Meiosis

1. PRESENTATION ON
MEIOSIS
Presented To
Dr. K. K. Panigrahi
Presented By
Sairendri Mishra(16C/15)

2. MEIOSIS
 INTRODUCTION:
 The form of cell division by which GAMETES, with
HALF the number of CHROMOSOMES, are
produced.
 DIPLOID (2n)  HAPLOID (n)( Reductional division)
 Meiosis is SEXUAL reproduction.
 TWO divisions (MEIOSIS I and MEIOSIS II).
 It maintain the constancy in the cromosome number from
generation after generation.
 In the absence of meiotic cell division, the chromosome
number of a species would be doubled in every generation,
due to the fussion of male and female gametes, an
impossible biological situation .

3. PRE-MEIOTICINTERPHASE
 CHROMOSOMES (DNA)replicate in the S phase
 Each duplicated chromosome consist of two identical SISTER
CHROMATIDS attached at theirCENTROMERES.
 CENTRIOLE pairs also replicate.
 Nucleus and nucleolus visible

4. FIRST MEIOTIC DIVISION(MEIOSIS-1)
Cell division that reduces the chromosome
number by one-half.
Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I

5. PROPHASE -1
Longest and most complex phase (90%).
 It consists of 5 substages, those are
i. Leptotene
ii. Zygotene
iii. Pachytene
iv. Diplotene
v. Diakinesis

6. LEPTOTENE
 i. There is a marked increased in the nuclear
volume
ii. There is chromosome condensation so that
they become visibile as fine threads like a
ball of knitting wool. Each chromosome
of two chromatids

7. ZYGOTENE
 zygotene is the second stage of prophase-1
 A diploid cell contains 2 sets of crhomisomes. The 2 chromosomes
similar in size,form,structure are called homologous chromosomes.one
of the homologous chromosome is parental & other one is maternal.
 During zygotene, these htwo homologous chromosomeologous
chromosome stars pairing together.these homologous chromosome come
to lie side by side in pairs &this pairing is called synapsis.the complex
formed by apair of synapsed chromosome is called bivelent.
 Electron microscope studies of this stage shows that synapsis is
accompanied by a structure called synaptonemal complex which is
thought to stabilise the 2 homologous chromosomes till the crossing over
is completed.

8. HOMOLOGOUS CHROMOSOMES
eye color
locus
eye color
locus
hair color
locus
hair color
locus

9. PACHYTENE
 In this stage the synapsed chromosome become thick &short .the
chromatids of the homologous chromosomes now become clearly visible
as tetrad.
 During pachytene crossing over takes place between non-sister
chromatid of homologous chromosomes.The exchange of genetic
material between non-sister chromatid of homologous chromosomes is
called crossing over
 Crossing over leads to recombination of genetic material which involves
a mutual exchange of the corresponding segments of non-sister
chromatids of homologous chromosomes. It takes place by breakage and
reunion of chromatid segments.
 The site where crossing over occures forms a recombination nodule. The
recombination is an enzyme mediated process. An enzyme called
recombinase is involved during this phase.
 By the end of pachytene , the recombination between the homologous
chromosome is complete & the two chromatids are linked at the site of
crossing over.

10. DIPLOTENE
 During this stage the crossing over is completed and the two hmologous
chromosomes begins to seperated from each other. The following events
occur during this phase.
 The synptonemal complex formed during the zygotene dissolves during
diplotene.thereore te homologous chromosomes separate except in region
of co.
 The point of attachment between the homologous chromosomes after
dissolution of the synaptonemal complex is called chiasmata. It becomes
visible during diplotene stage. The two homologous chromosomes begin
to separate from each other but remain attach at the chiasmata.
Chiasmata mark the sites where co. occur during pachytene. The
homologous chromosomes do not separate at chiasmata and hence tere
seen as X shaped structure

11. DIAKINESIS
 It represent transition to metaphase-1.The following event occur during
diakinesis.
 Spindle assembles to prepare homologous chromosomes for separation. It
assembles at the poles, so that it separates the homologous chromosomes
into two cells.
 Terminalisation of chiasmata: due to tight condensation of chromosomes ,
the chiasmata disappear from the chromosomes by slipping off or sliding
from the tip of the chromosomes & from there it either slips off or remain
at the tip.
 Nucleolus disappears & the nuclear envelop disintegrates.

13. METAPHASE-1
i. Bivalents are arranged at the metaphase plate
ii. Centomeres of the two homologues of each bivalent
on the either side of the equatorial plate.
iii. Metaphase terminates as soon as homolgous
chromosomes begin to separate from each other and
migrate to opposite poles of the cell.

14. ANAPHASE-1i. Separation of the two homologous chromosomes of each bivalent marks
the beginning of anaphase stage.
ii. One chromosome from each bivalent begins to migrate to one pole,
the other migrates to the opposite pole.
As a result the numbner of chromosomes at each pole is exactly half (h)
each pole receives one homologue from each of the bivalents present in
cell. Thus the reduction in chromosomes number is not only a
one but a qualitative one as well. Thus at the end of AI, the
present is somatic cells are effectively and precisely separated into two
identical groups.

15. TELOPHASE-1
i. The chromosomes uncoil only partially.
ii. Nuclear envelope becomes organized around the two groups of
chromosomes.
iii. Nucleolous also reappears

16. CYTOKINESIS
 The cytoplasm of each cell divides into two halves, with a single
haploid nuclear in each half. The two halves of each cell do not
separate, but they stay together, and this two celled structure is
known as dyad.

18. INTERKINESIS OR INTRAMEIOTOC INTERPHASE:
Interkinesis is a metabolic phase between
telophase-1 & prophase-2.
During this phase chromosomes are elongated but
do not form chromatin fibres.
There is no replication of DNA during this phase ,
but centrioles pairs replicate in animal cell.
The RNA and protein required during meiosis-11
are syntesised during this phase.

19. MEIOSIS -2
 During Meiosis II, two sister chromotids of each chromosome separate
and migrate to the opposite pole. As a result, the number of chromosomes
in each of the two haploid nuclei remains the same (i.e haploid)., at the
end of this division. The second division of meiosis is often referred to as
equational division. Sometimes, it is called as 'Meiotic Mitosis'. The
second meiotic division is also divided into four stages.
 i. Prophase II
 ii. Metaphase II
 iii. Anaphase II and
 iv. Telophase II
 The 4 stages of meiosis –II is same as mitosis .

20. CYTOKINESIS;
Dyad divides into two parts. One parent cell
produces from haploid daughter cells after meiosis.
 The four daughter cells present together and are
known as tetrad.

22. THANK YOU