The Other Cell Division:
Making Sex Cells
Meiosis – A Source of Distinction
Ever wonder why you don’t look exactly like
either your mother or father? Or why you
and your siblings are not identical?
It’s all in MEIOSIS!
2 Major Roles of Meiosis
1. Make GAMETES (egg and sperm);
diploid (2n) cells make haploid (n)
cells
2. Genetic Variation
GAMETES
Meiosis takes a cell with two copies of every
chromosome (diploid) and makes cells with
a single copy of every chromosome
(haploid).
This change (diploid  haploid) is critical if
two gametes combine to make a new
individual
In meiosis, one diploid cells produces four
haploid cells.
Genetic Variation
Meiosis scrambles the specific forms
of each gene that each sex cell (egg
or sperm) receives.
Increases genetic diversity
(accomplished through independent
assortment and crossing-over).
Genetic diversity is important for the
evolution of populations and species.
Meiosis
Parent cell –
chromosome pair
Chromosomes
copied
1st division - pairs split
2nd division – produces
4 gamete cells with ½
the original no. of
chromosomes
Homologous Chromosomes
Homologous
Chromosomes are:
1. The same size
1. The same shape
1. Have the same
genes
1. Different forms of
gene
Meiosis I : Separates
Homologous Chromosomes
● Interphase
● Each of the chromosomes
replicate
● The result is two genetically
identical sister chromatids
which remain attached at their
centromeres
Prophase I
● Each pair of sister chromatids move to
their homologous pair and join together
(synapsis) in a group of four called a
tetrad.
● This is when crossing over can occur.
● Crossing Over is the exchange of
segments during synapsis.
Metaphase I
● The chromosomes line up at the equator
(metaphase 1 plate) attached by their
centromeres to spindle fibers from
centrioles.
● Still in homologous pairs
Anaphase I
● The spindle guides the movement
of the chromosomes toward the
poles
● Sister chromatids remain
attached
● Move as a unit towards the same
pole
● The homologous chromosomes are
separated to opposite poles
Telophase I
● This is the end of Meiosis I.
● The cytoplasm divides, forming two
new daughter cells.
● Each of the newly formed cells has half
the number of the parent cell’s
chromosomes (23 unique chromosomes)
● but each chromosome is already
replicated (sister chromatids) and the
daughter cells are ready for Meiosis II
Cytokinesis
● Occurs simultaneously with Telophase I
● Forms 2 daughter cells
● Plant cells – cell plate
● Animal cells – cleavage furrows
● NO FURTHER REPLICATION OF GENETIC
MATERIAL (S Phase) PRIOR TO THE
SECOND DIVISION OF MEIOSIS
Meiosis II : Separates sister
chromatids
●There is no Interphase.
●Results in 4 haploid daughter cells
(gametes)
Prophase II
●Each of the daughter cells forms
a spindle, and the sister
chromatids move toward the
equator
Metaphase II
●The chromosomes are positioned
on the metaphase plate in a
mitosis-like fashion
Anaphase II
● The sister chromatids finally
separate
● The sister chromatids of each pair
move toward opposite poles
● Now individual chromosomes, we
no longer call them chromatids!
Telophase II and Cytokinesis
● Nuclei form at opposite poles of the
cell and cytokinesis occurs
● After completion of cytokinesis there
are four daughter cells
● All are haploid (n)
● This is the whole point of meiosis!
Figure 13.7 The stages of meiotic cell division: Meiosis II
One Way Meiosis Makes
Lots of Different Sex Cells
(Gametes) – Independent
Assortment
Independent assortment
produces 2n distinct gametes,
where n = the number of unique
chromosomes.
In humans, n = 23 and 223 =
8388608.
That’s a lot of diversity by this
mechanism alone.
Another Way Meiosis Makes Lots of Different
Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gamete
types produced by independent assortment.
The Key Difference Between Mitosis and Meiosis is
the Way Chromosomes Uniquely Pair and Align in
Meiosis
Mitosis
The first (and
distinguishing)
division of meiosis