Meiosis – The spice of life
Meiosis – The SPICE of life
Meiosis
• A cell division that separates homologous
pairs of chromosomes forming haploid
sex/germ cells
•Whaaaa?
• Let’s back it up a few steps 
Parents give you your genes
A. Gene – a unit of hereditary info consisting of a
specific nucleotide sequence of DNA, via a gamete
Genes are packaged into chromosomes.
B. A Karyotype
• is a display of all a
cell’s chromosome
pairs in order of
shape and size
C. Chromosomes come in pairs…in
somatic cells that is!
-Homologous chromosomes
mom
dad
(one from mom and one from
dad) have the same
-size
-centromere location
-shape
-gene type location (locus)
-humans have 46
chromosomes, or 23 pairs!
Typical Karyotypes
Normal Female Karyotype
Normal Male Karyotype
Autosomes 1-22
Sex chromosomes – 23
These are all double stranded chromosomes
These are all double stranded chromosomes
1. How many pairs of homologous chromosomes are found in a typical
human cell?
2. Which chromosome pair is not homologous in humans?
3. Why are chromosomes found in pairs? (hint – sexual reproduction)
4. What must happen to homolog chromosomes in cells used for
• This karyotype has 2 copies of every chromosome, and we call
that diploid (2n). The “n” stands for a set of chromosomes.
This means it came from a somatic (or body) cell.
Y
I
K
E
S
!
***ALMOST
ALL your cells
are diploid –
skin, muscle,
bone, fat,
nerve
• If two somatic cells fertilized each other, what would
the offspring’s karyotype look like, or what would the
chromosome number be?
What does meiosis do to the # of
chromosomes?
• It reduce the chromosome number to one
chromosome from each homologous pair! The
chromosome number is now haploid (n). The
type of cell formed is a gamete (aka germ, sex,
reproductive , sperm, egg)
• Humans – somatic cell = 46 chromosomes
• Humans – germ cell = ____
23 chromosomes
• When is a human cell’s ploidy (chromosome
set) restored to diploid?
Fertilization
There are 3 ways to define MEIOSIS
***Assume you start meiosis with a somatic cell***
1. What happens to homologous chromosomes?
Meiosis separates homologous pairs of chromosomes into new cells
2. What cell type is formed?
Meiosis makes gametes (sex, reproductive cells)
3. What happens to the ploidy/chromosome
number?
Meiosis reduces the diploid # (2n) to haploid # (n),
aka (reduction division)
II. Meiosis follows the cell cycle
• Interphase (G1, S, G2) followed by TWO cell
divisions – Meiosis I and Meiosis II.
• Don’t forget that chromosomes duplicate
during the S phase!
Overview of Meiosis
MEIOSIS II: Separates
sister chromatids
MEIOSIS I: Separates
homologous chromosomes
Prophase I
Metaphase I
Sister
Chiasmata
chromatids
Anaphase I
Telophase I
and
Cytokinesis
Prophase II
Metaphase II Anaphase II
Telophase II
and
Cytokinesis
Sister
chromatids
remain
attached
Spindle
Homologous
chromosomes
Homologous Cleavage
chromofurrow
somes
separate
Sister
chromatids
separate
Haploid
daughter
cells
forming
Microtubules
attached to kinetochore
Meiosis I = “reductional division” as
chromosome number goes from diploid
to haploid
Meiosis II = “equational division” as
sister chromatids separate (start
double-stranded, centromeres break
and become single-stranded)
A. Importance of Meiosis I
• In Prophase I of Meiosis I,
crossing over occurs. This
is when the doublestranded homologous
pairs connect and
exchange DNA (some of
mom’s genes go onto
dad’s chromosomes and
vice versa). This is also
known as recombination.
Crossing Over and Synapsis in Prophase I
1
3 Crossover – DNA exchanged
Paternal
sister
chromatids
2
2. Synapsis occurs – When
homologous pairs “associate”
(loosely bond together)
4
4. Chiasmata - points
where crossing over
occurred
B. Steps of Meiosis I
Prophase I
Unique Events:
1. Synapsis: Homologous Chromosomes Pair up
2. Crossing over: Homologous chromosomes exchange parts
Metaphase I
Homologous Chromosomes line up opposite one another
Centromeres are not lined up on the metaphase plate
(different arrangement than mitosis)
Anaphase I
Complete DS Chromosome moves the poles.
CENTROMERES DO NOT SPLIT
Telophase I
Cytokinesis
Meiosis II ***The two cells are haploid! But they
have double-stranded chromosomes!
Prophase II
May be skipped (human males)
Will not start until fertilization (human females)
Metaphase II
Chromosomes line up at equator
***Centromeres line up on the metaphase plate (like mitosis)
Anaphase II
CENTROMERES SPLIT
***Double Stranded chromosomes become single stranded
Telophase II
Cytokenesis
Maturation of gametes into 4 HAPLOID daughter cells
III. How Meiosis Increases
Genetic Variety in Offspring
A. Crossing over during Prophase 1 –
recombination of genes!
B. Independent Assortment of chromosomes
during Metaphase 1 & II.
-this means how they align on the ‘metaphase plate)
C. Random fertilization to form zygote
IV. Who does Meiosis?
1. Adults always diploid
2. Meiosis only used to
make gametes
1. Adults can be diploid or
haploid
1. Meiosis used to produce
haploid adult
2. Gametes made by mitosis
2. Gametes made by mitosis
3. Spores made by meiosis
3. Only a zygote is diploid
TAKE-HOME POINTS
Ѻ Sexual reproduction GREATLY INCREASES
genetic variation!
Ѻ Sexual reproduction is possible via meiosis,
formation of gamete cells with half the
chromosomes (haploid) than somatic cells
(diploid)
Crossing
over in
Prophase I
Slide 16
Independent Assortment of
Homologous Chromosomes
Slide 16
Meiosis Cell Cycle
Meiosis
Phase
Slide 10