10.2 – Meiosis
Sexual Reproduction
Function: creates gametes
(sperm, egg, pollen) for
sexual reproduction.
Meiosis: Cell division for sexual reproduction.
Males - testes
Sex Cells
Females - ovaries
Homologous Chromosomes
1. A normal human body cell contains 46
chromosomes in 23 pairs
a) 23 from mother; 23 from father
2. Each pair of chromosomes are called
HOMOLOGOUS CHROMOSOMES
3. In a homologous pair, one chromosome is
from the mother and one is from the father
Haploid vs. Diploid
1. Haploid cell – a cell with only one copy of
each chromosome (23 total in humans)
a) ½ the number of chromosomes as the parent or
normal body cells.
b) n  haploid; n = 23
2. Diploid cell – a cell with 2 copies of each
chromosome (46 total; 23 pairs in humans)
a) 2n  diploid; 2n = 46
Diploid
1. All body cells are diploid (2n)
a) Skin, liver, bone, lung, etc.
i. Mother  23 chromosomes
ii. Father  23 chromosomes
46
chromosomes
total
2. Body cells are called SOMATIC cells
3. Diploid body cells divide via mitosis to
form two identical diploid daughter cells
Haploid
1. Sex cells are called GAMETES (sperm and egg)
a) Gametes join to form a ZYGOTE
i. sperm + egg = zygote
2. Gametes are haploid (n)
a) Sperm = n
b) Egg = n
3. n + n = 2n
Meiosis
1. Meiosis is the process of making haploid
gametes from a diploid cell
a) Each haploid gamete will have HALF the
genetic material as the diploid parent cell
b) 2n  n
Meiosis
1. Meiosis happens in two stages:
a) Meiosis I
b) Meiosis II
2. There are two cell divisions
a) 4 gametes are created
Phases of Meiosis
Interphase
1st Division
2nd Division
1. Prophase I
1. Prophase II
2. Metaphase I
2. Metaphase II
3. Anaphase I
3. Anaphase II
4. Telophase I
4. Telophase II
Cytokinesis
Chromatin
Interphase
1. During Interphase:
a) Cell grows in size
b) DNA is duplicated
Nuclear membrane
Prophase I
1. During Prophase I:
a) Chromatin condenses into chromosomes
b) Nuclear membrane dissolves
c) Spindle forms
d) Homologous chromosomes pair up and
CROSSING OVER occurs
2. Crossing Over: homologous chromosomes pair up
and randomly exchange genetic information
during Prophase 1
Mistakes in Meiosis
Genetic Recombination:
AKA Crossing Over
1. Occurs during meiosis,
chromosomes actually cross
over each other, genetic
material gets switched!
2. This leads to genetic
variability!
3. Can be an endless number of
different chromosomes possible!
Homologous
Chromosomes
(Crossing Over)
Spindle
Metaphase I
1. During Metaphase I:
a) Homologous chromosomes are pulled to
the middle of the cell by the spindle
Spindle
Homologous
Chromosomes
Independent Assortment
1. Chromosomes line up randomly
during metaphase
2. There is a 50% chance of the
maternal or paternal chromosomes
going to either side of the cell
= FATHER
= MOTHER
Anaphase I
1. During Anaphase I:
a) Homologous chromosomes are pulled towards
opposite ends of the cell by the spindle
Spindle
Chromosomes
Telophase I
1. During Telophase I:
a) Nuclear envelope reforms
b) Spindle dissolves
c) Cell stretches and cytoplasm divides
(cytokinesis)
Chromosomes
NO Interphase!
1. There is NO Interphase between
Meiosis I and Meiosis II
2. Each gamete must receive HALF of
the parent’s genetic material so no
duplication is necessary
Prophase II
1. During Prophase II:
a) Nuclear
membrane
dissolves
b) Spindle re-forms
2. Meiosis II is the
division of the two
diploid cells
resulting from
Meiosis I
a) 4 haploid cells
will be created
Spindle
Chromosomes
Metaphase II
1. During Metaphase II:
a) Chromosomes are pulled to the
middle of the cell by the
spindle
Spindle
Chromosomes
Anaphase II
1. During Anaphase II:
a) Sister chromatids are separated
and pulled to opposite ends of the
cell by the spindle
Chromosomes
Spindle
Telophase II
1. During Telophase II
a)Nuclear membrane re-forms
b)Spindle dissolves
c) Cells stretch and prepare for
cytokinesis
Chromosomes
Cytokinesis
1. The cytoplasm divides resulting in a
total of 4 unique haploid cells
2. All gametes are different because of
crossing over during Prophase I
and independent assortment during
Metaphase I & II
Meiosis: IPMAT cytokinesis; 2nd phase of PMAT
producing 4 (haploid-single) daughter cells
I
II
Prophase
Synapsis occurs and
crossing over of
chromosomes
same
Metaphase
Pairs of chromosomes line
in the plates
2 celled-pair of
chromosomes line in the
plates
Anaphase
Homologous pairs separate
Sister chromatids separates
Telophase
Division occurs
Division occurs
Genetic Diversity
1. Meiosis increases genetic diversity because
every sperm or egg is a little bit different. No
two offspring are exactly the same
a) Crossing Over
b) Independent Assortment
2. Genetic diversity causes some organisms to be
more fit to survive and reproduce (evolution)
How does meiosis create
genetic diversity?
1) crossing over
2) random assortment of chromosomes
and later …
3) random pairing of gametes
Meiosis and Gametes
Nondisjunction
1. Nondisjunction - an error in Meiosis when
homologous chromosomes fail to separate
properly during Anaphase I or II
2. The resulting gametes will have an extra
chromosome or be missing a chromosome
a) 3 copies instead of 2
b) 1 copy instead of 2
3. Most zygotes will not survive
Nondisjunction
during Meiosis I
Nondisjunction
during
Meiosis II
Non-Disjunction
1. In humans, a person with an extra 21st
chromosomes will have Down’s Syndrome
Down Syndrome
1. Trisomy – when an organism has 3 copies of a
chromosome where it should only have two
2. Trisomy is caused by nondisjunction
3. Trisomy of the 21st chromosome results in DOWN
SYNDROME
a) 3 copies of the 21st chromosome
Trisomy 21
causes Down
Syndrome
Mitosis
vs.
1. Makes EXACT copies
a) No new genetic
diversity
2. 2n  2n (diploid 
diploid)
3. Happens in body
(somatic) cells
4. One cell division
5. Asexual
Meiosis
1. Makes UNIQUE
gametes
a) Increases genetic
diversity
2. 2n  n (diploid 
haploid)
3. Happens in gamete
producing cells
(GONADS: testes and
ovaries)
4. Two cell divisions
5. Sexual
Diploid Cell – full
set of
chromosomes
= Sexual
Reproduction
Same as Mitosis
Haploid Cells –
½ set of
chromosomes
Also called
Gametes
Ex. Sperm, egg,
pollen
2 daughter cells are identical to parent
cell.
Chromosomes do NOT duplicate here.
Each cell divides- chromosomes
separating randomly!
The Result: 4 daughter cells
with ½ the number of
chromosomes
needed for an individual.
Sperm
are
created
Eggs are
created
Resulting Cell when 2
gametes fuse = Fertilization
Mitosis vs. Meiosis
Mitosis
Function
Type of Reproduction
# of Cells Produced
Type of Cell Produced
# of Cellular Divisions
# of Chromosomes in
Daughter cells
Meiosis
Mitosis vs. Meiosis
Function
Type of Reproduction
Mitosis
Meiosis
Create new cells for Create gametes for
growth & replace
sexual
old cells
reproduction
Asexual
Sexual
# of Cells Produced
Type of Cell Produced
2
Diploid
4
Haploid
# of Cellular Divisions
1
2
# of Chromosomes in
Daughter cells
Same as parent cell
½ as parent cell
somatic cell - undergoes mitosis, non-sex cell, diploid, identical with the parent
germ cell - undergoes meiosis, sex-cell, haploid, non-identical with the parent,
brings variety of genetic information
A germ cell is a reproductive cell, like an ovum, pollen, sperm, or spore. It is
made in special tissues and contains the haploid chromosome number, which,
in humans, is 23.
"Somatic cells" includes all the other cells; somatic cells are "body cells"
(SOMA=body). They are diploid; human somatic cells have 46 chromosomes