186
kinetochore.
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188
Polymerization at these plus ends then results in growth of the
189
microtubules away from the kinetochore.
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191
The minus ends of these kinetochore microtubules are
192
eventually cross-linked to other minus ends and focused by
193
motor proteins at the spindle pole.
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Bi-orientation Is Achieved by Trial and Error
196
197
The success of mitosis demands that sister chromatids in a pair attach
198
to opposite poles of the mitotic spindle
so that they move to opposite
199
ends of the cell when they separate in anaphase. This mode of
200
attachment is called bi-orientation.
201
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The sister kinetochores are constructed in a back-to-back orientation
203
that reduces the likelihood that both kinetochores can face the same
204
spindle pole.
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Nevertheless
incorrect attachments do occur
207
mechanisms have evolved to correct them.
208
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Incorrect attachments are corrected by a system of trial and error that is
210
based on a simple principle: incorrect attachments are highly unstable
211
and do not last
whereas correct attachments become locked in place.
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Alternative forms of kinetochore attachment to the spindle poles
214
Bi-orientation Is Achieved by Trial and Error
215
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Bi-orientation Is Achieved by Trial and Error
217
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The tension-sensing mechanism depends on the protein
219
kinase Aurora-B.
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Aurora-B is associated with the kinetochore and is thought to
222
generate the inhibitory signal that reduces the strength of
223
microtubule attachment in the absence of tension.
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It phosphorylates several components of the microtubule
226
attachment site
including the Ndc80 complex
227
site’s affinity for a microtubule plus end.
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229
When bi-orientation occurs
the resulting tension somehow
230
reduces phosphorylation by Aurora-B
thereby increasing the
231
affinity of the attachment site.
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How tension might increase microtubule attachment
234
to the kinetochore
