MEIOSIS
Prepared by:
NEMIA L. ALERTA
Biology Teacher
Janiuay National Comprehensive High School
Revie
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Chromosome
•Made up of DNA and proteins (histones)
•carries the genes and
•functions in the transmission of
hereditary information that controls
our hereditary characteristics
• In what stage of the cell cycle,
chromosomes becomes duplicated or
double stranded?
• Are there differences between two
sister chromatids?
– Chromosomes duplicate during the S-
phase of the interphase
– There is no differences in two sister
chromatids. Sister chromatids are
identical twin.
How will you describe homologous
chromosomes?
Homologous Chromosomes
• A pair of chromosomes that control the
same traits.
– One came from a male parent and the other
came from a female parent.
Homologous Chromosomes
• similar in structures, shape and size and control same
traits but not identical.
• For example, color of the eyes. Both father and mother have
genes for the color of eyes in chromosome. They may differ;
father might have genes for brown colored eyes while the
mother has a gene for black color.
• This pair of chromosomes that contains genes controlling the
same traits is called homologous chromosomes.
• Every species has a
characteristic number of
chromosomes;
Example:
• Humans have 23 pairs of chromosomes
• Total individual chromosomes = 46
(22 pairs of autosomal, or
nonsex,
chromosomes; and
one pair of sex
Example:
•One set from each parent.
• Father has one set (23 chromosomes)
• Mother has one set (23 chromosomes)
•Humans have 46 total number
of chromosomes arranged in
23 pairs in every cell.
In mitosis
• In every cell division, the cell produces
two identical cells with exactly the same
genetic materials.
• They have the same DNA / genes contained in
the chromosomes.
Meiosis produces gametes
or Sex cells
•Sex cells - contain half the number of
the species chromosome number
•Sex cells have one set of
chromosomes, thus are called
haploid (n)
•In humans, 23 chromosomes are
found in each sex cell.
In Human egg cell
23 chromosomes
In Human sperm cell
23 chromosomes
How does this happen?
From 2n to n?
•Meiosis is a special type of cell
division that produces gametes
(sex cells) with half of the
species chromosomes number.
Through FERTILIZATION
• the union of the egg
and sperm restores
the 2n number or
diploid number of
chromosomes.
• This results in a
zygote, the first cell
formed by fertilization
Comparison between Mitosis and
Meiosis
Description Mitosis Meiosis
No. of cell division 1 2
No. of cells produced 2 4
Chromosome number in 2 2
the original or mother
cell n n
Chromosome number of 2 N
daughter cells
n
Cells produced Body cells Sex cells
Lets take a look now on the
PROCESS OF
MEIOSIS
Definition of Meiosis
• a process of nuclear division;
• reduces the number of chromosomes by half
• produces haploid cells from diploid cells.
• Males – produces haploid sperms
• Females- produces haploid eggs
Meiosis:
• Has 2 cell divisions
– Meiosis I and Meiosis II
• The chromosome number is reduced to half (1/2)
• Crossing over and recombination of genes
happen.
• Four daughter cells are produced at the
end of meiosis II.
•Before meiosis occurs, the cell
has to undergo INTERPHASE
Cell at Interphase
2n =
4
Before meiosis begins,
genetic material is
duplicated.
Interphase
Like mitosis, the cell has to undergo G1 phase, S phase, and G2
phase.
Remember that in the S-phase, the DNA in the cell is exactly copied
or chromosomes resulting in two identical full sets of
chromosomes.
Outside of the nucleus, there are two centrosomes, each
containing a pair of centrioles, these structures are critical for the
process of meiosis in the animal cells destined to become sex
cells.
Meiosis I
• Centrioles move apart;
• spindle formation and nuclear
envelope breakdown occur as
in mitosis.
• Chromosomes become visible
• During early prophase I,
homologous chromosomes
paired and crossing over
occurs:
Major Events in Prophase I
• Chromosomes condense
• Synapsis occurs - Homologous chromosomes
come together or pair up and match up in
pairs (tetrads or bivalents).
– A pair of homologous chromosomes lines up
closely together) and a tetrad is formed. Each
tetrad is composed of four chromatids.
Crossing over occurs.
Homologous
chromosomes exchange
their genes resulting in
the recombination of
genes between two
homologous
• Pairs of homologous
chromosomes are arranged
at the metaphase plate or
at the center of the pole;
• Both chromatids of each
homologous chromosome
are attached to the spindle
fibers of opposite poles.
METAPHASE I
•Tetrads align at the
metaphase plate, not
individual chromosomes
Note that the centromeres of
homologous chromosomes are oriented
toward the opposite cell poles which are
attached to the spindle fibers.
•Homologous
chromosom
es
separate ;
• Double-stranded
chromosomes
move towards
opposite poles.
ANAPHASE I
• In anaphase I, the homologous
chromosomes separate,
• One of each pair travels to each of the
two poles of the cell,
• This reduces the chromosome number
from 2n to 1n.
• Note that the sister chromatids
stay together.
• Double-stranded
chromosomes reach the
opposite poles;
• When telophase I begins, each
half of the cell has a complete
haploid set of double-stranded
chromosomes. Each
chromosome is composed of
two sister chromatids.
• Cytokinesis (division of the
cytoplasm) usually co-occurs
with telophase I, forming two
haploid daughter cells.
• In animal cells like in the
picture, a cleavage furrow
forms. (in animal cells, a cell
plate forms.)
TELOPHASE I
•Two daughter cells are formed during
telophase I.
•Chromosomes are still double-stranded
•These usually go immediately into the
second cell division (Meiosis II) to
separate the sister chromatids to singe-
stranded chromosomes.
MEIOSIS II
Events are similar to
mitosis.
• Spindle fibers form
• Centrioles
move apart;
• Nuclear
membrane and
nucleolus
disappear
• The double-stranded
chromosomes line
up in the center of
the pole.
• Note: chromosome
number is haploid
but chromosomes
are still duplicated
Anaphase II
• The double-stranded chromosomes separate.
Telophase II
• Four cells with a haploid number of
are produced. All chromosomes are single-
chromosomes
stranded
Telophase II and cytokinesis
Comparison between Mitosis and
Meiosis
Description Mitosis Meiosis
No. of cell division 1 2
No. of cells produced 2 4
Chromosome number in 2 2
the original or mother
cell n n
Chromosome number of 2 N
daughter cells
n
Cells produced Body cells Sex cells
Sources of Genetic Variation
• As a result of meiosis followed by fertilization,
there are three sources of genetic
recombination:
1) Independent alignment of paired
chromosomes along the metaphase I plate
Sources of Genetic
Variations
2) Crossing-over during prophase I
3) Combining of chromosomes of
genetically different gametes
Identify the phases
1.
2.
3.
4.
5
.
6.
7.
8.
9.
10.
Clarify/explain
1. What are the major differences of mitosis
and meiosis based on your activity?
2. Why do you think daughter cells produced in
mitosis should have the same number of
chromosomes with the species chromosome
number (2n) or parent cell?
3. Why do you think daughter cells produced in
meiosis should have haploid chromosome (n)
number as compared to the species
chromosome number (2n) or parent cell?
Comparison of mitosis and meiosis
Comparison of mitosis and meiosis
Comparison of mitosis and meiosis
Comparison of mitosis and meiosis
Comparison of mitosis and meiosis
Elaborate/extend
4. Why do you think synapsis and crossing
over in prophase I of meiosis should occur?
5. Infer the role of mitosis and meiosis;
6. Infer the possible effects, if something
wrong with meiosis like non-disjunction of
one or more chromosomes in anaphase I or
II.
Generalizations:
• The differences in the behavior of the
chromosomes in the different phases of
mitosis and meiosis brought differences in
the chromosome number of their daughter
cells.
Generalizations:
• Mitosis produces two genetically identical
daughter cells with diploid chromosome
number which is genetically the same with the
parent cell.
• Mitosis enables cells to reproduce and
regenerate tissues in the body.
• Mitosis is responsible for the production of all
body cells that comprise an organism.
• Therefore, all body cells have the same
genetic material.
Generalizations:
• Meiosis involves two divisions giving rise to four daughter
cells with haploid chromosome numbers, each possessing a
unique set of genetic material.
• This is a result of the exchange of chromosome segments
during the process of crossing over in prophase I.
• Through meiosis, gametes are produced with haploid
chromosome numbers.
• Therefore, the species chromosome number is maintained
during fertilization and can remain the same from one
generation
next. to the
Gametogenesis
•Spermatogenesis
•Oogenesis
Spermatogenesis
Oogenesis
