MUNI
SCI
Cell division
RNDr. Jan Škoda, Ph.D.
Department of Experimental Biology
Bi1700en Cell Biology / 07 - Cell division (20 Apr 2022)
Outline
-Cell division in prokaryotes: binary fission -Cell division in eukaryotes
- Mitosis -Cytokinesis
- Meiosis
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Omnis cellula e cellula
BIOLOGY: THE ONLY SCIENCE WHERE MULTIPLICATION AND DIVISION MEAN THE SAME THING.
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Law of cell lineage: each cell (stems) from another cell
Cell division in prokaryotes:
binary fission
/ 07 - Cell division (20 Apr 2022)
Binary fission
1. Duplication of the chromosome
2. Cell growth/elongation
3. Formation of septum (cross wall)
4. Separation of daughter cells
distinct walls
Cell separation
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Fission mediated by FtsZ ring
FtsZ - microtubule homolog
Assembles between nucleoids
Directs formation of the septum
Recruits the downstream components of the divisome
W bands zones
Bi1700en Cell Biology / 07 - Cell division (20 Apr 2022)
Replication of the circular prokaryotic chromosome begins at the origin of replication and continues in both directions at once.
Origin of replication
Prokaryotes have a single, circular chromosome
FtsZ protein
The cell begins to elongate. FtsZ proteins migrate toward the midpoint of the cell.
(OCT)
The duplicated chromosomes separate and continue to move away from each other toward opposite ends of the cell. FtsZ proteins form a ring around the periphery of the midpoint between the chromosomes.
Cleavage furrow ■
• FtsZ ring
The FtsZ ring directs the formation of a septum that divides the cell. Plasma membrane and cell wall materials accumulate.
Septum
Septum
After the septum is complete, the cell pinches in two, forming two daughter cells. FtsZ is dispersed throughout the cytoplasm of the new cells.
FtsZ distribution during cell cycle
Slow growth
Replication initiation
/. * - AN
FtsZ       DnaA-ATP MreB
Fast growth
Replication initiation
In media that support slow growth, replication initiation and Z-ring assembly occur sequentially (top panel), whereas in media that support fast growth, FtsZ can form the Z-ring a few minutes after cell birth, potentially coinciding with replication initiation (bottom panel).
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Mechanisms of fission
-Gram-negative bacteria: membrane constriction & formation of septum -Gram-positive bacteria: FtsZ treadmilling & septal cell wall synthesis
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Inheritance of cell poles
- £. coll. symmetrically dividing
- Inheritance of old poles: allows asymmetric segregation
of proteins or other cellular components during cell divisions
- Aging in unicellular organisms: cells with exhibits a diminished growth rate, decreased offspring production, and an increased incidence of death
- Lineage survival: cells with new poles maintain fitness
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Bacterial transformation and conjugation
-Transformation:
incorporation of foreign DNA
-Conjugation: exchange of genetic material
(mobile plasmids, retained as plasmids)
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Cell division in eukaryotes
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/-\
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Chromatin organization during the cell cycle
INTERPHASE M PHASE INTERPHASE
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Duplication of genetic material: S-phase
INTERPHASE
telomere
replication origin
centromere —
replication bubble
MITOSIS
INTERPHASE
i r
f
portion of mitotic spindle
duplicated chromosomes In separate cells
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Chromatin organization
core histories linker DNA       of nucleosome '-1 \
"beads-on a-string" nucleosome includes form of chromatin      -200 nucleotide pairs of DNA
NUCLEASE DIGESTS LINKER DNA
released nucleosome core particle
I
Jí-
DISSOCIATION WITH HIGH CONCENTRATION OF SALT
octameric histone core 147-nucleotide-pair DNA double helix
A
DISSOCIATION
H2A H2B
- Nucleosomes -> „beads on a string" /11 nm fibre
-30 nm fibre (solenoid)
H3
H4
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Chromatin organization
Short region of DNA double helix
"Beads on a erring/ form of chromatin
30-nm chromatin fibre of packed nucleosomes
Section of chromosome in an extended form
Condensed section ofchrornosome
r r i L i r L- mitom chromosome
1 r4Q0 rtm
1 |xm
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Metaphase chromosome structure
- Monocentric chromosomes
lj
0.1 |xm
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Metaphase chromosome structure
- Monocentric chromosomes
TELOMERE SATELLITE
SECONOARY CONSTRICTION NUCLEOLAR ORGANIZER (NOR)
Sisters chromatids
Short arm
P(As)
Centromere
Long arm q(Ac)
Telomeres
PRIMARY CONSTRICTION
KIETOCHORE FOR ATTACHMENT OF SPINOLE FIBRES
/   %  t *
J S íí i
#1 9% 9l
%2* * \ '\
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Metaphase chromosome structure
- Monocentric chromosomes
Centromere
Chromatids
Primary Constriction
A
Satellite
Arm
Short arm
\
— Long arm
Secondary Constriction
A
Metacentric Submetacentric Acrocentric Telocentric
-Classified according the centromere position
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Monocentric versus holocentric chromosomes
Monocentric
Holocentric
-Monocentric: one kinetochore
Holocentric: multiple kinetochores
- Lack primary constriction
- Microtubules bind one chromatid at multiple sites (kinetochores)
■ Chromatids move in parallel (not V-shape) Present in some insects, plants, arachnids, and nematodes
- Further reading
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DNA content - cell cycle and division
Mitosis
Diploid
(2n) mother
cell
J
4c
DNA synthesis
/ \
r Division ^
2c
2c
L
Identical diploid (2n) daughter cells
2c
Meiosis
(T\ Diploid KZJ (2n) meiocyte
Division I
2c
L
1
1
Nonidentical haploid (n) cells
c = DNA mass of one (haploid) set of chromosomes when each chromosome is formed by one chromatid
n = haploid number -chromosomes in a gamete
- half of the usual complete sets of chromosomes in somatic cells
x= chromosome number - one complete set of chromosomes
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Mitosis
Bi1700en Cell Biology / 07 - Cell division (20 Apr 2022)
cytoplasm nucleus-(—
chromosome duplication
MITOSIS
CYTOKINESIS
Mitosis
Fundamental role
- Passing the identical genetic information to the next generation of cells
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Stages of mitosis
- Initiated after progression through the G2/M checkpoint
- Fully replicated undamaged DNA?, sufficient cell size?, (animal cells: duplicated centrosomes?)
- Prophase
- Prometaphase
- Metaphase vs. -Anaphase -Telophase
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II
Prophase
PROPHASE
centrosome
condensing replicated chromosome, consisting of two sister chromatids held together along their length
At prophase, the replicated chromosomes, each consisting of two closely associated sister chromatids, condense. Outside the nucleus, the mitotic spindle assembles between the two centrosomes, which have replicated and moved apart. For simplicity, only three chromosomes are shown. In diploid cells, there would be two copies of each chromosome present. In the photomicrograph, chromosomes are stained orange and microtubules are green.
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Promethaphase
PROMETAPHASE
centrosome at spindle pole
kinetochore microtubule
fragments of nuclear envelope
Prometaphase starts abruptly with the breakdown of the nuclear envelope. Chromosomes can now attach to spindle microtubules via their kinetochores and undergo active movement.
chromosome in active motion
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Nuclear envelope breakdown
DEPHOSPHORVLATION OF LAMINS
Progressive
phosphorylation of lamins
by active M-Cdk (cycline B-Cdk1; progression through the G2/M checkpoint)
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Mitotic spindle - three types of microtubules
spindle pole replicated
astral microtubules  kinetochore microtubules interpolar microtubules
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Metaphase
METAPHASE
centrosome at spindle pole
kinetochore microtubule
At metaphase, the chromosomes are aligned at the equator of the spindle, midway between the spindle poles. The kinetochore microtubules attach sister chromatids to opposite poles of the spindle.
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Anaphase
■ ANAPHASE
daughter chromosomes
shortening kinetochore microtubule
At anaphase, the sister chromatids synchronously separate to form two daughter chromosomes, and each is pulled slowly toward the spindle pole it faces. The kinetochore microtubules get shorter, and the spindle poles also move apart; both processes contribute to chromosome segregation.
spindle pole moving outward
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Anaphase: sister-chromatid separation
Cdc20
securin
inactive separase
T
inactive APC/C |
active APC /C
cohesin complex
M-Cdk
active separase
mitotic spindle
^2
metaphase
UBIQUITYLATION AND DEGRADATION OF SECURIN
I
cleaved and dissociated cohesins
anaphase
Separase cleaves cohesins
Allows sister-chromatid separation
Normally prevented by securin
Spindle checkpoint: securin is a target of APC/C ubiquitin ligase
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Anaphase: sister-chromatid separation
Anaphase: sister-chromatid separation
ANAPHASE A
shortening of kinetochore microtubules; movement of daughter chromosomes to poles; forces generated mainly at kinetochores
(1) a sliding force is generated between interpolar microtubules from opposite poles to push the poles apart; the interpolar microtubules also elongate;
(2) a pulling force acts directly on the poles to move them apart
microtubule growth at plus end of polar microtubules
Synergy between: -Anaphase A
- Kinetochore microtubules shortening
- Microtubule flux (depolymerization at both ends)
-Anaphase B
- Growth of interpolar microtubules from opposite poles interconnected by motor proteins
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Spindle microtubule flux
- Depolymerization of spindle microtubules can occur at both + & -
- Poorly understood, mediated by motor proteins
end
k i net o chore
O   ® q depolymerization O        of plus end of
kinetochore microtubules
í
c m
4-1
VI
TJ
\
time
(B) | (C)
Fluorescent tubulin time-lapse microscopy
■——— spindle pole
N • , TUBULIN ".* REMOVAL
TUBULIN ADDITION
.\« TUBULIN | " ADDITION
I — speckles moving ■ poleward
TUBULIN REMOVAL
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Telophase
TELOPHASE
set of daughter chromosomes at spindle pole
contractile ring starting to contract
overlap microtubules
centrosome
During telophase, the two sets of daughter chromosomes arrive at the poles of the spindle and decondense. A new nuclear envelope reassembles around each set, completing the formation of two nuclei and marking the end of mitosis. The division of the cytoplasm begins with contraction of the contractile ring.
nuclear envelope reassembling around individual chromosomes
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Nuclear envelope reassembly
DEPHOSPHORVLATION OF LAMINS
Dephosphorylation of lamins by protein phosphatases
- Protein phosphatase 1 (PP1), PP2A ■ Active due to the deactivation of Cdks (degradation of cyclins by APC/C)
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II
Microtubules / DNA
Bi1700en Cell Biology / 07 - Cell division (20 Apr 2022)
Cytokinesis
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Cytokinesis in an animal cell (zygote)
(mag X 30)
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Cytokinesis - animal cells
-Contractile ring at the original equatorial plane - Actin and myosin movement
i_i
25 |xm
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Cytokinesis - animal cells
6 CYTOKINESIS
contractile ring creating cleavage furrow
completed nuclear envelope surrounds decondensing chromosomes
During cytokinesis, the cytoplasm is divided in two by a contractile ring of actin and myosin filaments, which pinches the cell in two to create two daughters, each with one nucleus.
re-formation of interphase array of microtubules nucleated by the centrosome
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Cytokinesis - animal cells
- Midbody
- Tether between dividing cells
- After complete separation - signaling functions, cell polarization
10 Lim
dense matrix material plasma membrane
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Cytokinesis - plant cells
preprophase band of microtubules remains of interpolar cortical array of
and actin filaments spindle microtubules interphase microtubules
microtubules
Phragmoplast guides assembly of the cell plate (new cell wall)
GA-derived vesicles filled with polysaccharides and glycoproteins
Fusion - outward growth of the early cell plate
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Cytokinesis - plant cells
Wall of Cell plate Daughter
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Cytokinesis - centripetal vs. centrifugal
centifugal
Animal cells -Centripetal division
-Cleavage furrow
Plant cells
-Centrifugal division
- Central early cell plate growth
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Distribution of organelles into daughter cells
- Most organelles are not synthesized de novo
Recent studies: GA, ER, peroxisomes, lysosomes, vacuoles may form de novo - Mitochondria cannot be synthesized de novo
- Biosynthesis and fragmentation / fission of pre-existing organelles
■ Equal distribution during mitosis
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Meiosis
Bi1700en Cell Biology / 07 - Cell division (20 Apr 2022)
Reproduction of eukaryotes
Asexual reproduction
-The offspring inherit the full set of genes of their single parent
Sexual reproduction - Involves fusion of gametes
-Original sets of genes of both parents are reshuffled and combined in a genetically unique offspring
-Selective advantage in a rapidly changing environment
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Meiosis
-Two subsequent cell divisions with only one round of DNA replication:
- Meiosis I (heterotypic division)
- Meiosis II (homotypic division)
Function
- Reduction of chromosomes: haploid gametes from germ cells
- Independent assortment of the maternal and paternal homologs: 2n combinations
- Recombination of genes: crossing-over of homologous chromosomes
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Three major roles of meiosis
(A)    diploid organisms
germ-line cells
somatic cells
MEIOSIS
haploid egg       haploid sperm
'-1-1
FERTILIZATION \
I
diploid zygote MITOSIS
t
^^^^ diploid organism
- Reduction of the sets of chromosomes
Haploid gametes from diploid germ-line cells
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Three major roles of meiosis
three pairs of homologous chromosomes
maternal paternal
INDEPENDENT ASSORTMENT OF MATERNAL AND PATERNAL HOMOLOGS DURING MEIOSIS I ♦
MEIOSIS II
(A)      possible gametes
- Independent assortment of the maternal and paternal homologous chromosomes
-Genetic variability: 2n combinations
- Humans: 223 = 8,388,608 different combinations (different gametes)
Further variability: one of these gametes of one parent fuses with 01 from the possible combinations from the other parent - Further variability: genetic recombination during crossing-over
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Three major roles of meiosis
one pair of homologous chromosomes
maternal paternal
CROSSING-OVER DURING PROPHASE I \
MEIOTIC DIVISIONS I AND II
Recombination of genes: exchange of genetic material between homologous chromosomes during crossing over
Shuffles DNA regions of maternal and paternal chromosomes
- Increases genetic variability of gametes
(B)
possible gametes
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Stages of meiosis
Premeiotic interphase
Meiosis I - heterotypic
- Prophase I
Leptotene Zygotene
Pachytene: crossing-over
Diplotene
Diakinesis
- Metaphase I
- Anaphase I: separation of homologs
- Telophase I & Cytokinesis
Meiosis II - homotypic
- Prophase II
- Metaphase II
- Anaphase II: separation of chromatids
- Telophase II & Cytokinesis
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Heterotypic vs. homotypic division
(A)      MEIOSIS                    (B) MITOSIS
paternal homolog /jT^\
* J^~~"- maternal homolog 'J \  DNA REPLICATION
2n, 4c
I DNA REPLICATION
2n, 4c
1
I
o
PAIRING OF DUPLICATED HOMOLOGS
HOMOLOG PAIRS LINE UP ON THE SPINDLE
SEPARATION OF HOMOLOGS ATANAPHASE I
DUPLICATED CHROMOSOMES LINE UP INDIVIDUALLY ON THE SPINDLE
SEPARATION OF SISTER CHROMATIDS AT ANAPHASE II
SEPARATION OF SISTER CHROMATIDS ATANAPHASE
haploid daughter cells
n, 1c
diploid daughter cells
2n, 2c
Bi1700en Cell Biology / 07 - Cell division (20 Apr 2022)
Stages of meiosis
MEIOSIS I
Prophase I: Prophase I: Prophase I: Prophase I: Prophase I:
Leptonema Zygonema Pachynema Diplonema Diakinesis Metaphasel Anaphase I Telophase I
Chromosomes, each Homologous chromo- Homologous chromo-     Homologous Paired chromosomes Paired chromosomes Homologous chromo- Chromosome movement
consisting of two sister somes begin to pair, somes are fully paired,    chromosomes condense further and align on the equa- somes disjoin and is completed and new
chromatids, begin separate, except become attached torial plane in move to opposite nuclei begin to form,
tocondense. atchiasmata. to spindle fibers. the cell. poles of the cell.
MEIOSIS II
Prophase II
Metaphasell        Anaphase II
Chromosomes, each consisting of two sister chromatids, condense and become attached to spindle fibers.
Chromosomes align on the equatorial plane in each cell.
Sister chromatids disjoin and move to opposite poles in each cell.
Telophase II Cytokinesis
Meiosis in lily anther
Chromosomes decondense and new nuclei begin to form.
The haploid daughter cells are separated by cytoplasmic membranes.
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Meiosis I: Prophase I - Leptotene
- Condensation of chromosomes - each formed by two chromatids
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Meiosis I: Prophase I - Zygotene
Homologous chromosomes begin to pair
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Meiosis I: Prophase I - Zygotene
Formation of synaptonemal complex
paternal sister
chromatids
maternal sister
chromatids
LEPTOTENE
I r,
PACHYTENE
chromatid 1\Xf{|
yj M assembling
chromatid 2 chromatid 3
chromatid 4
synaptonemal complex
INTERPHASE
I I ZYGOTENE
DIPLOTENE FOLLOWED BY DIAKINESIS
Specific protein complexes forming:
- Central element
- Transverse filaments
- Lateral elements
Synaptonemal complex
Chromatin fibers of homologue 1
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Meiosis I: Prophase I - Pachytene
- Homologous chromosomes are fully paired
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Meiosis I: Prophase I - Pachytene
replicated paternal
replicated maternal
chromosome 1    chromosome 1
*» centromere
sister
chromatids
bivalent
chiasma
- Formation of bivalents (= tetrads referring to chromatids)
-Crossing-over between chromatids of homologous chromosomes
- Results in chiasmata *A
p
S 9
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Meiosis I: Prophase I - Diplotene
Homologous chromosomes separate, except at chiasmata
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Resolution of chiasmata
- Fully resolved during Anaphase I
Prophase I
Recombined Homologous (Maternal and
Paternal) Chromosomes
Metaphase I T
Anaphase
Loss of Sister Chromatid Cohesion at Chromosome Arms
Homologous Kinetochore Co-orientation
1 Bivalent
Retained in The Oocyte
Brought to The First Polar Body
2 Univalents
• Meiotic Sister Chromatid Cohesin Complex
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Meiosis I: Prophase I - Diakinesis
- Further condensation of paired chromosomes and their attachment to spindle microtubules
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Meiosis I: Metaphase I
■ Paired chromosomes align at the equatorial plane in the cell
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Meiosis I: Anaphase I
Reduction of chromosome sets: haploid cells
- Homologous chromosomes separates (chiasmata resolved) and are moved to opposite poles of the cell - independent assortment
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Meiosis I: Telophase I
-Chromosome movement is completed, formation of new nuclei - Followed by Cytokinesis I
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Meiosis II: Prophase II
Haploid cells
- Each chromosome consists of two chromatids (some with recombined regions)
-Chromosomes condense and become attached to spindle microtubules
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Meiosis II: Metaphase II
<
-Chromosomes align at the equatorial plane in each eel
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Meiosis II: Anaphase II
-Sister chromatids separate and are moved to opposite poles of the cells
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Meiosis II: Telophase II
-Chromosomes decondense, formation of new nuclei - Followed by Cytokinesis II
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Meiosis endpoint
-4 haploid cells
-separated by plasma membranes, + cell walls in plants/fungi
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(A) MEIOSIS
kinetochore microtubule
J      kinetochore \
I
cohesin complexes
haploid daughter cells
-(HI
METAPHASE I
ANAPHASE I METAPHASE II ANAPHASE II
TELOPHASE II
(B) MITOSIS
cohesin complex
chromatid - centromere
kinetochore microtubules
^      JfN kinetochore
METAPHASE
ANAPHASE
diploid daughter cells
/ \
i| i
TELOPHASE
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Alteration of generations in plants and algae
Sporophyte (2n) produces haploid spores by meiosis
Spores grow into haploid gametophytes (n) that produce gametes by mitosis
Icf
Meiosis
♦
t
Mitosis
■-v-
Fertilization
Gametes •
Mitosis
t
Mitosis
Gametophytes
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Yeast mating: Saccharomyces cerevisiae
Ascus containing four haploid ascospores
Nutrients Nutrients
Germination
V
Germination
Vegetative life cycle (haploid)
Sporulation
Meiosis
Vegetative life a cycle
Switch between haploid & diploid life cycle based on conditions
- Meiosis initiated upon nutrient starvation
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Gametogenesis in animals
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Meiotic aberrations and genetic consequences
Unequal crossing-over
- Prophase I: loss (deletion) of genetic material in one chromatid and gain (duplication) in other
Nondisjunction
- Homologue chromosomes (Anaphase I) or sister chromatids (Anaphase II) fail to separate, e.g., aberrations in spindle apparatus or centromeres
- Leads to aneuploidy: monosomy or trisomy syndromes
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