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Lecture 22: Mechanisms of cell division
“Omni cellulae e cellulae…”
(all cells come from cells)Virchow circa 1855
Prokaryotes
Eukaryotes = M phasemitosis - separation of chromosomescytokinesis - splitting of the cytoplasm
Prokaryotes divide by septation and fission
Localized insertion of new membrane and cell wall components
Septum
Single chromosome attached to membrane…
Replicate DNA…
Daughter chromosomes attached to membrane…
Localized wall and membrane growth segregates daughter chromosomes…
Septum
Mitochondria and plastids divide by fission!
Eukaryotic cell grow and replicate DNA and organelles during “Interphase” (= G1 + S + G2)
Appearance:Nuclear envelope intact and chromatin dispersed (decondensed)
Interphase MT array
Key functions:Most cell growth and organelle biogenesis occurs during interphase
DNA replicates (in S)
Centrosomes duplicate
ECB Panel 19-1
Intact nuclear envelope
Centrosome and radial MT array
Dispersed chromatin
G1G1 MG2S
Animal cells inherit a single centrosome (centriole pair) from previous division
Centrosome duplication is initiated late in G1 by G1-S Cdk trigger
Parent centrioles separate (disjoin) and centrioles replicate in a template-dependent process during S phase
Daughter centrosomes function as spindle poles during M-phase
Animal cells must duplicate their centrosomes prior to division
Parent centrosome contains a pair of
centriolesDisjunction
Daughter centrioles
Centriole replication
2. “Prometaphase”(before the
middle)Nuclear envelope disassemblesChromosomes attach to spindle
3. “Metaphase”(the middle)
Chromosomes align at metaphase plate
5. “Telophase”(the end)
Nuclear envelope reassembles Cytokinesis begins
1. “Prophase”(the beginning)
Chromosomes condenseSpindle assembly begins
Overview of “mitosis” and “cytokinesis” in eukaryotes
Mitosis (nuclear division) consists of five phases:
Cytokinesis (cytoplasmic division)• Actin and myosin II in animal cells
• MT phragmoplast in plant cells
Division must partition all organelles (ribosomes, ER, Golgi, lysosomes, mitochondria, centrosomes, etc) and components to “daughters”…Organelles with high copy number (mito, chloro, ribosomes, ER, Golgi) segregate by mass action
Organelles with low copy number (chromosomes, centrosomes) use specific segregation mechanisms
4. “Anaphase”(after the middle)
Chromosomes segregate
19.2-animal_cell_division.mov
19.1-plant_cell_division.mov
Gr = MTsBl = DNA
Mitosis begins with “prophase”
Centrosome separation beginning
Nuclear envelope intact
Condensing chromosomes attached at centromeres
Duplicated centrosomes separating to act as spindle poles; driven by MT motors (kinesins)
Intact nuclear envelope
Centrosomes
Condensing chromosomes
Condensing chromosomes
Intact nuclear envelope
ECB Panel 19-1
ECB Panel 19-1
Chromosomes condense during prophase
Duplicated chromosomes consist of two “sister” chromatidsEach contains a single DNA molecule, 2 DNA molecules are identicalSister chromatids are linked at centromere by cohesin proteins(earlier, in S, cohesion along entire chromatid length)
Early prophase Metaphase chromosomeECB 19-3
Centromere
2 “sister chromatids” 2 “sister chromatids”
Cohesins
“Prometaphase” begins with nuclear envelope breakdown
M phase Cdk (MPF)-dependent phosphorylation of nuclear lamins induces disassembly of nuclear lamina and envelope breakdown
Chromosomes are “dumped” between poles of spindle
Chromosomes attach to “spindle” by “capturing”spindle MTs
Remnants of nuclear envelope
Chromosomes capture spindle MTs
ECB Panel 19-1
Spindle MTs are captured by kinetochores
Sister chromatids are linked at centromere by cohesin proteinsEach chromatid assembles a kinetochore at the centromereKinetochores capture + ends of MTs (1-40) to attach chromatids to opposite spindle polesSpindle MTs are 20X more dynamic than interphase MTs (stabilizing MAPs phosphorylated and removed: catastrophins)
Metaphase chromosomeECB19-3
Centromere
2 “sister chromatids 2 “sister chromatids
Cohesins
Kinetochore Inner plate Outer plate
Kinetochore MTs(1-40 KT MTS make “KT fiber”)
+ +
ECB 19-9
MTs attached to kinetochore
“Prometaphase” chromosomes are under tension
ECB panel 19-1
Spindle poles
Spindle poles
Chromosomes may initially capture MTs from a single pole
Eventually, chromosomes attach to MTs from both poles (sister chromatids to opposite poles)
In “congression,” chromosomes oscillate between spindle poles
Pulled and/or pushed?
MT assembly and disassembly or motors?
Both kinesins and dynein localized to “kinetochore”…
“Metaphase”Metaphase
plate
Chromosomes align at the “metaphase plate”: a plane ~equidistant from each spindle pole.
Alignment results from a balancing of forces from each spindle pole
MCB figure 19-35C
Metaphase plate
Pole
Pole
Pole
Pole
Metaphase plate
ECB Panel 19-1
Metaphase spindles in animal cells include 3 classes of MTs
Overlapping interpolar MTsSpindle pole (centrosome)
Astral MTs Kinetochore fiber
“Kinetichore fibers” (= 1-40 kinetochore MTs) link chromosomes to the poles…
Interpolar MTs extend from the spindle poles and interdigitate in central spindle; interacting MTs become stabilized
Dynamic “Astral MTs” extend radially from spindle poles…
Kinetochore
Sister chromatids Note that all MTs have plus-ends away from centrosome/spindle pole
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See ECB 19-7 and 19-13
19.4-mitotic_spindle.mov
MCB figure 19-35D
Chromosome segregation occurs during anaphase
Metaphase plate
Pole
Pole
Pole
Pole
Metaphase plate
Anaphase A: Chromatids separate and move towards poles at ~1 m/min-1
Anaphase B: Poles separate
ECB Panel 19-1
Sister chromatids separate in anaphase
Anaphase A and B may temporally overlap
Anaphase promoting complex (APC, last lecture) triggers degradation of cohesin proteins linking sister chromatids at centromere
Chromatids jump apart and then move more steadily
Astral MTs Kinetichore fiber
Sister chromatids
Sister chromatids move towards opposite spindle poles in Anaphase A and spindle poles separate in anaphase B
ECB 19-17
Multiple forces may contribute to chromosome movements in Anaphase A
Overlapping polar MTsMotorsSpindle pole (centrosome)
Astral MTs Kinetichore fiber
1. Shortening (disassembly) of MT at kinetochore may provide poleward force
2. Minus-end directed motor (dynein?) at kinetochore may provide poleward force
ECB 19-17
Both?
Multiple forces may contribute to pole separation during Anaphase B
Overlapping polar MTsMotors
Astral MTs
1. Pulling forces generated by astral MTs interacting with (attaching to) cell cortex
2. Motor-dependent sliding of anti-parallel polar MTs in spindle mid-zone (coupled with elongation of mid-zone MTs) Which motor do you guess?
3. Both #1 and #2
A “spindle checkpoint” monitors chromosome attachment
APCAPC
Extra chromosome
Missing chromosome
Metaphase
Cytokinesis
Anaphase
X X
Unattached kinetochore
Normal chromosome segregation
With checkpointIf there were no checkpoint
Delay provides time for normal attachment
Unattached kinetochore inhibits APC
Lamins are dephosphorylated
Nuclear envelope reformation begins around individual chromosomes which then fuse to form daughter nuclei
Interphase MT array reforms
Contractile ring at division furrow…
Cytokinesis in animal cellsNuclear envelope reforming around individual chromosomes… Contractile ring of
actin/myosin forming…
Daughter chromosomes reach separated poles
Nuclear position determines division plane but mechanism unclear
Actin filaments (red)
Myosin filaments (green)
ECB Panel 19-1
Cytokinesis begins as actomyosin contractile ring divides cytoplasm
Sliding of actin and myosin creates “purse-string” contraction
Summary of mitosis and cytokinesis in animal cells
1. ProphaseNuclear envelope intact…Chromosomes condense…Centrosomes separate…MTs VERY dynamic…
2. PrometaphaseNE disassembles…Kinetichores capture MTs…Chromosomes attach to spindle…Congression movements…
Interphase (G1-S-G2)Cell growth..Organelle biogenesis…DNA replication (S-phase)…Centrosome duplication…MTs dynamic…
3. MetaphaseChromosomes align at metaphase plate…
4. Anaphase Chromatids segregate…A: chromatids to poles…B: poles separate…
5. TelophaseNE re-assembles…Chromosomes decondense…Cytokinesis begins…
CytokinesisContractile ring of actin and myosin divides cell…Daughter cells enter G1…
Mitosis and cytokinesis in higher plants
During interphase, MTs are organized in circumferential bands
These guide cellulose deposition (next slide)
Spindle poles are less focused and spindles are barrel-shaped
Higher plants lack centrioles and conventional centrosomes
Centrioles lost when cell motility lost?
Before mitosis, a “pre-prophase band” of MTs marks the future division site
How it forms is unclear
And it disappears before mitosis!
Cell wall
Pre-prophase band of MTs
-TB, etc
ECB 19-22
Cell wall dictates direction of cell elongation in interphase plant cells
Cellulose in wall
Cell only grow (elongate) perpedicular to cellulose
Microtubule in cytoplasm
Microtubules in cytoplasm are colinear with cellulose in wall
Thought that MTs somehow control orientation of cellulose deposition
ECB 21-6
Microtubules may guide cellulose synthase movement in plane of membrane
Rigid cell wall requires a distinctive mode of cytokinesis in plants
Division is completed with fusion of the cell plate with the plasma membrane and cell wall
Circumferential MT array is re-assembled
Cell plate (membrane and nascent cell wall)
phragmoplastVesicles fuse to ends of growing
cell plate
During late mitosis, golgi vesicles containing cell wall precursors are transported towards the division site
Vesicle fusion initiates assembly of the “cell plate” (the division membrane and nascent wall)
As vesicles fuse, the cell plate grows outward towards the cortex
The “phragmoplast,” a ring of MTs surrounding the cell plate guides deposition of cell plate
ECB 19-22
A comparison of mitosis and cytokinesis in animals and plants
Centrosomes act as spindle poles Higher plant cells lack “classical” centrosomes…
“Anastral” spindle is barrel-shaped
Contractile ring of actin and myosin…
Plant cells surrounded by rigid wall
MT “phragmoplast” organizes growing cell plate
PlantsAnimals
Cyto
kinesi
sM
itosi
s
phragmoplast
A comparison of chromosome segregation in diverse taxa
Adapted from MBoC figure 18-40
There are many variations on the theme!