- Meiosis is a type of cell division that produces gametes, such as sperm or egg cells, with half the normal number of chromosomes. It involves two rounds of division instead of one, resulting in four daughter cells from one parent cell. - During meiosis I, homologous chromosome pairs align and separate, reducing the chromosome number by half. Meiosis II is similar to mitosis, where sister chromatids separate to produce four haploid gametes. This ensures genetic variation between offspring and maintains the chromosome number across generations.

2.  Multicellular organisms grow by the addition
of new cells
 New cells arise by the division of pre-existing
cells (Virchow 1858)
 Duplication of DNA occur well before cell
division begins
 The mode of cell division is fundamentally
similar in all organisms
 This shows unity of life

3. A somatic cell exists in two main states
INTERPHASE
A long un-dividing state
MITOTIC PHASE (M PHASE)
A short dividing state

4.  The cell grows by synthesizing biological
molecules such as carbohydrates, lipids,
proteins and nucleic acids
 It lasts for 10-30 hours
 It is divided into 3 periods
First gap phase
G1
Synthetic phase
S
Second gap
G2

6.  Gap between previous mitosis and beginning
of DNA synthesis
 Carbohydrates, lipids, proteins and RNA are
synthesized
 No change occur in the DNA content of the
cell
 Muscle and nerve cells never divide again and
remain in G0 phase

7.  Duplication of each chromosome by
replication of new DNA molecule
 Each chromosome now consists of two
identical sister chromatids
 A diploid (2n) cell thus become tetraploid (4n)
 Synthesis of histone proteins, mRNAs and
new nucleosomes also occur in this phase
 This lasts for 6-8 hours

8.  A gap between DNA synthesis and nuclear
division
 RNA transcription and protein synthesis
continue
 Centrioles, mitochondria and golgi apparatus
are doubled
 Proteins for spindle and asters are
synthesized
 This lasts for 2-5 hours

9.  It is aimed at orderly distribution of the already
duplicated chromosomes to the daughter cells
 Many structural and physiological changes occur
in the cell during mitosis
 Nuclear envelope breaks down and chromatin is
packed into visible chromosomes
 ER and Golgi apparatus breakdown into small
vesicles
 Microtubules dissociate into tubulin dimers
 Cell activities like gene expression, protein
synthesis, secretion and cell motility, stops
 Cell’s entire attention is devoted to the process
of division

10. Cell division occur in 3 ways
Amitosis
Mitosis Meiosis

11.  First described by Robert Remak in RBC of chick
embryo
 It occurs without the formation of spindle and
appearance of chromosomes
 The nucleus elongates and develops a
constriction round its middle
 The constriction gradually deepens and cuts the
nucleus into 2 daughter nuclei
 Similar constriction occur in the cytoplasm
between two nuclei and divide the cell into two

12.  Amitosis is rare
 It occurs in
 Mammalian cartilage
 Degenerating cells of
diseased tissue
 Foetal membranes

13.  A common method of cell division in
eukaryotes
 Occurs in somatic cells
 Lasts for 30 minutes to 3 hours

14. Mitosis
Karyokinesis Cytokinesis
Prophase AnaphaseMetaphase Telophase

15. Early prophase
 The cell becomes more or less rounded and
cytoplasm turns more viscous
 Short radiating microtubules assemble
around centrioles
 Two pairs of centrioles of start moving to the
opposite ends of the cell
 Microtubules called astral rays are not in
contact with centrioles

16.  Between the asters,
long microtubules
assemble on one
sideof the nucleus
called mitotic spindle
 Chromosomes first
appear as long thin
thread gradually
change into short thick
rodlets
 Chromosomes are fully
replicated at all points
along their lenght

17.  Chromosomes finally assume their
characteristics forms and sizes
 Nucleoli become smaller and finally
disappear and nucleolar materials are
dispersed into nucleoplasm
 Nuclear envelope begins to breakdown into
small vesicles which disperse into cytoplasm
Middle prophase

18.  Chromosomes and
other nuclear content
are released into the
cytoplasm
 Centriole pairs are
pushed into the
opposite ends of cells
by growing spindles
 Spindle and asters are
together called as
mitotic apparatus
Late prophase

19.  Lasts for 2-10 minutes
 Chromosomes move to
the equatorial plane of
spindle
 Chromosomes soon
get aligned at the
middle of the spindle in
the form of plate called
metaphase plate
 the Chromosomes are
balanced at the
metaphase plate by
two chromosomal
fibres that connect the
sister kinetochores to
the opposite poles

20.  Sister chromatids of each chromosome
slightly separate at the primary constriction
so that their kinetochores stretch towards the
opposite poles of the spindle
 Chromosomes are pulled intoV J I shapes
depending upon the position of kinetochores
 Anaphase ends when all the chromatids
reach the opposite poles

21.  Movement of
chromosomes is called
anaphase A
 Extension of poles is
called anaphase B
 Chromosomal
microtubules generate
the force for poleward
movements of
chromosomes

22.  Long and complex phase lasts for an hour
 Chromosomes at each pole unfold and long and
slender
 Finally they become undistinguishable as in
interphase cell
 Nuclear envelope is reconstructed around each
chromosome
 All the envelops fuse to form an envelope
around entire set of chromosomes
 Nucleolar material dispersed in the cytoplasm
return to nucleolar organizer and form nucleolus

23.  Spindle begin to
disappear by
depolymerisation
of microtubules
 Asters become
small till only short
microtubules are
left
 Centrioles take up
their characteristic
interphase position

24.  Two daughter nuclei formed in telophase are
identical
 The accuracy of karyokinesis depends upon
two features
 The arrangement of spindle microtubules to
form two distinct poles in the cell
 The connection of two chromatids of each
chromosome to the opposite poles of spindle
to ensure their delivery to opposite poles

25.  Cell typically divide by a process called
furrowing or cleavage
 Short spindle microtubule become structure
less material called mid body
 Midbody extends completely across the cell
 Then furrow appears in the plasma
membrane at the level of mid body
 With the contractile force, furrow gradually
deepens

26.  Membranes fuse
 Original cytoplasm and
two daughter nuclei
form two independent
daughter cells
 New cells are half the
size of mother cells
 These enter the G1
phase of next cell cycle

27.  It is confined to particular cell and takes place
at a particular time
 Cells of sexually reproducing organism
undergo meiosis
 It produces gametes in animals and spores in
higher plants
 The cells in which meiosis takes place are
called meiocytes (oocytes, spermatocytes,
sporocytes)

28.  Meiosis consists of two division that takes
place in the rapid succession with the
chromosomes replicating only once
 One parent cell produces four daughter cells
 Each having half number of chromosomes
and half amount of nuclear DNA
 It is known as reduction division
 Two divisions are known as Meiosis I and
Meiosis II

29. It has four phases
 Prophase I
 Metaphase I
 Anaphase I
 Telophase I

30.  It is more complex than mitotic prophase because of
recombination that occurs in it
 It may extents over weeks, months or years
 It has five sub-stages

31.  Chromosomes appear
as thin thread by
condensation
 Each chromosome is
double consisting of
chromatids due to
DNA replication during
interphase
 Chromatids are closely
adhere together

32.  Homologous
chromosomes come to
lie side by side in pairs
 Pairing is called synapsis
 A regular space of 0.15to
0.2µm wide exists
between synapsed
homologous
chromosomes
 The space have highly
specialized fibriller
organelle synaptonemal
complex

33.  Chromatids of synapsed
chromosomes slightly separate
and become visible
 Chromosome with two
chromatids called dyad
 Group of four homologous
chromatids called tetrads
 Two chromatids of same
chromosome is called sister
chromatids
 Those of two homologous
chromosomes is called non
sister chromatids
 Crossing over occurs in this
stage
 It involves mutual exchange of
corresponding segments of non
sister chromatids of
homologous chromosomes

34.  Homologous
chromosomes
separate at different
places
 This is called
disjunction
 Chromosomes do not
separate at certain
points called
chaismata

35.  Chromosomes condense again into short
thick rodlets
 Centrioles move apart in pairs to the opposite
ends of the cell
 Asters form around each centriole
 Spindle develop between the centrioles
 Nucleolus disintegrate
 Nuclear envelope breaks down
 Tetrads are released in the cytoplasm

36.  Tetrads scattered in the
cytoplasm move to the
equator of spindle
 They align in two parallel
metaphase plate
 Each homologous
chromosome has two
kinetochores, both are
connected to same spindle
pole
 Two kinetochores of a
homologous chromsomes
act as a functional unit in
metaphase-1

37.  The homologous
chromosomes
separate, while sister
chromatids remain
associated at their
centromeres

38.  The nuclear membrane and nucleolus reappear,
cytokinesis follows and this is called as diad of
cells
 Although in many cases the chromosomes do
undergo some dispersion, they do not reach the
extremely extended state of the interphase
nucleus.
 The stage between the two meiotic divisions is
called interkinesis and is generally short lived.
 Interkinesis is followed by prophase II, a much
simpler prophase than prophase I.

40.  Prophase II
 Meiosis II is initiated immediately after
cytokinesis, usually before the chromosomes
have fully elongated.
 In contrast to meiosis I, meiosis II resembles a
normal mitosis.
 The nuclear membrane disappears by the end
of prophase II
 The chromosomes again become compact.

41.  Metaphase II: At this stage the chromosomes align at
the equator and the microtubules from opposite poles
of the spindle get attached to the kinetochores of
sister chromatids
 Anaphase II: It begins with the simultaneous splitting
of the centromere of each chromosome (which was
holding the sister chromatids together), allowing
them to move toward opposite poles of the cell
 Telophase II: Meiosis ends with telophase II, in which
the two groups of chromosomes once again get
enclosed by a nuclear envelope; cytokinesis follows
resulting in the formation of tetrad of cells that is four
haploid daughter cells

43. Meiosis is the mechanism by which conservation
of specific chromosome number of each species
is achieved across generations in sexually
reproducing organisms, even though the
process, per se, paradoxically, results in
reduction of chromosome number by half. It also
increases the genetic variability in the
population of organisms from one generation to
the next.Variations are very important for the
process of evolution.