Meiosis- The
Formation of Sex Cells
I. Introduction to Meiosis
A. Purpose - to make sex cells
for reproduction.
B. Why can’t mitosis do this?
1. Mitosis would make
too many chromosomes.
2. Ex. If a fruit fly has 4
chromosomes, each of its
children would need to have 4
chromosomes.
A. Mom and dad each give
4 chromosomes, offspring has
8 and would die.
3. The organism would
have double the number of
chromosomes which would
cause the organism to die.
4. Mom gives 2 and dad
gives 2, new organism has 4
just like parents.
C. Diploid Chromosome the total number of
chromosomes the
individual should have.
1. Ex. Human = 46
Ex. Lettuce = 18
2. Diploid number = 2n
D. Haploid Chromosome
Number - half the number of
chromosomes an organism
should have.
1. Ex. Human = 23
2. Ex. Lettuce = 9
3. Haploid Chromosome # = n
II. Workings of Meiosis
A. Meiosis is - the formation of
sex cells (eggs and sperm)
1. Gametes - another name
for sex cells.
A. Gametes have haploid
chromosome number
B. Meiosis is the making of
eggs and sperm with half
the number of
chromosomes as the parent
cell.
1. Female egg = 23
2. Male sperm = 23
C. Two Main Stages of
Meiosis
1. Meiosis I
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I
2. Meiosis II
a. Prophase II
b. Metaphase II
c. Anaphase II
d. Telophase II
D. There is only one interphase
III. Meiosis I
A. Interphase
1. Replication of
chromosomes occurs.
2. This is the only
interphase.
B. Prophase I
1. Chromatids appear
2. Centrosome splits into
two centrioles.
3. Centrioles move to the
opposite poles.
4. Spindle fibers appear
5. Nucleolus disappears
6. Nuclear membrane
disappears
7. Tetrads form - two pairs
of homologous
chromosomes joining
together.
8. Crossing-over occurs here
- genes are exchanged
between homologous
chromsomes.
a. Creates variation in
new offspring
C. Metaphase I
1. Tetrads line up at the
equator.
D. Anaphase I
1. Tetrads separate.
2. Chromatids are pulled
to the opposite poles.
E. Telophase I
1. Chromatids still
visible.
2. Nucleolus and Nuclear
Membrane reforms.
3. Centrioles replicate
4. Two new cells.
IV. Meiosis II
A. No interphase begins this
phase.
1. 2 cells with the diploid
chromosome number begin
this stage.
2. Meiosis II will always
have two cells in each phase.
B. Prophase II
1. Nuclear membrane and
nucleolus disappear.
2. Centrosome splits into two
centrioles.
3. Centrioles move to the
poles.
4. Spindle fibers form.
5. Chromatids are visible
and attach to spindle fibers.
C. Metaphase II
1. Chromatids line up on
the equator.
D. Anaphase II
1. Centromeres separate.
2. Chromosomes are
pulled to opposite poles.
E. Telophase II
1. Centrioles replicate
2. Nuclear membrane
and nucleolus reappear.
3. Spindle fibers
disappear.
4. 4 new cells are formed
a. Only have half the number
of chromosomes as the parent
cell.
b. Males - all 4 remain alive
and viable.
c. Females - one grows rest
die off.
V. Meiosis-Mitosis Comparison
A. Mitosis
1. 4 phases
2. Produces 2 diploid
cells
3. Daughter cells
identical to parent
B. Meiosis
1. 8 phases
2. 4 haploid cells produced
3. Daughter cells not
identical to parent
4. Contain tetrads and
crossing-over.