Meiosis: Formation
of gametes
Utilizing
http://www.biology.com
BioCoach Activity
“Meiosis”
Introduction to Meiosis
Meiosis occurs only in sexually
reproducing organisms.
 In animals, where the somatic (body)
cells are diploid, the products of
meiosis are the gametes.
 Some plants, algae and fungi have
more complex cycles involving both
mitosis and meiosis.

Introduction to Meiosis

A two-step cell-division
process in sexually
reproducing eukaryotes.

Results in cells (typically
gametes) with one-half the
chromosome number of the
original parental cell.

One diploid cell (2N) results
in 4 haploid cells (N).
Introduction to Meiosis

Meiosis consists of two successive nuclear
divisions:
meiosis I
meiosis II

Each division consists of :
 Prophase
 Metaphase
 Anaphase
 Telophase
Premeiotic Interphase
in Animal Cells
All chromosomes are duplicated
 The two centriole are produced by the
duplication of a single centriole

Meiosis I: Prophase I

Chromosomes become visible as
tetrads


A tetrad is a pair of homologous
chromosomes (ex. both copies of chr #3)
Crossing-over occurs
 Homologous chromosomes
exchange portions of their
chromatids
 Produces new combinations of
alleles
Meiosis I: Prophase I



Nucleolus disappears
Meiotic spindle forms
Nuclear envelope disappears
Prophase I consumes
90% of the time for meiosis
Crossing Over During Meiosis

crossing-over results in a physical
exchange of equivalent segments of
maternal and paternal homologous
chromosomes
Crossing
Over
During
Meiosis
Prophase I
Meiotic prophase I
Meiosis I: Metaphase I



The centrioles are at opposite poles of
the cell
The pairs of chromosomes (tetrads)
are arranged along the center of cell
Spindle fibers are attached to one
chromosome of each pair
Metaphase I
Meiotic metaphase I
Meiosis I: Anaphase I

Chromosomes of each tetrad separate
and move to opposite poles
Note: Sister chromatids remain attached
and move together to the pole
Anaphase I
Meiotic anaphase I
Meiosis I: Telophase I
Homologous chromosome pairs reach
the poles of the cell
 Nuclear envelopes form around them
 Cytokinesis follows to produce two
cells

Telophase I
Meiotic telophase I
Product of Meiosis I
Two cells with a haploid number of chromosomes is
produced. Each cell has two copies of each
chromatid.
The two cells have sets of chromosomes that are
different from each other and from the parent cell
that entered meiosis.
Animation
Meiosis II
There is no interphase between the two
stages (chromosomes are not
duplicated).
Meiosis II: Prophase II
Centrioles duplicate
 Nuclear envelope breaks down
 Spindle apparatus forms

Prophase II
Meiotic prophase II
Comparing Prophase I
and Prophase II
Meiosis II: Metaphase II

Spindle apparatus attaches to each
chromosome

Single chromosomes align along
center of each cell
Metaphase II
Meiotic metaphase II
Comparing Metaphase I
and Metaphase II
Meiosis II: Anaphase II
Centromeres separate
 Sister chromatids — now individual
chromosomes — move toward the
opposite poles of the cell.

Anaphase II
Meiotic anaphase II
Comparing Anaphase I
and Anaphase II
Meiosis II: Telophase II
Nuclear envelope forms around each
set of chromosomes
 Cytokinesis occurs
 Four daughter cells are produced, each
with a haploid set of chromosomes.

Telophase II
Meiotic telophase II
Comparing Telophase I
and Telophase II
Product of Meiosis II
Four cells with a haploid number of chromosomes is
produced. Each cell has a single copy of each
chromatid.
The four cells have sets of chromosomes that are
genetically different from each other and from the
parent cell that entered meiosis.
Animation
Mitosis and Meiosis Reviewed
Compare meiosis to mitosis
How are they alike?
•In interphase prior to both,
replication of DNA occurs
•Both are methods of cell division
•Both include Prophase, Metaphase,
Anaphase and Telophase
•Cytokinesis occurs in both
Compare meiosis to mitosis
How are they different?
Meiosis
Two cell divisions
Sexual Reproduction
--makes gametes
Results in 4 haploid
cells
Mitosis
One cell division
Asexual reproduction
--makes body cells
Also for growth and
regeneration
Results in 2 diploid
cells
Compare meiosis to mitosis
How are they different?
Meiosis
Two parts - eight
phases
DNA in daughter cells
differs from parent
cell
Crossing over occurs
Mitosis
One part – four
phases
DNA in daughter cells
is same as parent cell
Crossing over
doesn’t occur
Compare anaphase in mitosis to
anaphase I in meiosis
Mitotic Anaphase
Meiotic Anaphase I
How are they different?