MEIOTIC CELL DIVISION
Mr. Binu Babu
MBA, M.Sc. (N)
Mrs. Jincy Ealias
Most cells of the human body undergo cell
division, the process by which cells
• The two types of cell division—
– Somatic cell division (mitotic division or
– Reproductive cell division (meiotic division or
Somatic cell division
• A cell undergoes a nuclear division called
• A cytoplasmic division called cytokinesis
• This two process produce two genetically
identical cells, each with the same number and
kind of chromosomes as the original cell.
• Somatic cell division replaces dead or injured
cells and adds new ones during tissue growth.
Reproductive cell division
• It is the mechanism that produces gametes,
the cells needed to form the next generation
of sexually reproducing organisms.
• This process consists of a special two step
division called meiosis in which the
number of chromosomes in the nucleus is
reduced by half.
Meiosis is a form of cell division by which gametes,
with half the number of chromosomes, are
• Diploid (2n) haploid (n)
• Meiosis is sexual reproduction. It is a two
divisions process (meiosis I and meiosis II).
• Sex cells divide to produce gametes (sperm or
• Gametes have half of the chromosomes.
• Occurs only in gonads (testes or ovaries).
• Meiosis is similar to mitosis with some
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
• During interphase the cell replicates its DNA, it also
produces additional organelles and cytosolic components in
anticipation of cell division.
• Interphase is a state of high metabolic activity but cell does
• Interphase consists of three phases
– G1:- it is the interval between the mitotic phase and the S
– S or synthesis:- DNA replication occurs
– G2 is the interval between the S phase and the mitotic
G0:- Cells that remain in G1 for a very long time, perhaps
destined never to divide again.
Cell division that reduces the chromosome number
• Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I
It is the longest and most complex phase (90%)
in meiosis. In this phase
• The chromosomes condense.
• Synapsis occurs: homologous chromosomes come
together to form a tetrad.
• Tetrad is two chromosomes or four chromatids
(sister and nonsister chromatids).
• Pair of chromosomes (maternal and paternal) that are
similar in shape and size.
• Homologous pairs (tetrads) carry genes controlling the
same inherited traits.
• Each locus (position of a gene) is in the same position on
• Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 01 pair of sex chromosomes
• Karyotype:- A method of organizing the chromosomes
of a cell in relation to number, size, and type.
• Crossing over (variation) may occur between non
sister chromatids at the chiasmata.
• Crossing over: segments of non sister chromatids
break and reattach to the other chromatid.
• Chiasmata (chiasma) are the sites of crossing
Crossing Over - variation
chiasmata: site of
XX chromosome - female XY chromosome - male
• It is the shortest phase in meiosis
• Tetrads align on the metaphase plate.
• The members of each homologous pair of
chromosomes separate as they are pulled to
opposite poles of the cell
• The paired chromatids, held by a
centromere, remain together.
• Each pole now has haploid set of chromosomes.
• Cytokinesis occurs and two haploid daughter cells
• No interphase II
(or very short - no more DNA replication)
• Meiosis II is similar to mitosis
Prophase - II
• During early prophase - II, the chromatin
fibers condense and shorten into
• The condensation process may prevent
entangling of the long DNA strands as they
move during mitosis.
Metaphase - II
• During this phase, the microtubules of the
mitotic spindle align the centromeres of the
chromatid pairs at the exact center of the
• This midpoint region is called the metaphase
Anaphase - II
• During this phase, the centromeres split,
separating the two members of each
chromatid pair, which move toward
opposite poles of the cell.
• Once separated, the chromatids are termed
• As the chromosomes are pulled by the
microtubules of the mitotic spindle during
anaphase, they appear V-shaped.
Telophase - II
• The final stage of meosis II, telophase II,
begins after chromosomal movement stops.
• The identical sets of chromosomes, now at
opposite poles of the cell, uncoil and revert
to the threadlike chromatin form.
• A nuclear envelope forms around each
chromatin mass, nucleoli reappear in the
identical nuclei, and the mitotic spindle
• After the completion of telophase - II
• After completion of meiosis – II four
haploid daughter cells produced.
• Gametes = sperm or egg