Download The Formation of Sex Cells

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 two complete sets of genes,
one from each parent
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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 for 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 male and 4
from the female
Each body cell of a
human has 46
chromosomes, 23 from
the male and 23 from the
female
Homologous Chromosome Pairs
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What are these chromosomes made of?
What species is this individual? How do you know?
What gender is this individual?
Why are there two of each chromosome?
III. Phases of Meiosis
Overview: Diploid cells destined to become gametes
go through two rounds of cell division
A. DNA replication
1. occurs before the first round of cell division
(recall the S phase of mitosis)
B. Meiosis I
1. 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
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This is different from mitosis!
Tetrad
B. Meiosis I cont.
2. the tetrads may exchange portions of their
chromatids, in a process called crossingover
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genes are exchanged between
homologous chromosomes
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this introduces variation!
3.  During metaphase I, there are a variety of
ways that the chromosomes can line up.
This is called “Independent Orientation”
and it introduces more variation!
4.. homologous chromosomes separate and the
cell divides (recall cytokinesis)
5. two new diploid (2N) cells are formed, each
having a unique set of genes
Images of Meiosis I
note the similarities and differences to mitosis
Meiosis and
Variation
movie
C. Meiosis II
1. Performed by the two new cells
produced in Meiosis I
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each of the new diploid cell’s
chromosomes contains 2 unique
chromatids
2. DNA does not replicate
3. Prophase through cytokinesis
occurs
4. Four unique haploid (N) cells
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
Spermatogenesis
Oogenesis
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
Square Dance