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!