Download MeiosisPPT

The Formation of Sex Cells
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You simply cannot combine an egg and sperm if they contain the
normal number of chromosomes!
The chromosome # must be reduced to half! This is Meiosis.
I. Overview of Meiosis
A. All organisms inherit one of each gene from each
parent, resulting in paired genes.
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In animals, mom’s set of genes comes from an egg
cell; dad’s set of genes comes from a sperm cell
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In plants, the genes come from pollen and ovule
B. Sex cells, or gametes contain half of the normal
amount of chromosomes
1. Meiosis is the process of creating haploid (N)
gametes from diploid (2N) cells
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occurs in eukaryotic cells only
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required by sexual, but not asexual, reproduction
II. Chromosome Number
A. Eukaryotic sexual reproduction results in
organisms with two sets of chromosomes
1. the two sets are said to be homologous
a. chromosomes in one set have a version of
themselves in
the other set
B. A cell with both sets of homologous chromosomes
is said to be diploid (2N)
1. Ex. somatic cells (liver cell, heart cell, skin cell etc.)
C. A cell with one set of chromosomes is said to be
haploid (N)
1. Ex. gametes (egg and sperm or ovule and pollen)
Each body cell of a fruit
fly has 8 chromosomes,
4 from the father and 4
from the mother
Each body cell of a
human has 46
chromosomes, 23 from
the father and 23 from
the mother
Homologous Chromosome Pairs
III. Phases of Meiosis
Overview: Diploid cells destined to become gametes
(germ cells) go through two rounds of cell
division
1. DNA replication
a. occurs before the first round of cell division
(recall the S phase of mitosis)
2. Meiosis I
a. newly synthesized chromosomes pair up with
their homologes forming a tetrad
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Ex. The chromosome containing the gene for eye color
from mom will pair up with the chromosome containing
the gene for eye color from dad
Tetrad
This is different from mitosis!
2. Meiosis I cont.
b. the tetrads may exchange portions
of their chromatids, in a process
called crossing-over
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genes are exchanged between
homologous chromosomes
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this introduces variation!
c. homologous chromosomes separate and
the cell divides (recall cytokinesis)
d. two new haploid (N) cells are formed,
each having a unique set of genes
Meiosis I and
Meiosis II
Start: PAIRED
chromosomes,
each with 2 sister
chromatids
vs. Mitosis:
Start: UNPAIRED
chromosomes, each
w/ 2 sister chromatids
2 sister chromatids
split (like Mitosis)
QuickTime™ and a
decompressor
are needed to see this picture.
End: PAIRED
chromosomes,
each with 1
sister
chromatid
End: UNPAIRED chromosomes, each with 1 sister
chromatid (RE-PAIRING occurs at ferilization).
Images of Meiosis I
note the similarities and differences to mitosis
QuickTime™ and a
Animation decompressor
are needed to see this picture.
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
3. Meiosis II
a. Performed by the two new
cells
produced in Meiosis I
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each of the new haploid cell’s
chromosomes contains 2 unique
chromatids
b. DNA does not replicate
QuickTime™ and a
Animation decompressor
are needed to see this picture.
c. Prophase through cytokinesis
occurs
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similar to mitosis
d. Four unique haploid (N) cells
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
are formed
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two from each of the cells
produced in meiosis I
Males
vs.
Females
there is a difference!
For animals,
meiosis in males
produces 4 sperm
from every germ
cell vs. one egg for
females
3 polar
bodies donate
cell materials
(except
DNA) and
disintegrate
IV. Mitosis vs. Meiosis
1. Mitosis
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Asexual reproduction
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Begins with a diploid cell (2N)
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Results in the formation of 2 diploid cells (2N)
that are genetically identical to each other and
to the original cell
2. Meiosis
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Required for sexual reproduction
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Begins with a diploid cell (2N)
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Results in the formation of 4 haploid cells (N)
that are genetically different from each other
and the original cell
Compare
Mitosis and
Meiosis
Link to
Meiosis
Animation