CELL CYCLE, MITOSIS & MEIOSIS SMG

Dr. Saji Mariam George
Associate Professor (Retired)
Assumption College Autonomous
Changanacherry
CELL CYCLE, MITOSIS AND MEIOSIS

CELL CYCLE
• The sequence of events a cell undergoes from
the time of its formation by division to its
own division into daughter cells.
• Duration varies – depends on the type of cell
and external factors – temperature, supply
of food, O2 etc.
• A growing cell undergoes a cell cycle.

• Divided into 4 periods – G1,
S, G2 and M (Mitosis )
• G1, S, G2 phases are
collectively called
Interphase.
• In continuously dividing
cells, a cell passes through 2
main phases of cell cycle –
Interphase & Mitotic Phase.
https://teachmephysiology.com

INTERPHASE
• Resting phase or stage
between 2 mitotic
divisions.
• No division of
chromosomes or
cytoplasm
• Metabolically active
• Longest phase – sub
phases - G1, S and G2
https://quizlet.com

a ) G1 Phase (Post mitotic gap phase or First
Gap phase )
• The time “gap” between the end of Mitosis
and the start of DNA synthesis.
• Most variable in length – days, months or
years.
• Include the synthesis and organization of the
substrate and enzymes necessary for DNA
synthesis.

• It is marked by the transcription of rRNA,
tRNA, mRNA and synthesis of different types
of proteins.
• Regulation of the duration of the cell cycle
occurs primarily by arresting it at a specific
point of G1 and the cell in the arrested
condition is said to be in G0 state – the cell is
withdrawn from cell cycle.

b) ‘S’ Phase
• The synthesis of DNA occurs in the ‘S’ phase
(Synthetic period) .
• DNA content of the nucleus is doubled.
• Additional histones are also synthesized.

c ) G2 phase (Post DNA synthesis phase or
premitotic gap phase or Second Gap phase.)
• The gap between the end of DNA synthesis
and the beginning of mitosis.
• Synthesis of RNA and protein continues, but
DNA synthesis stops.

• The time taken for S phase, G2 phase and
Mitosis is approximately equal. The length of
G1 is usually long.
• The nucleus of an interphase cell contains
very long chromatin fibres , which is suited
for transcription and replication.

Cell Cycle
Image:https://slideplayer.com

M phase or MITOSIS
(Gr. Mitos = Thread)
• The entry of a cell into M phase is initiated by
a protein called maturation – promoting
factor.
• The term Mitosis was coined by the German
Biologist Walther Flemming to describe the
thread like chromosomes.
• The type of division occurring in somatic cells
and results in growth.

• A cell divides to form two identical daughter
cells which are similar to the parent cell with
regard to the number and kind of
chromosomes - Equational division.
• Involves a series of changes in the nucleus as
well as cytoplasm – indirect division.
• Involves two main events – Karyokinesis
(Division of nucleus) and Cytokinesis
(Division of cytoplasm).
Karyokinesis - involves 4 stages – Prophase,
Metaphase, Anaphase and Telophase.

1. PROPHASE
• First phase of Mitosis
The important events during Prophase :
• The cell becomes spheroid and viscous.
• The extended form of interphase chromatin
is converted into much shorter, thicker
structures by a process of chromosome
compaction ( chromosome condensation) -
DNA supercoiling play an important role in
chromosome condensation.

• Each chromosome contains two chromatids.
• As the prophase progresses , the chromatids
become shorter and thicker and the primary
constriction becomes clearly visible.
• The nucleolus gradually disappears.

• Spindle is formed in the
cytoplasm.
• Nuclear envelope
gradually disintegrates.
Image:https://lh6.ggpht.com

Mitotic apparatus - Spindle
Image:https://www.bio.umass.edu

PROMETAPHASE
• The nuclear membrane completely
disintegrates, releasing the nuclear contents
into the cytoplasm.
• A clear zone known as equator appears
between the mid – line of the spindle and the
nucleus.
• The chromosome move towards the equator.

• The chromosomes of a prometaphase cell are
moved by a process called congression towards
the centre of the mitotic spindle.
• The forces required for chromosome
movements during prometaphase are generated
by motor proteins associated with both the
kinetochores and arms of the chromosomes.
• The chromosomes are maintained in the
equatorial plane by a “ tug – of – war” between
balanced pulling forces exerted by chromosomal
spindle fibres from opposite poles.

2. METAPHASE
• The chromosomes are aligned at the equatorial
portion of the spindle.
• Usually the arms of the chromosomes lie on the
equator of spindle.
• Occasionally , only the centromeres lie on the
equatorial plane and arms are directed towards
the poles.
• Smaller chromosomes are usually central in
position while larger ones are peripheral.

Metaphase
Image:https://home.cc.umanitoba.ca

• Some spindle fibres are attached to the
centromere of each chromosome –
Chromosomal fibres.
• Some spindle fibres extend from one pole of
the cell to another pole – Continuous fibres.
• Some spindle fibres occur in between the
chromosomes – interzonal fibres or
interchromosomal fibres.

3. ANAPHASE
• The centromere of each chromosome divides into
two.
• The chromatids of each chromosome are separated
and form two daughter chromosomes.
• The daughter chromosomes migrate towards the
opposite poles of the cell – achieved by the
contraction of chromosomal fibres and the stretching
of interchromosomal or interzonal fibres - Spindle
fibres shorten to one third to one fifth of the original
length.

Anaphase
Image:https://search.library.wisc.edu

4. TELOPHASE (Telos =end)
• Final stage of Mitosis
• The chromosomes start to unfold and become less
condensed.
• Nucleolus reappears
• Nuclear envelope is re-formed from the Endoplasmic
reticulum(ER).
• Spindle fibres gradually disappear by the
depolymerisation of tubulin sub units of microtubules.
• Two daughter nuclei are formed.

Telophase
Image:https://search.library.wisc.edu

CYTOKINESIS
• Process of separation of cytoplasm.
• In plant cells, cytokinesis starts with the formation of
the phragmoplast , which comprises interzonal
microtubules and Golgi vesicles.
• This structure is transformed into the cell plate
between the group of chromosomes.
• Cell plate grows from the middle towards the
periphery and finally joins the cell wall. In animal
cells, there is a constriction at the equator that finally
results in the separation of the daughter cells.

Cytokinesis
Cytokinesis in a Plant cell Cytokinesis in an animal cell
Image:https://celldivisionandreproduction.weebly.comImage:https://slideplayer.com

SIGNIFICANCE OF MITOSIS
• Maintainance of chromosomal continuity and diploid
chromosome number.
• The two daughter cells receive exactly the same number
and kind of chromosomes as that of parent cell.
• Helps the cell in maintaining its proper size .
• Provides opportunity for the growth and development of
the body of the organisms.
• Provides new cells for repair.
• Helps the organisms in asexual reproduction.

MEIOSIS
• The term Meiosis was coined by Farmer & Moore (1905)
• Special type of cell division present in the germ cells of all
sexually reproducing organisms.
• Consists of two divisions which takes place one after
another.
• The chromosome number is reduced to half – helps to
maintain a constant chromosome number of a species.
• Helps to maintain the regularity of reproductive cycle in
sexually reproducing organisms.

TYPES OF MEIOSIS
1. Gametic or terminal
meiosis - In animals
and lower plants
meiosis occurs before
the formation of
gametes.
Image: https://quizlet.com

2. Zygotic meiosis – This
occurs in lower plants.
Fertilization is
immediately followed
by meiosis.

3.Sporogenic or
Indeterminate meiosis
– Found in higher
plants – occur at the
time of spore
formation.

• Two divisions in rapid succession, with the
chromosomes replicating only once – Meiosis
I & II and each division involves 4 stages –
Prophase, Metaphase, Anaphase , Telophase.
• A cell produces 4 daughter nuclei, each
having half the number of chromosomes –
reduction division.

MEIOSIS I
(FIRST MEIOTIC DIVISION)
• Heterotypic division
• Stages – Prophase I, Metaphase I, Anaphase I
and Telophase I
Prophase I
• Most important stage
• Longest stage – sub stages Preleptotene
(preleptonema), Leptotene(Leptonema),
Zygotene(Zygonema),
Pachytene(Pachynema),
Diplotene(Diplonema) and Diakinesis.

PRELEPTOTENE (PRELEPTONEMA)
• Early prophase
• Increase in nuclear volume
• Chromosomes are thin and difficult to
observe.

LEPTOTENE (LEPTONEMA ; Greek. Leptos =
Thin , nema = thread)
• Chromosomes become more clear – appear
as thin threads.
• Looks single even though DNA duplications
have already occurred and have two
chromatids.

• In some cases , granular chromomeres are
visible – Number, shape, size and location of
chromomere is characteristic of individual
chromosomes and therefore helps in their
identification.
• Chromosomes take up a specific orientation
inside the nucleus . The ends of the
chromosomes converge towards one side of
the nucleus where the centrosome lies –
Bouquet stage .

Bouquet stage
• This is an arrangement of
chromosomes in such a way
that the telomeres bunch
together in a confined area
of the nuclear periphery
with centromeres at a polar
position.
• This arrangement resembles
a bouquet.
• This help for the pairing of
homolgous chromosomes
within the cell.
Image:http://scienceyoucandigest.blogspot.com

Leptotene (Leptonema)
Images:https://biology4isc.weebly.com

ZYGOTENE ( ZYGONEMA, Gr. Zygon = Adjoining)
• Pairing of homologous chromosomes, Synapsis
takes place.
• A pair of homologous chromosomes lying
together is a bivalent.
• The two homologues do not fuse during pairing ,
but remain separated by a space – 0.15 to 0.2
µm which is occupied by the synaptonemal
complex.
• Pairing starts at random .
• Pairing is specific, point for point and
chromomere for chromomere in each
homologue.

Zygotene(Zygonema)

Zygotene….Synaptonemal Complex
• Formed during pairing of homologous chromosomes.
• It is mainly composed of proteins
• It is tripartite, ribbon like structure.
• It consists of a dense central element with a dense lateral
element on either side.
• Each lateral element is attached on the inner side of a
homologous chromosome.
• The space between the central and lateral elements is
traversed by series of transverse units or LC fibres.

Synaptonemal Complex
Image:https://bio.libretexts.org

Synaptonemal Complex…..
Image: http://cc.scu.edu.cn

Synaptonemal Complex…
Images:http://bio3400.nicerweb.com

PACHYTENE (PACHYNEMA , Gr. Pachus = thick)
• Chromosomes become shorter and thicker
• In the middle of the pachytene, chromosome
split length wise to form chromatids.
• Two chromatids of the same chromosome
are called sister chromatids and those of two
homologous chromosomes are called non-
sister chromatids.
• Each chromatid pair is united by a
centromere.

• Pachytene chromosome thus consists of 4
chromatids closely joined together in one
complex unit known as a bivalent.
• The most important event, crossing over
occur during pachytene.
• Crossing over involves the exchange of
chromosomal segments between non – sister
chromatids of the homologous
chromosomes.

• Crossing over takes place by breakage and
reunion of chromatid segments.
• Points of interchange of crossing over are
known as chiasmata (Sing. Chiasma).
• Crossing over result in the shuffling of genes
which leads to genetic recombinations.

Pachytene (Pachynema)

Pachytene – Crossing over
Image:http://fig.cox.miami.edu

DIPLOTENE (DIPLONEMA)
• Paired chromosomes begin to separate, but
they are held together at the chiasmata .
• A chiasma formed at the ends chromosomes
– terminal chiasma.
• Chiasmata formed along the lengths of
chromosomes – interstitial chiasmata.
• The number and position of chiasmata varies
with the length of the chromosomes - there
is at least one chiasma per bivalent
chromosome.

• Terminalisation of chiasma takes place – the
chiasma begin to be displaced along the
length of the chromosomes.
• The terminal chiasma slips off the ends of the
chromosomes and its position is taken up by
an interstitial chiasma, which is now called
the terminal chiasma.
• When terminalisation is complete, the
homologues remain in contact through the
terminal chiasma.

Diplotene (Diplonema)

DIAKINESIS (Gr. Dia = Across, Kinesis = movement)
• The chromosomes become more condensed.
• The bivalents are more evenly distributed in the
nucleus and migrate towards the periphery.
• Terminalisation is complete and the homologues
remain in contact with each other by their
terminal chiasma.
• Nucleolus disappear.

Diakinesis

PROMETAPHASE I
 Condensation of chromosomes reaches its
maximum.
Nuclear envelope disappears.
Spindle formation begins.

METAPHASE I
• The chromosomes (bivalents) are arranged
at the equator .
• Centromeres of homologous chromosomes
point towards opposite poles and lie on
either side of the equatorial plate.

Metaphase I

ANAPHASE I
• The homologous chromosomes of each pair (bivalents)
with its two chromatids and undivided centromere moves
towards the opposite poles of the cell.
• This results in the reduction of the chromosome number
into half.
• Movement of chromosomes towards the pole is at
random.
• The chromosomes with single or a few terminal chiasmata
usually separate more quickly than longer chromosomes
with many interstitial chiasma.

Anaphase I

TELOPHASE I
• Begins when the group of chromosomes arrive at
their respective poles.
• The chromosomes undergo despiralization and
become elongated.
• The nuclear membrane is reformed.
• Nucleolus also reappear.
• Two daughter nuclei with haploid number of
chromosomes are formed at each pole.

Telophase I

CYTOKINESIS
• After karyokinesis, cytokinesis takes place.
• In most plant cells, a cell plate is formed in
between the two nuclei resulting in two
daughter cells.
• In animal cells, the cell membrane constricts
in the middle and two daughter cells are
formed

• In some plant cells, cytokinesis does not
takes place until both meiotic divisions are
completed.
• Following Telophase is a short interphase.
• At interphase between the two meiotic
divisions, there is no duplication of DNA.

MEIOSIS II
(SECOND MEIOTIC DIVISION)
STAGES :
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

PROPHASE II
• Short duration
• Nuclear membrane and nucleolus disappear
• The chromosomes with two chromatids
become short and thick
• Spindle is formed

Prophase II

METAPHASE II
• Chromosomes get arranged on the equatorial
plate.
• The microtubules of the spindle are attached
with the centromere of the chromosomes.
• The centromere divide , thus separating the
two chromatids of each chromosome.

Metaphase II

ANAPHASE II
• Daughter chromosomes
move towards the
opposite poles.

TELOPHASE II
• Nuclear membrane and
nucleoli reappear.
• The chromosomes
despiralise and
elongate .

• After the karyokinesis,
in each haploid meiotic
cell, cytokinesis occurs
and thus four haploid
cells are formed.

SIGNIFICANCE OF MEIOSIS
1. Maintainance of Chromosome number –
Meiosis halves the chromosome number in
the gametes so that fertilization may restore
the original diploid number in the zygote.
2. By crossing over, the meiosis provides an
opportunity for the exchange for the genes
and this causes the genetic variation among
the species.

COMPARISON BETWEEN
MITOSIS & MEIOSIS
MITOSIS
• Occur in somatic cells
• Involves a single division , resulting
in 2 daughter nuclei which are
identical to that of the parent cell.
The chromosome number remains
constant at the end of mitosis. The
genetic constitution of the
daughter cells is identical to that
of parent cells.
• Occurs in both sexually as well as
asexually reproducing organisms.
MEIOSIS
• In germ cells (Reproductive cells)
at the time of gametogenesis.
• Involves 2 successive divisions ,
resulting in 4 daughter nuclei
with chromosome number
reduced to half. Chromosome
number is reduced from diploid to
haploid. The genetic constitution
of the daughter cells differ from
that of the parent cell.
• Occurs only in sexually
reproducing organisms.

MITOSIS
• A short process
• DNA replication takes place
during interphase
• Prophase is simple and of
short duration.
MEIOSIS
• A long process
• Takes place during Interphase
I , but not in Interphase II
• Prophase is very long and
complex. It comprises five
substages viz. Leptotene,
Zygotene, Pachytene,
Diplotene and Diakinesis.

MITOSIS
• Prophase chromosome
appear double from the
very beginning.
• No pairing or synapsis
takes place .
• No formation of
synaptonemal complex
• Each chromosome consists
of two chromatids united
by centromere.
MEIOSIS
• Prophase I chromosome appear as
single in the beginning.
• Pairing or synapsis takes place
between homologous
chromosomes.
• Synaptonemal complex is formed
between synapsed homologous
chromosomes.
• The two homologous
chromosomes form bivalents or
tetrads. Each bivalent has four
chromatids and two centromeres.

MITOSIS
• No crossing over and
genetic recombination.
• Metaphase-The
centromere of the
chromosome remain
directed towards the
equator and the arms of
the chromosome remains
directed towards the
poles.
MEIOSIS
• Crossing over takes place
which leads to genetic
recombination.
• Metaphase-The
centromeres of the
chromosomes remain
directed towards the poles
and the chromosomal arms
remain directed towards
the equator.

MITOSIS
• Anaphase involves the
separation of chromatids of
each chromosome.
• The chromosomes separate
simultaneously during
Anaphase.
• Telophase occurs in all
cases.
MEIOSIS
• Anaphase I involves
separation of homologous
chromosomes. The
chromatids move apart in
Anaphase II.
• Short chromosomes separate
early. Separation of long
chromosome is delayed.
• Telophase I is eliminated in
some cases.

CELL CYCLE, MITOSIS & MEIOSIS SMG

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