10 – cell cycle and cell division

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Cycle& Cell Division
 According to Rudolph Virchow cells originates from pre existing cells through cell
division. ―Omnis cellula‖.
 Specific characters like growth and reproduction is only possible through the process of
cell division.
 Protozoans and other unicellular organism is only able to reproduce to form a new
generation through cell division.
 But in multicellular organism- new cells are formed by cell division.
REASON OF CELL DIVISON :-
Why cell divide ?
 To understand this phenomenon, Hertwig 1903postulated a theory called - ―Kernplasm
theory‖.
 According to this theory when the karyoplasmic Index [K.I.] falls [lower down] then cell
divides.
 The volumes [ratio] of karyoplasm and cytoplasm is termed K.I. – K.I. = Vn/Vc
Here - Vn = Volume of karyoplasm. Vc = Volume of cytoplasm.
 K. I. of new cells is high. It means karyoplasm of a cell is higher than cytoplasm.
Therefore , nucleus of the cell easily control all the activities of the cytoplasm.
 The amount of cytoplasm increase according to increase in the age of a cell. Eventually
cell reaches the position where nucleus is unable to easily control all activities of
cytoplasm.
CELL CYCLE:-
A complete life cycle of a cell is divided into 2 phases :-
Every dividing cell passes through four phases or stages, which collectively constitute
the cell cycle (Mitotic cell cycle or Meiotic cell cycle).
CELL CYCLE
[A] INTERPHASE [B] DIVISION PHASE

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Four phases of cell cycle are:
(i) G1-phase or post-mitotic phase or pre-DNA synthesis phase or first growth period
or first gap period:
 This phase is having high metabolic activities.
 Synthesis of proteins, RNA and ribosomes occurs during this phase.
 Time taken for completion of this phase is about 30-40% of the total cell cycle (mitotic
cycle).
 Decision of cell division occurs during this phase.
(ii) S-phase or DNA synthesis phase:
 G1-phase is followed by S-phase.
 This phase is characterized by synthesis of DNA (i.e., DNA replication) as well as
histone proteins.
 During this phase ‗DNA polymerase‘ enzyme (responsible for DNA replication) is
functionally active.
 Time taken for completion of this phase is 30-50% of total cell cycle.
 A cell would proceed to division (mitosis) without any interruption, when it has entered
S-phase.
(iii) G2-phase or post DNA synthesis phase or second gap period or second growth period:
 It follows S-phase.
 Time taken for completion of this phase is 10-20% of a total cell cycle.
 In this phase division of mitochondria and chloroplasts, division of centrioles
 synthesis of proteins for spindle apparatus occur.
 Preparation of cell division occurs in this phase.
All these 3 phases collectively constitute interphase or resting phase, which is actually
not resting phase but is most metabolically active phase.
(iv) M-phase or mitotic phase (or meiotic phase in meiosis) or actual cell division phase:
 G2-phase is followed by this actual dividing phase, i.e., M-phase.
 Time taken for completion of this phase is 5-10% of the total cell cycle and hence it is
shortest of all the 4-phases.
G1 + S + G2
Interphase or
resting phase
(long)
M phase Cell cycle

Actual dividing
phase
(short)
+

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Correct sequence of different phases of cell cycle is: G1, S, G2 and M.
G1-S-G2- M(PMAT)-G1-S-G2-M
Time taken by these different phases varies in different organisms and also in different
tissues of same organism, e.g., in Vicia faba (Broad bean or Bakla), duration of different
phases of cell cycle are:
G1-phase 12 hrs
S-phase 6 hrs
G2-phase 12 hrs
M-phase or mitotic phase 1 hr
Therefore, cell cycle here is of 31 hrs (Interphase— 30 hrs and mitotic phase—1 hr)
 in Yeast cell cycle is of 90 minutes, in human 24 hrs.)
NOTE- Meiotic cell cycle has also interphase just like mitotic cycle consisting of G1, S
and G2-phases; however here, G2-phase is either totally absent or very short, i.e.,
meiotic division (M-phase) occurs just after completion of DNA-synthesis (S-phase).
I. Mitosis or Mitotic cell division or Equational cell division
 Mitosis or mitotic division is meant for multiplication of cells. Development of complete
organism from zygote is by means of mitotic cell division. So mitosis is necessary for
maintenance, growth, repair and continuity of life.
 Mitosis generally takes place in vegetative or somatic cells.
 In this process, one parent cell divides into two daughter cells, where chromosome
number remains the same as in parent cell, i.e., daughter cells exactly resemble with
parent cell both quantitatively as well as qualitatively.
 mitosis leads to increase in number of cells without any change in genetic make-up.
 Mitosis can be easily observed in meristematic cells of plant. Root tips (e.g., onion root
tips) are considered to be the best material for study of mitosis.
 W. Flemming (1882) gave the term mitosis (Gr.: Mitos-thread + osis-state). However,
mitosis was first observed in plants by E. Strasburger (1875) and in animals by W.
Flemming (1879).
 The actual mitotic-phase (M-phase) is completed in two steps:
(A) Karyokinesis: i.e., division of nucleus into two.
(B) Cytokinesis: Karyokinesis is followed by cytokinesis, i.e., division of cytoplasm into
two cells or wall formation.
(A) Karyokinesis:
Karyokinesis (division of nucleus) consists of the following four stages:
(i) Prophase (ii) Metaphase (iii) Anaphase (iv) Telophase
(i) Prophase:
 It is known to be the longest and the most complex phase of cell division because it
lasts for about 50 min of total duration of mitotic phase.
 This is the first stage of mitosis that follows by the S and G2-phase of interphase.
 This phase is known for the initiation of condensation of chromosomal material, during
the process of chromatin condensation becomes untangled,

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 and finally the centriole (already duplicated during S-phase of interphase) begins to
move towards the opposite poles of the cell.
a. Early Prophase
 During this phase, condensation of chromosomal material takes place in order to form
a compact mitotic chromosomes composed of two chromatids which are attached
together at centromere.
 The most conspicuous change that take place during prophase is the formation of
mitotic spindle. The initiation of mitotic spindle assembly, the microtubules and the
proteinaceous components of the cell cytoplasm helps in the completion of the
process.
 The mitotic spindle is formed between the two pairs of centrioles that migrate towards
the opposite poles of the cell.
b. Late Prophase
 At the end of the prophase, i.e., during late prophase the nucleolus disintegrates
gradually and the nuclear envelope disappear.
 This disappearance marks the end of the prophase.
 If we view cells under the microscope, during the prophase the cell will not show
nucleolus, nuclear envelope, Golgi complex, endoplasmic reticulum, etc.
(ii) Metaphase:
 It is the phase that starts after the disintegration of nuclear envelope in the late
prophase.
 The chromosomes spreadout through the cytoplasm of the cell and are seem to be
slighly shortest and thickest of all. The chromosome can be easily observed under the
microscope

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 At this stage, the chromosomes become maximally distinct due to further contraction
and thus size of the chromosomes is measured at mitotic metaphase.
 Each chromosome at this stage is made up of two longitudinal threads (sister
chromatids) and are held together by the centromere in the centre.
 At the surface of each centromere disc-shaped structures called kinetochores are
present, which helps in the attachment of spindle fibres to the chromosomes.
 The chromosomes finally arrange themselves at the equator in one equitorial plane
known as metaphase plate.
 Attachment of spindle fibres to kinetochores of chromosomes.
 Movement of chromosomes to spindle equator and its
alignment along the metaphase plate through spindle fibres to both poles.
Note:
Kinetochore are the small disc-shaped structures at the surface of the centromeres,
which serve as the sites of attachment of spindle fibres to the chromosomes that are
moved into position at the centre of the cell.
Chromosomes are attached to the polar fibres at the kirtetochores, through kinetochore
fibres.
 In animal cells, spindle apparatus is formed due to division of centrosome or centriole,
but in plants, spindle is still formed even in absence of centrosome in plant cells.
 Mechanism of the formation of spindle is, however, not clear in plant cells.
 Amphiastral mitosis - In animal cells and lower plants, centrioles are present and asters
(delicate fibres or microtubules), radiate from the centriole, which form spindle fibres.
This type of mitosis is called Astral or Amphiastral mitosis.
 Anastral mitosis - In higher plants, there are no centrioles and hence no asters are
formed, so this type of mitosis is called Anastral mitosis.
NOTE-Mitotic Poison: Colchicine
Colchicine, an alkaloid is extracted from the corms of autumn crocus (Colchicum autumnale),
acts as a poison for mitosis as it does not allow the formation of mitotic spindle to takes place
by preventing the assembly of microtubules. But does hot affect replication of chromosomes.
Thus, meristematic cells treated with this chemical show doubling of chromosomes. It usually
causes arrest at metaphase of mitosis.
NOTE- Spindlefibres are chemically made of 97% tubulin protein (protein of
microtubules) and 3% RNA. Further spindle microtubules are polar with their polymerising ―+‘
ends (fast growing) facing centre and non-polymerising ‗-‘ ends (slow growing) facing poles.

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(iii) Anaphase:
 This is the shortest stage of mitosis.
 Chromosomes divide at the point of centromere or kinetochore and thus two sister
chromatids are formed, which are now called chromosomes.
 Interzonal fibres appear between the daughter centromeres (i.e., formed after division of
centromere).
 In anaphase, sister chromatids separate and move along the kinetochore microtubules
toward opposite ends of the cell
 The microtubules shorten by depolymerizing at their kinetochore ends

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 Different shapes of chromosomes are observed at anaphase (V or J or L or I-shaped),
depending upon position of kinetochore or centromere.
(Hence shape of chromosomes is determined at mitotic anaphase).
(iv) Telophase:
 Chromosomes reach opposite poles of the spindle.
 Chromosomes now become decondensed, uncoiled and not clear in outline (fuzzy).

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 Disintegration of spindle fibres occurs at this stage and nuclear membrane reappears
around each group of chromosomes. Nucleoli also make their appearance, thus forming
two daughter nuclei.
(B) Cytokinesis
Karyokinesis results in formation of two nuclei inside a cell and now it is followed by
division of cytoplasm (Cytokinesis), thus forming two cells (daughter cells).
Cytokinesis is by 2 methods:
(i) Cell furrow method: This is characteristic of animal cells. Due to absence of rigid
cell wall here, the more flexible plasma membrane forms the outer layer of cell. A
circular constriction or invagination appears at centre or equator, which deepens
gradually and finally two daughter cells are separated.
(ii) Cell plate method: This is characteristic of plant cells. Here, vesicles provided by
Golgi apparatus unite to form phragmoplasts, which join to form cell plate. Cell plate
is first laid down in centre and then proceeds towards periphery (i.e., centrifugal
plate-formation). Cell wall materials are now laid down on both sides of cell plate
and thus forming two daughter cells.
Thus by mitotic division or mitosis, two daughter cells (exactly similar to parent cell)
are formed from a single parent cell.
Period of active mitosis or mitotic division ranges from 10 minutes to several hours.
Further higher the temperature, lesser will be the time taken.
Colchicine is called ‗Mitotic poison‘ as it causes temporary cessation of mitotic division
by inhibiting spindle formation.
Uncontrolled cell division and increase in cell size leads to tumour formation or
cancers. Main cancer causing agents (carcinogens) are radiations and chemicals like
benzpyrene, aflatoxin, mustard gas vapours and vinyl chlorides.
Significance or role of mitosis
1. Growth:
Mitotic divisions of vegetative or somatic cells are responsible for growth, maintenance
and repair.

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2. Genetic stability:
Mitotic cell division results in daughter cells, which are both quantitatively and
qualitatively similar to parent cell and thus responsible for genetic stability.
Meiosis or Meiotic cell division or Reduction cell division
 Farmer and Moore (1905) coined the term meiosis (Meio, to reduce) for the type of cell
division in which a parent cell gives rise to four daughter cells, which are having half the
chromosome number as compared to parent cell.
 Meiosis was first of all observed and studied by Van Beneden (1887), Strasburger (1888),
Sutton (1900) and Winiwarter (1900).
 Meiosis is associated with reproductive or germinal cells.
 Meiosis is the phenomenon which occurs in any life cycle that involves the process of
sexual reproduction. The production of offspring by sexual reproduction involves the
fusion of two gametes (each having a complete haploid set of chromosomes).
 Thus, meiosis is known to be the specialised form of cell division which reduces the
chromosome number in such a way that each daughter nuclei receive only one set of
each kind of chromosome, {i.e., maternal and paternal). It results in the production of
haploid daughter cells. In meiosis, the nucleus divides twice but the replication
chromosome takes place only once.
 Thus, it is also known as the reductional division. In case of diploid organisms, meiosis
takes place during the formation of spores or gametes whereas, in haploid organisms it
takes place during germination of zygote.
 Meiosis ensures the production of haploid phase in the life cycle of sexually
reproducing organism whereas, the fertilisation restores diploid phase.
 During this division, the homologous chromosomes of each pair separates from each
other and reaches separate daughter cells which thereby reduces the number of
chromosomes from diploid to haploid, i.e., from 2n to n. Thus, it is known as heterotypic
division.
 This reduction division may take place at any time in life cycle. Further the daughter
cells resulting from meiosis are genetically distinct from parent cells.
Meiosis always occurs in diploid cells called meiocytes.
 The anthers of onion or Tradescantia are considered to be the best material for study of
meiosis.
 The stage of reduction division or meiosis varies from plant to plant.
Stages of meiosis:
Meiosis consists of two successive divisions as:

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Homologous Chromosomes
There are two sets of chromosomes in a
diploid cell undergoing meiosis, one set
contributed by the male parent and the other
by the female parent. There are always two
similar chromosomes, having the same size,
shape and position of centromere. In some
organisms, the chromosomes give beaded
appearance due to the presence of
chromomeres (swollen area).
(A)First meiotic division or meiosis I or
actual reduction division or heterotypic
division:
 It is accompanied with chromosome
number reduction without any division
of chromosome, i.e., in this the
chromosome number is reduced to half.
Parent cell (2n) 2cells(n)
(B)Second meiotic division or meiosis II or
homotypic division or equational
division:
 It involves separation of chromatids of
the chromosomes. Here 4 cells are
produced from 2 cells and the
chromosome number remains constant.
So it is actually mitotic or equational
division and hence sometimes called
meiotic mitosis.
2cells(n) 4cells(n)
 In meiotic cell cycle, there is interphase like mitosis (i.e., G1 + S + G2), but here, G2-
phase is either very short or altogether absent, i.e., meiotic division starts soon after S-
phase (DNA synthesis phase).
(A) First meiotic division or Meiosis I
 It is actual reduction division and is completed in 4 phases or stages.
(a) Prophase I (b) Metaphase I (c) Anaphase I (d) Telophase I
(a) Prophase I:
It is longest and most complex phase of meiosis I. It is further divided into 5 sub-stages
as:
(i) Leptotene or leptonema
(ii) Zygotene or zygonema
(iii) Pachytene or pachynema
(iv) Diplotene or diplonema
(v) Diakinesis

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(i) Leptotene or leptonema:
 Size of nucleus increases.
 The chromosomes appear as thin, uncoiled thread-like structures.
 One half of these chromosomes are of male parent and the other half of female parent.
 Chromosomes having similar characters or genes (which can pair) are known as
homologous chromosomes. They are similar in shape and size.
 In some plants, e.g., Lilium, all the chromosomes are pointed at one end, which is called
synizesis or bouquet formation, but its significance is not clearly known.
(ii) Zygotene or zygonema:
 Pairing or synapsis of homologous chromosomes takes place
 which is due to small amount of DNA replication at this stage (Stern and Hotta, 1969).
 Synapsis or pairing leads to formation of bivalents.
 Synapsis occurs in zipper-like fashion and may begin at centromere or at ends of
chromosomes or at any point.
 Thickening and shortening of chromosomes occur.
(iii) Pachytene or pachynema:
 It is the longest sub-stage of prophase I.
 Chromosomes become further shorter and thicker.
 Each chromosome of homologous pair is having 2 chromatids at this stage and thus
bivalent or homologous pair of chromosomes is now having four chromatids (four
stranded) which is called tetrad stage or tetravalent stage.
 Crossing over (Exchange of chromosomal segments between non-sister chromatids of
homologous chromosomes) occurs at this stage (at molecular level, however, expression
at diplotene).
 Out of 4 chromatids of tetrad, only two are cross-overs and two are non-cross overs.
Chromosomes at pachytene are more longer even than at mitotic metaphase, so
morphological details of chromosomes can be best studied at this sub-stage.
(iv) Diplotene or diplonema:
 Chromosomes become still more shorter and thicker.
 As forces of attraction between homologous chromosomes or bivalents reduce, so they
begin to separate from each other and this separation of homologous chromosomes is
called terminalization (as separation starts from centromere towards ends).
 The separation is not complete, but the homologous chromosomes remain attached at
one or more points and these points of contact are called chiasmata.
 Chiasmata are the points where crossing over takes place (but now it is considered that
chiasmata are not causes but are consequence of crossing over).

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 Number of chiasmata may be 1 or more depending upon the length of the chromosomes.
 Nuclear membrane and nucleolus begin to disappear at this sub-stage.
(v) Diakinesis:
 In this last sub-stage of prophase I, further contraction and shortening of
chromosomes occur and chromosomes become minimum in size.
 Terminalization is almost complete.
 Both nucleolus and nuclear membrane completely disappear at this sub-stage (or in
early metaphase).
 Characteristic difference from diplotene is that chromosomes are shorter in diakinesis.
Synaptonemal Complex (SC):
o SC is a feature of meiotic prophase I, which was first observed by Moses (1956).
o SC is a tripartite (three-layered) protein framework (made of 2 lateral elements and one
central element) found between two paired homologous chromosomes. Ubiquitin is
the characteristic protein forming SC.
o Synaptonemal complex appears in zygotene, persists in pachytene (more
conspicuous or clear) and disappears during diplotene sub-stage of prophase I.
o Function of synaptonemal complex: It is believed that SC helps in proper pairing of
homologous chromosomes and also in producing recombinations. This function is
supported by period of appearance and disappearance of SC.

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(b) Metaphase I :
 The chromosomes become more condensed and distinct.
 Spindle apparatus appears, the spindle fibres get attached to centromeres of bivalents
and are arranged on equator due to congression movements.
 The arrangement of chromosomes on equatorial plate is such that centromeres are
towards poles and chromosomal ends are towards equator.

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(c) Anaphase I:
 This process of separation of homologous chromosomes is known as disjunction. The
separated chromosomes are univalents and are also called dyads.
 On reaching at the end of the anaphase, the two groups of chromosomes are produced
(with each having half number of chromosomes).
 The sister chromatids remains attached at their centromeres on the separation of the
homologous chromosomes .
(d) Telophase I:
 Homologous chromosomes reach at their respective poles!
 Reappearance of nuclear membrane and nucleolus takes place
 Telophase I may or may not be followed by cytokinesis or wall formation.
 Resting period or interphase may or may not be present between meiosis I and
meiosis II.
(B) Second meiotic division or Meiosis II
Meiosis-II is known by another term, i.e., homotypic division, because in this division

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chromosome number remains same, as produced in meiosis-I. It is initiated immediately after
cytokinesis. It is often known an equational division. Meiosis-lf also resembles a normal
mitotic division in contrast to meiosis-I because it distributes chromatids to daughter cells
(like mitosis).
Prophase-ll
This is known to be the very short stage out of the all. During this process the chromosomes
again become compact in organisation.
 The centrioles duplicate themselves by the separation of the two members of the pair.
 Each chromosome comprising two chromatids become visible in the nucleus. These
chromosomes further become thick and short in size.
 Nuclear envelope breaks down and the formation of spindle apparatus takes place and
nucleoli disappears.
Metaphase-ll
 The chromosomes align at the equator or the metaphase plate, in the similar way as in
mitosis.
 Chromosomes get attached to the fully formed spindle apparatus and the kinetochores of
sister chromatids for each chromosome face the opposite poles and each is attached to
the kinetochore microtubule coming from the pole of that side.
Anaphase-ll
 The centromere of each chromosome splits, that was holding the sister chromatids
together.
 The shortening of chromosomal microtubules take place, the two chromatids of each
chromosome start moving away from each other and finally reaches the opposite poles of
the spindle (now called chromosomes).
Telophase-ll
 This is known to be the last stage of second meiotic division and show changes equally
opposite to that of the prophase-II. Meiosis ends up with the progress of this particular
phase, i.e., telophase-II.
 Formation of a nuclear envelope (from ER) around each set of chromosomes.
(ii) Nucleoli reappears due to the synthesis of ribosomal RNA (rRNA) and ribosomal DNA
(rDNA).
Cytokinesis

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 This occurs after every nuclear division. It includes separation of cytoplasm and
organelles in the two halves of the cell.
 Thus, the two daughter cells formed have half the number of chromosomes and the
amount of nuclear DNA.
 Both the cells undergo divisions and give rise to four cells. These haploid cells are
arranged tetrahedrally and are collectively called tetrahedral tetrad.
 It usually send equal amounts to each daughter cells but sometimes division is highly
unequal (especially in egg production).
Note:
Cytokinesis may or may not occur between meiotic divisions, however, in most of the
organisms it is of rare occurrence after Mitosis and Meiosis-ll.
Significance of Meiosis-ll
(i) It maintains the same chromosome number in the sexually reproducing organisms.
From a diploid cell, haploid gametes are produced which in turn fuse to form a diploid
cell. Haploid gametes are formed due to reduction of chromosomes to its half.
(ii) It restricts the multiplication of chromosome number and maintains the stability of the
species.
(iii) Maternal and paternal genes get exchanged during crossing over. It results in
variations among the offspring.
(iv) All the four chromatids of a homologous pair of chromosomes segregate and go over
separately to four different daughter cells. This leads to variation in the daughter cells
genetically.
(v) Paternal and maternal chromosomes assort independendy. Thus, cause reshuffling of
chromosomes and traits controlled by them.
Differences between Mitosis and Meiosis

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III. Free cell formation:
This type of cell division is characteristic of endosperm tissue or cells and is also
found in development of gametophytes and embryos in gymnosperms.
In this method, single nucleus undergoes free nuclear divisions (not accompanied by
wall formation) forming a large number of free nuclei. After stoppage of nuclear division, wall
formation occurs from periphery towards centre (i.e., centripetal wall formation) and thus
forming a cellular tissue.
IV. Amitosis:
(A -not + mitos- thread), i.e., division of nucleus without recognizable or visible
chromosomes is called amitosis. It is also called direct nuclear division. Amitosis was first of
all observed by R. Remak (1855) in RBC of chick embryo.
In this type of cell division, usual features of mitosis like nuclear membrane
disappearance and spindle formation, etc., do not occur but nucleus divides by a constriction
into two unequal nuclei. Further this direct nuclear division may not be followed by wall
formation.
Amitosis is found in some lower algae, fungi (Thallophytes) and also in mature cells of
higher plants, which are about to disintegrate.
V. Budding:
This type of cell division is common in Saccharomyces (yeast). Here, small outgrowths
or protuberances (buds) arise from cell and part of nucleus after dividing by mitosis or

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amitosis migrate into these outgrowths, which now develop into new cells. These buds or
cells on separation lead independent life.
With regard to time and place of occurrence of meiosis in life cycle of an organism,
meiosis is of 3 types:
(a) Gametic or Terminal meiosis:
In some lower plants, many protozoans
and all animals, reduction division or meiosis
occurs during formation of gametes before
fertilization and this type of meiosis is called
gametic or terminal meiosis. This type of life
cycle having diploid adult and gametic meiosis is
called diplontic cycle.
(b) Zygotic or Initial meiosis:
In fungi, some algae and some protozoans,
meiosis occurs immediately after fertilization in
the zygote and thus the resulting adult
organisms are haploid. This meiosis is called
zygotic or initial. Such life cycle with haploid
adult and zygotic meiosis is called haplontic
cycle.
(c) Intermediate or Sporogenic meiosis:
In all higher plants and some lower plants,
meiosis occurs at time of spore formation (while
gametes are formed by mitosis) and this is called
sporogenic or intermediate meiosis.

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10 – cell cycle and cell division

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