CELL DIVISION- PART - 2 -MEIOSIS.pptx

CELL DIVISION – PART -2 (MEIOSIS)
DR. C. BEULAH JAYARANI
M.Sc., M.A, M.Ed, M.Phil (Edn), M.Phil (ZOO), NET, Ph.D
ASST. PROFESSOR,
LOYOLA COLLEGE OF EDUCATION, CHENNAI - 34

Cell Division…
 The process by which a cell
divides into two or more cells.
 Cell division is the means of
reproduction in organisms that
reproduce asexually, as by fission
or spore formation.
 In organisms that reproduce
sexually, cell division is the
source of all tissue growth and
repair.
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Types of cell division……
• Prokaryotes (bacteria)
undergo a vegetative cell
division known as binary
fission, where their
genetic material is
segregated equally into
two daughter cells.
CELL
DIVISION
1.
PROKARYOTE
2.
EUKARYOTES

Cell division in eukaryotes
Vegetative division Reproductive division
Each daughter cell
is genetically
identical to the
parent cell mitosis.
The number of chromosomes in the
daughter cells is reduced by half to
produce haploid gametes - meiosis

Meiotic division…
 Meiosis ( which means lessening) is a
specialized type of cell division that reduces
the chromosome number by half, creating
four haploid cells, each genetically distinct
from the parent cell that gave rise to them.
 This process occurs in all sexually reproducing
single-celled and multicellular eukaryotes,
including animals, plants, and fungi.
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Interphase…
oPrior to meiosis, all chromosomes are duplicated in
a process similar to chromosome duplication prior
to mitosis.
oOutside the nucleus of animal cells have two
centrosomes, each containing a pair of
centrioles.
oThe two centrosomes are produced by the
duplication of a single centrosome during pre
meiotic interphase.

• The chromosomes duplicated coil and become shorter
and thicker and visible under the light microscope.
• The duplicated homologous chromosomes pair, and
crossing-over - genetic recombination (the physical
exchange of chromosome parts) occurs.
• Nucleolus disappears & the nuclear envelope
disappears at the end of prophase I, allowing the
spindle to enter the nucleus.
• In the cytoplasm, the meiotic spindle, consisting of
microtubules and other proteins forms between the
two pairs of centrioles as they migrate to opposite
poles of the cell.
• Prophase I is the longest phase of meiosis, typically
consuming 90% of the time for the two divisions.
PROPHASE - I
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Dr. C. Beulah Jayarani 8

• The centrioles are at opposite poles of the cell.
• The pairs of homologous chromosomes (the
bivalents), now as tightly coiled and
condensed as they will be in meiosis, become
arranged on a plane equidistant from the
poles called the metaphase plate.
• Spindle fibres from one pole of the cell attach
to one chromosome of each pair (seen as
sister chromatids), and spindle fibres from the
opposite pole attach to the homologous
chromosome (again, seen as sister
chromatids).
METAPHASE - I

ANAPHASE - I
• Anaphase I begins when the two chromosomes
of each bivalent (tetrad) separate and start
moving toward opposite poles of the cell as a
result of the action of the spindle.
• Notice that in anaphase I the sister chromatids
remain attached at their centromeres and move
together toward the poles.
• A key difference between mitosis and meiosis
is that sister chromatids remain joined after
metaphase in meiosis I, whereas in mitosis they
they separate
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• The homologous chromosome pairs complete their
migration to the two poles as a result of the action of
the spindle.
• Now a haploid set of chromosomes is at each pole,
with each chromosome still having two chromatids.
• A nuclear envelope reforms around each
chromosome set, the spindle disappears, and
cytokinesis follows.
• In animal cells, cytokinesis involves the formation of a
cleavage furrow, resulting in the pinching of the cell
into two cells.
• After cytokinesis, each of the two progeny cells has a
nucleus with a haploid set of replicated
TELOPHASE - I

Prophase ii
• While chromosome duplication took place prior to
meiosis I, no new chromosome replication occurs
before meiosis II.
• The centrioles duplicate.
• This occurs by separation of the two members of
the pair, and then the formation of a daughter
centriole perpendicular to each original
centriole.
• The two pairs of centrioles separate into two
centrosomes.
• The nuclear envelope breaks down, and the spindle
apparatus forms.
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METAPHASE II  Metaphase II is the second
stage in meiosis II.
 The cell is in metaphase II when
the chromosomes align
themselves along
the metaphase plate through
facilitation of the spindle fibers.
 The spindle fibers are now
attached to the two
contained in the centromere of
each chromosome.

ANAPHASE II
The genetic structures in
cells composed of
condensed DNA ,which
contain the genetic code
for an organism & move
apart to opposite ends

TELOPHASE II  In telophase II, nuclear
membranes form around each
set of chromosomes, and the
chromosomes decondense.
 Cytokinesis splits the
chromosome sets into new
cells, forming the final
products of meiosis:
 four haploid cells in which
each chromosome has just
one chromatid.

CONCLUSION…
In humans, meiosis produces genetically different haploid daughter cells, each with 23
chromosomes that consist of one chromatid.
These haploid cells become unfertilized eggs in females and sperm in males.
The genetic differences ensure siblings of the same parents are never entirely genetically
identical.
Meiosis is the process by which sexually reproducing organisms make their sex cells, sperm and
eggs.
During meiosis, a specialized cell called a germ cell splits to make four new sex cells, each with half
the number of chromosomes as the original germ cell.
Each of the four sex cells has a unique combination of DNA, meaning no two sperm or
eggs are genetically identical.
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REFERENCE
“SCIENCE READER”
Tamil Nadu text book
NCERT Books
Google Images
1/25/2024 Dr. C. Beulah Jayarani 17

CELL DIVISION- PART - 2 -MEIOSIS.pptx

CELL DIVISION- PART - 2 -MEIOSIS.pptx

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CELL DIVISION- PART - 2 -MEIOSIS.pptx