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C ell cycle and cell division
CELL CYCLE AND CELL DIVISION
Chapter Outline:
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Prerequisites
Learning objectives
Cell Cycle
M Phase
Significance of Mitosis
Meiosis
Significance of Meiosis
Summary
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C ell cycle and cell division
Prerequisites
Hi children, this is an interesting and familiar topic which we would like to know, the following facts as
prerequisites:
1. Growth is a fundamental feature of living organisms.
2. In unicellular organisms growth includes only cell enlargement.
3. In multicellular organisms growth includes cell division and cell elongation.
4. The life of multicellular organisms begins with a single celled zygote.
5. The zygote divides repeatedly and forms thousands of cells which aggregate to form different plant
organs. Some of the cells retain the capacity to divide. Such cells constitute a meristem. Therefore
in a plant, cell division is confined only to meristematic cells.
6. Cell division is the most important method of formation of new cells. New cells always arise from
pre existing cells.
Learning objectives
Before discussing this present chapter, we would like to know the concept of cell cycle and cell division
that how it plays an important role in our living system. This is a wonderful creation among the nature
that a single cell forms a structure, consisting of millions of cells.
Let us gain the knowledge over this concept as following learning objectives.
1. To understand and explain the concept of cell cycle as (a) interphase and (b) M-phase.
2. To understand the process of cell division and its significance.
3. To comprehend the process of interphase which is further proceeds as G1 phase,S- phase and
M-phase.
4. To recognize the process of mitosis in which nucleus divides first and is followed by the division of
cytoplasm.
5. To recognize and recall the different phases of karyokinesis in mitosis are (1) prophase (2)
metaphase (3) Anaphase and (4) Telophase.
6. To identify and explain the process of Meiosis (or) reduction division.
7. To recognize and recall the following terminology in meiosis-l and meiosis-ll as Cytokinesis,
Karyokinesis, Prophase-l and ll, Metaphase-l and ll, Anaphase-l and ll, Telophase-l and ll, Leptotene,
Zygotene, Pachytene, Diplotene, Diakinesis, Synaptonemal complex, Bivalent, Chiasmata etc.
8. To know the importance of crossing over.
9. To explain the difference between mitotic & meiotic cell division.
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C ell cycle and cell division
Cell Division is of 2 types
1) Direct cell division(Amitosis)
2) Indirect cell division
Direct cell divisions:
Budding
Furrowing
Indirect cell division:
Mitosis
Meiosis
DIRECT CELL DIVISION OR AMITOSIS
In direct cell division the cell divides directly by furrowing or budding into 2 to many cells. Amitosis is a type of
cell division which involves no spindle formation. The nucleus divides directly into two. The division of nucleus is
followed by cytoplasmic division.
This type of division is common in Bacteria, Amoeba & Yeast. They contain naked circular DNA molecule. The
nucleus elongates and becomes dumbbell shaped. As the constriction deepens the nucleus is divided into two
daughter nuclei. The nuclei may or may not be equal.
Amoeba
Amoeba
INDIRECT CELL DIVISION
In indirect cell division the cell divides systematically to produce 2 - 4 daughter cells. Spindle apparatus is formed.
Chromosomes appear. The daughter cells are equal in all respects. The indirect cell division is of 2 types. They
are Mitosis and Meiosis.
Mitosis
Indirect Cell
Division
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C ell cycle and cell division
Cell Cycle
The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and
eventually divides into two daughter cells is termed as cell cycle. Although cell growth is a continuous process,
DNA synthesis occurs only during one specific stage in the cell cycle. The replicated chromosomes are then
distributed to daughter nuclei by a complex series of events during cell division. These events are under genetic
control.
PHASES OF CELL CYCLE
The duration of cell cycle varies from organism to organism
and also from cell to cell type. The cell cycle is divided into
2 phases.
A) Interphase B) M - Phase (Mitotic phase)
INTERPHASE
Phases Of Cell Cycle
The stage of cell cycle in which the nucleus is not in a
state of division is called Interphase. The Cell spends most
of its life cycle in Interphase. Interphase is the period of
interval between two successive cell divisions. Though it is
called as resting phase, it is not a period of rest but a period
of great metabolic activity. It is the time during which the cell
is preparing for division by under growing cell growth and
DNA replication in an orderly manner.
Interphase
Interphase is divided into 3 further phases or sub stages.
1) G1 Phase (Gap 1) 2) S phase (Synthesis) 3) G2 Phase
(Gap 2)
1) G1 Phase: G1 phase corresponds to the interval between
Mitosis and initiation of DNA replication. During G1 Phase
the cell is metabolically active & continuously grows but
does not replicate its DNA. The events that occur in G1
phase include.
a) Increase in size of the cell.
b) Synthesis of RNA and proteins.
This process takes 5 hours.
2) S-Phase: This phase marks the period during which
DNA replication takes place. During this time the amount of
DNA/cell doubles, but there is no increase in chromosome
number. This process takes 7 hours.
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Phase
S-Phase
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C ell cycle and cell division
3) G2 – Phase: During G2 phase, the synthesis of proteins
& RNA continues. Various cell organelles are newly
synthesized. Energy sources (ATP) required for spindle
formation and movement of chromosomes is synthesized. It
denotes gap between S-phase and M-phase. This process
takes 3 hours.
G2 – Phase
Some cells in the adult animals do not appear to exhibit division (heart cells) and many other cells divide only
occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not
divide further exit G1 phase to enter an inactive stage called quiescent stage (Go) of the cell cycle. Cells in this
stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement
of the organism.
M- Phase
M-phase represents the phase when actually cell division
occurs. It takes one hour. (Mitos means thread)
Mitosis was first observed by Strass burger (1870) in plant
cells & by Fleming (1882) in animal cells.
Mitosis results in the formation of two identical daughter
cells. Therefore mitosis may be considered as equational
division. The chromosomal number of daughter cells is
equal to the chromosomal number of parent cells. The
daughter cells resemble the parent cell in all respects except
the size. Mitotic cell division occurs in vegetative or somatic
cells. Hence it is called somatic cell division. Mitosis occurs
actively in apical meristems of stem or root. It helps in the
growth of the organisms by increasing their size, shape and
volume, hence it is called growth division.
In Mitosis the nucleus divides first & is followed by division
of cytoplasm. Nuclear division is called Karyokinesis and
cytoplasm division is called Cytokinesis.
Strass burger
Fleming
apical meristems
KARYOKINESIS
(Karyon means Nucleus; Kinesis means division) karyokinesis is the division of nucleus. It can be divided into 4
phases. They are 1) Prophase 2) Metaphase 3) Anaphase and 4) Telophase.
PROPHASE
(pro means first) Prophase is the first stage of Mitosis in
the early prophase, the chromosomes are long slender and
spread extensively in the nucleoplasm. The chromosomes
gradually become short, thick and stout of condensation of
chromatin. In the Mid prophase, each chromosome splits
longitudinally into two parts called chromatids, but they
remain united at the centromere.
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Prophase
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In the late prophase, the Nuclear membrane decreases in
thickness and gradually dissolves. Nucleolus also decreases
in size and finally disappears. The chromosomes appear to be
randomly scattered in the cytoplasm.
At this stage, initiation of mitotic spindle takes place. The
microtubules and the proteinaceous components of the cell
cytoplasm, helps in this process.
Late prophase
METAPHASE
(Meta means between) By this stage the complete
disintegration of nuclear envelops & condensation of
chromosomes is complete. At this stage the morphology
of the chromosomes can be easily studied. The important
changes in this phase are
a) Formation of bipolar spindle fibres which are attached to
the Kinetochores of chromosomes.
Metaphase
b) Orientation of chromosomes on the equatorial region and forming an equatorial plate or Metaphasic plate. In
the Early Metaphase – a bipolar spindle apparatus is formed by the fusion of microtubules & chromosomes move
towards the equator. All the centromeres along with Kinetochores are oriented along the axis of equator & their
chromosomal arms are placed towards the poles. The spindle fibres start from poles extend towards equator and
attaches to the kinetochore region of chromosomes and form into an apparatus known as spindle apparatus. The
spindles are composed of microtubules and chemically consist of tubulin proteins. The plane of alignment of the
chromosomes at metaphase is referred to as the Metaphase plate or equatorial plate. These kinds of spindle
fibres are commonly observed.
a) CONTINUOUS SPINDLE FIBRES: They originate at one pole and extend to the opposite pole through the
equator without touching any chromosome.
b) CHROMOSOMAL SPINDLE FIBRES: They originate at both the poles and extend in opposite directions
towards the equator and attaches to the kinetochore region of chromosomes at the equator.
c) INTERZONAL SPINDLE FIBRES: They are found suspended nearer the equator on its either side.
ANAPHASE
(Ana means back) The Anaphase is characterized by the following events
1) Separation of two chromatids.
2) Migration of chromatids (daughter chromosomes)
to opposite poles.
The centromere divides, spindle fibres begin to contract as a result, the two chromatids are separated from
each other in each chromosome. The separated chromatids, along with centromere can be called daughter
chromosomes. Each daughter chromosome moves away from equatorial plate the movement of chromosomes
towards their respective poles takes place by pull & push mechanism of spindle fibres. Based on the position of
centromere, the chromosomes exhibit V, L, I and J shapes during their movement towards opposite poles.
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C ell cycle and cell division
Anaphase
V, L, I and J
TELOPHASE
(Telo means end) It is reverse of Prophase. The sets of
daughter chromosomes at the poles, Organize into daughter
nuclei. The chromosomes uncoil & all uncoiled chromosomes
together appear as a network called chromatin reticulum.
Nuclear envelope and nucleoli are again formed. Thus 2
daughter nuclei which are identical are formed. They contain
the same number of chromosomes as in their mother
nucleus.
Telophase
CYTOKINESIS
Mitosis accomplishes not only the movement of duplicated
chromosomes into daughter nuclei, but the cell itself divides
into two daughter cells by a process called cytokinesis & its
leads to the completion of cell division.
In animal cells cytoplasmic division takes place by furrow in
the plasma membrane.
In higher plants, cytokinesis takes place by cell plate
formation method. During this process, the spindle fragments
gather at the equator region & form a barrel shaped structure
called Phragmoplast. The vacuoles of Golgi complex
enter the phragmoplast & release pectins into it. Thus the
phragmoplast is changed into a liquid form of plate like
structure known as cell plate. The growth of cell plate takes
place centrifugally & gets connected to the parent wall. It
gradually undergoes physical & chemical changes & finally
forms into cement like layer known as middle lamellum thus
dividing the cytoplasm into 2 parts.
Cytokinesis
Cell plate
The cell organelles are equally distributed to both cells on either side of the middle lamellum. Primary cell wall
materials like cellulose & hemicellulose are deposited on either side of middle lamellum & finally new primary cell
walls are formed on either side of the middle lamellum. After the formation of two primary cell walls the parent
cell is divided into two daughter cells. The division of cytoplasm by the formation of cell plate is called cell plate
method.
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C ell cycle and cell division
In some organisms, Karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises
leading to the formation of syncytium( e.g. : liquid endosperm in coconut)
Cell wall
Coconut
DURATION OF MITOSIS: The mitotic division may be
completed in 2-3 hours.
Eg: 1) Root tip cells of onion – 100 min
2) Endosperm cells of pea – 180 min
Temperature also effects Mitotic division
In staminal hairs of Rhoeo – the nuclear division takes
30 min at 450c
75 min at 250c
135 min at 100c
The time required for different phases also varies.
Prophase is the longest phase.
Telophase – 2nd place
Anaphase – 3rd place
Metaphase – Shortest phase.
In cells of stigmatic hairs of some grasses
The time required to complete the process
At 1900c - prophase 36 – 40 min
Metaphase 7 – 10 min
Anaphase 15 – 20 min
Telophase 20 – 35 min
Significance of Mitosis
1. Growth and development in organism is caused by mitosis & it restores the surface or volume ratio of the
cell.
2. The daughter cells formed by mitosis are identical with mother cell. Hence it is important in conserving the
genetic integrity of the organism.
3. In unicellular organisms, mitosis helps in reproduction. vegetative and asexual propagation takes place
through mitosis.
4. It maintains constant number of chromosomes in all the cells of the body.
5. Mitosis helps in wound healing.
6. Replacement of the lost parts – cells of the upper layer of epidermis, cells of the lining of the gut & blood
cells.
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C ell cycle and cell division
MEIOSIS OR REDUCTION DIVISION
(Meioum means to diminish)
The type of cell division, in which the chromosomes number
is reduced exactly to half is called Meiosis. The term Meiosis
was coined by J.B. Farmer (1905). It occurs in reproductive
cells of sexually reproducing organisms.
The cells in which meiosis occurs are called meiocytes.
J.B. Farmer
In Meiosis the chromosomes present in meiocyte is reduced to half in Meiosis- I. & these reduced chromosomes
divided by mitotically in Meiosis II. In this the mother cell divides into four daughter cells and these cells are un
identical when compared to their parent cell.
Every organism has definite number of chromosomes in diploid condition (2n). All organisms begin their life from
zygotic stage. The zygote is formed by the fusion of gametes. Thus in diploid organisms every cell has 2 sets of
chromosomes each acquired from the male and female gametes. Chromosomes with similar characters, uniform
size & types belonging to parental sources are called homologous chromosomes. In the life cycle of diploid
organisms, the chromosome number is reduced to half at one stage or the other. If not, at every fertilization the
chromosome number would be doubled leading to abnormal number & causes unimaginable genetic disorders.
Since the number of chromosomes is reduced to half, it is called reduction division.
Zygotic stage
Sperm Egg
The key features of meiosis are
1) Meiosis involves two sequential cycles of nuclear cell division called Meiosis I and Meiosis II but only a single
cycle of DNA replication occurs.
2) Meiosis- I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at
the S- phase of interphase.
3) Meiosis involves pairing of homologous chromosomes & recombination between them.
4) Four haploid cells are formed at the end of Meiosis – II.
MEIOSIS- is divided in two :
1) Meiosis I
2) Meiosis II
MEIOSIS I – further divides into 1) Karyokinesis – l
2) Cytokinesis
MEOSIS II - further divides into 1) Karyokinesis – II
2) Cytokinesis
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Karyokinesis – l: Occurs in four stages
• Prophase – I
• Metaphase – I
• Anaphase – I
• Telophase – I
Prophase – I: Again has 5 stages
• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis
KARYOKINESIS – II: Has 4 stages
• Prophase –II
• Metaphase –II
• Anaphase –II
• Telophase – II
MEIOSIS-I / HETEROTYPIC DIVISION / REPRODUCTIONAL DIVISION
Karyokinesis – I: It is completed in 4 phases. In this the chromosome number is reduced to half. Cytokinesis
may take place immediately or it may be delayed till the completion of telophase II.
PROPHASE – l: Prophase of the first meiotic division is longer & complex. Based on the chromosomal behaviour,
it has been sub divided into 5 phases.
a. LEPTOTENE (or) LEPTONEMA: The nucleus &
nucleolus enlarges in size. The chromosomes are long and
slender.
b. ZYGOTENE (or) ZYGONEMA: It is characterized
by pairing of homologous chromosomes and this process
of association is called synapsis. The pairs of homologous
chromosomes are called bivalents. The pairing is bought
about in a zipper like fashion due to protein called
synaptonemal complex and may start at centromere or
chromosome ends or at any other position. Based on the
specific locations from which synapsis occur, it is further
classified into three types.
Leptotene(or) Leptonema
Zygotene (or) Zygonema
1. Proterminal Synapsis: Pairing starts at the polarized ends and progresses gradually towards the other
extremity.
2. Procentic Synapsis: Pairing starts at the centromere & proceeds towards the ends of the chromosomes.
3. Random or Intermediate Synapsis: Pairing of chromosomes occurs simultaneously at various places
(randomly) along the length of the chromosomes. Zygotene is also characterized by the enlargement of
nucleolus and initiation of spindle formation.
C) Pachytene or Pachynema or Genetic Recombination stage: It is a long lasting stage
(weeks or years) the chromosomes divides into two chromatids thus in each bivalent, four chromotids are seen.
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They are called Pachytene tetrads. In a bivalent, the
chromatids of the same chromosome are called sister
chromotids and those of two different chromosomes as non
sister chromatids. The non sister chromotids exchange their
parts mutually at one, two or many places.
Pachytene
Such points where chromatids physically contact each other are called Chiasmata. During the formation
of chiasmata, the chromotid arms first break due to the action of enzyme called “endonuclease” The broken
chromatid arms mutually exchange with each other and get united by the action of an enzyme called Ligase.The
formation of chiasmata lead to exchange of genetic material and result in the recombination of genetic characters.
This phenomenon is known as ‘Crossing Over ‘. It is responsible for the origin of new species and thus leads to
evolution.
D) DIPLOTENE (or) DIPLONEMA: The beginning
of diplotene can be recognized by the dissolution of the
synaptonemal complex by the repulsion activity between
homologous chromosomes, As a result, the bivalents of
homologous chromosomes are separated from each other
except at the sites of chiasmata regions. At this stage the
chiasmata are clearly visible as ‘X’ shaped links between
non sisters chromatids of homologous chromosomes.
Diplotene
In oocytes of some vertebrates diplotene can last for months or years.
E)
DIAKINESIS: This stage is marked by the
‘TERMINALISATION OF CHIASMATA‘ This displacement
of chiasmata towards the terminal position is called
Terminalisation. The bivalents become very thick & short
and migrate to the periphery of the nucleus. The nucleolus
begins to disappear, the nuclear membrane disrupts and the
chromosomes are released into the cytoplasm. Diakinesis
represents transition to metaphase.
Diakinesis
Metaphase – I: It is characterized by the appearance
of bipolar spindle apparatus. The bivalent chromosomes
move to the equator of the cell and arrange themselves
with their centromeres directed towards opposite poles &
their arms towards the equator. The movement of bivalent
chromosomes towards the equator is called chromosomal
congression. The homologous chromosomes are fused by
the chiasmata at the terminal ends.
ANAPHASE – I: It is characterized by the movement of
each homologous towards the respective poles without
the division of the centromere. This phenomenon is called
Seggregation or disjunction of chromosomes. As a result,
the diploid number of chromosomes is reduced to half.
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Metaphase: I
Anaphase –I
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TELOPHASE – I: The nuclear membrane and nucleolus
reappear, cytokinesis follows and this is called diad of
cells. The stage between two meiotic divisions is called
Interkinesis and is generally short lived. Interkinesis is
followed by prophase II.
MEIOSIS II / HOMOETYPIC DIVISION / SECOND
MEIOTIC DIVISION:-
Telophase I
Meiosis II is equivalent to Mitosis. It is completed into 2
stages. They are Karyokinesis - II & cytokinesis.
Prophase ll: Is similar to mitotic prophase. The nuclear
membrane, nucleolus appears and chromosomes are
organized. Each chromosome has two chromatids which
remain attached at centromere. All the chromosomes will be
free in the cytoplasm.
Prophase ll
METAPHASE II: It is similar to mitotic metaphase but
dissimilar to it in the formation of 2 spindle apparatus
perpendicular to the one formed at the metaphase I.
Metaphase II
ANAPHASE II: It begins with simultaneous splitting of
centromere of each chromosome allowing them to move
toward opposite poles of the cell.
Anaphase II
TELOPHASE II: The daughter chromosomes arrived at the
poles become thin and transform into chromatin. Nuclear
membrane and nucleolus are reorganized. Thus at the end
of Telophase –II, four daughter nuclei are formed.
CYTOKINESIS: The process is similar as Mitotic division.
Telophase II
SIGNIFICANCE OF MEIOSIS
1) It helps in the maintenance of a constant chromosome number from one generation to the next.
2) Due to crossing over, genetic recombinations are caused which help in the origin of new species and lead to
evolution.
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C ell cycle and cell division
MITOSIS:
1)
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It occurs in haploid and diploid cells.
It occurs in somatic cells.
Nucleus divides once.
Daughter cells are identical.
Two daughter cells are formed.
Prophase is simple.
Pairing of chromosomes does not occur.
Both chiasmata & crossing over are absent.
The chromosome numbers of daughter nuclei are unchanged.
MEIOSIS:
1) It occurs only is diploid cells.
2) It occurs in reproductive cells.
3) Nucleus divides twice.
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Daughter cells are un identical.
Four daughter cells are formed.
It is complicated & shows five sub stages.
Homologous chromosomes pair to form bivalents.
Both chiasmata & crossing over occur between non sister chromatids.
Chromosome number of daughter nuclei is reduced to half.
Summary
1. According to cell theory. Cells arise from pre existing cells. The process by which this occurs is called cell
division.
2. Any sexually reproducing organism starts its life cycle from a single-celled zygote.
3. Cell division does not stop with the formation of the mature organism but continues throughout its life cycle.
4. The stage through which a cell passes from one division to the next is called the cell cycle.
5. Cell cycle is divided into two phases called (i) interphase -a period of preparation for cell division, and (ii)
Mitosis (M- phase) - the actual period of cell division.
6. Interphase is further subdivided into G1, S and G2.
7. G1 phase is the period when the cell grows and carries out normal metabolism. Most of the organelle
duplication also occurs during this phase.
8. S phase marks the phase of DNA replication and chromosome duplication.
9. G2 phase is the period of cytoplasmic growth.
10. Mitosis is also divided into four stages namely prophase, metaphase, anaphase and telophase.
11. Chromosome condensation occurs during prophase. Simultaneously, the centrioles move to the opposite
poles. The nuclear envelope and the nucleolus disappear and the spindle fibres start appearing.
12. Metaphase is marked by the alignment of chromosomes at the equatorial plate.
13. During anaphase the centromeres divide and the chromatids start moving towards the opposite poles.
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14. Once the chromatids reach the two poles, the chromosomal elongation starts, nucleolus and the nuclear
membrane reappear. This stage is called the telophase.
15. Nuclear division is then followed by the cytoplasmic division and is called cytotokinesis.
16. Mitosis thus, is the equational division in which chromosome number of the parent is conserved in the
daughter cell.
17. In contrast to Mitosis, meiosis occurs in the diploid cells, which are destined to form gametes. It is called the
reduction division since it reduces the chromosome number by half while making the gametes.
18. In sexual reproduction when the two gametes fuse the chromosome number is restored to the value in the
parent.
19. Meiosis is divided into two phases namely Meiosis-l and Meiosis II. In the first Meiotic division the homologous
chromosomes pair to form bivalents and undergo crossing over.
20. Meiosis -l has along prophase, which is divided further into five phases. These are leptotene, zygotene,
pachytene, diplotene and diakinesis.
21. During Metaphase- I the bivalents arrange on the equatorial plate. This is followed by anaphase-I.
22. Anaphase - l: In whish homologous chromosomes move to the opposite poles with both their chromatids.
Each pole receives half the chromosome number of the parent cell.
23. In Telophase I, the nuclear membrane and nucleolus reappear.
24. Meiosis- II is similar to Mitosis.
25. During anaphase- II the sister chromatids separate. Thus at the end of meiosis four haploid cells are formed.
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