MEIOTIC CELL DIVISION
Mr. Binu Babu Mrs. Jincy Ealias
MBA, M.Sc. (N) M.Sc. (N)
Asst. Professor Asst. Professor
Most cells of the human body undergo cell
division, the process by which cells
reproduce themselves.
• The two types of cell division—
– Somatic cell division (mitotic division or
mitosis)
– Reproductive cell division (meiotic division or
meiosis)
Somatic cell division
• A cell undergoes a nuclear division called
mitosis
• 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
produced.
• 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
egg).
• Gametes have half of the chromosomes.
• Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
• Meiosis is similar to mitosis with some
chromosomal differences.
Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
sperm
n=23
n=23
egg
2n=46
zygote
Spermatogenesis
n=23
human
sex cell
sperm
n=23
n=23
2n=46
haploid (n)
n=23
diploid (2n) n=23
n=23
meiosis I meiosis II
Interphase
• 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
not grow.
• Interphase consists of three phases
– G1:- it is the interval between the mitotic phase and the S
phase
– S or synthesis:- DNA replication occurs
– G2 is the interval between the S phase and the mitotic
phase
G0:- Cells that remain in G1 for a very long time, perhaps
destined never to divide again.
Meiosis I
Cell division that reduces the chromosome number
by one-half.
• Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I
Prophase 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).
Prophase I - Synapsis
Homologous chromosomes
sister chromatids sister chromatids
Tetrad
Homologous Chromosomes
• 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
homologues.
• 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
• 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
over.
Crossing Over - variation
nonsister chromatids Tetrad
chiasmata: site of variation
crossing over
Sex Chromosomes
XX chromosome - female XY chromosome - male
Metaphase I
• It is the shortest phase in meiosis
• Tetrads align on the metaphase plate.
Anaphase I
• 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.
Telophase I
• Each pole now has haploid set of chromosomes.
• Cytokinesis occurs and two haploid daughter cells
are formed.
Meiosis II
• 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
chromosomes.
• 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
mitotic spindle.
• This midpoint region is called the metaphase
plate.
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
chromosomes.
• 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
breaks up.
• After the completion of telophase - II
Cytokinesis occurs.
• After completion of meiosis – II four
haploid daughter cells produced.
• Gametes = sperm or egg
