Meiosis
Unit 4
Chromosome Number
Example: Drosophila
1. Has 8 chromosomes
2. 4 from each parent
3. The two sets are
homologous, they have a
corresponding chromosome
from each parent.
4. A diploid cell contains both
sets of homologous
chromosomes (2N)
5. The gametes of a sexually
reproducing organism has
only 1 set of chromosomes
making them haploid (N)
Phases of Meiosis
1. Meiosis is a process of reduction division
in which the number of chromosomes per
cell is cut in half through the separation of
homologous chromosomes in a diploid cell
2. Meiosis has two stages
O Meiosis I
O Meiosis II
Meiosis I
1. Prior to meiosis I each
chromosome is replicated
2. Prophase I
O Each chromosome pairs up with its
corresponding homologous
chromosome to form a tetrad (4
chromatids in a tetrad)
O Chromosomes may exchange
portions of chromatids in crossing
over
O results in new combinations of alleles
Meiosis I
3. Metaphase I
O Spindle fibers
attach to the
chromosomes
Meiosis I
4. Anaphase I
O The fibers pull the
homologous
chromosomes
toward opposite
ends of the cell
Meiosis I
5. Telophase I and
Cytokinesis
O Nuclear membranes
form. The cell separates
into 2 cells.
6. This results in 2 haploid
daughter cells, each
with ½ the number of
chromosomes as the
original cell.
Meiosis II
The two cells
produced by meiosis I
now enter a second
meiotic division.
2. Unlike meiosis I,
neither cell goes
through chromosome
replication.
3. Each of the cell’s
chromosomes has 2
chromatids.
1.
Meiosis II
4. Prophase II
O Spindle fibers form
Meiosis II
5. Metaphase II
O The chromosomes
line up in the center
of cell.
Meiosis II
6. Anaphase II
O The sister
chromatids
separate and
move toward
opposite ends of
the cell.
Meiosis II
7. Telophase II and
Cytokinesis
O Nuclear membranes
form and each cell
separates into 2 new
daughter cells.
O Meiosis II results in 4
haploid daughter cells.
Gamete Formation
1. Male
Gametes
O 4 equal-
sized sperm
cells
2. Female
Gametes
O 1 large
egg cell 3
polar
bodies
Comparing Mitosis & Meiosis
Mitosis
O Cells produced by mitosis
have the same number of
chromosomes and alleles as
the original cell.
O Mitosis allows an organism
to grow and replace cells.
O Some organisms reproduce
asexually by mitosis.
2. Mitosis results in the
production of two genetically
identical diploid cells.
1.
3.
Meiosis
O Cells produced by meiosis
have half the
number of chromosomes
as the parent cell.
O These cells are genetically
different from the diploid
cell and from each other.
O Meiosis is how sexually
reproducing organisms
produce gametes.
4.
Meiosis produces four
genetically different
haploid cells.
Review
1. During meiosis, the number of
chromosomes per cell is cut in half through
the separation of
Daughter cells
b) Homologous chromosomes
c) Gametes
d) Chromatids
a)
2. The formation of a tetrad occurs during
Anaphase I
b) Metaphase II
c) Prophase I
d) Prophase II
a)
3. If a human body cell contains 46
chromosomes, a single sperm cell should
have
46 chromosomes
b) 23 chromosomes
c) 92 chromosomes
d) Between 23 and 46 chromosomes
a)
4. In many female animals, meiosis results in
the production of
Only 1 egg
b) 1 egg and 3 polar bodies
c) 4 eggs
d) 1 egg and 2 polar bodies
a)
5. Compared to egg cells formed during
meiosis, daughter cells formed during
mitosis are
Genetically different, while eggs are
genetically identical
b) Genetically different, just as egg cells are
c) Genetically identical, just as eggs cells are
d) Genetically identical, while egg cells are
genetically different
a)