Transcript Meiosis

Meiosis
The Diploid Cell
Cell with 2 sets of chromosomes
Contains chromosomes for each homologous pair
Somatic Cells = Body Cells: Diploid
All human cells except sex cells are diploid
Fertilization restores the diploid number
2n, where n is the number of chromosomes an
individual has
Humans: n = 23, 2n = 46
The Haploid Cell
Cell containing one of each kind of chromosome
Ex. sperm and egg cells (gametes)
1 set of chromosomes
Half the number of chromosomes of diploid
n
Sperm + Egg = Zygote
Haploid = one of each kind
Diploid = two of each kind
+
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2n = diploid
or
n = haploid
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Mitosis vs. Meiosis
Mitosis: t = two: Diploid
2n=46
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Meiosis: o = one: Haploid
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n=
23
Meiosis
Gametes are produced in specialized body
cells
Sperm and Egg Cells
2 divisions: Meiosis I and Meiosis II
Meiosis occurs in sex cells, not body cells
4 daughter cells
Each cell has half the chromosomes of the
parent
Meiosis I
Interphase occurs: the
cell grows and DNA
replicates
Meiosis I begins
Original cell produces
two new cells
Like Mitosis
Prophase I
DNA Coils tightly into chromosomes
Spindle fibers appear
Each chromosome lines up next to the
homologue
Synapis occurs: pairing of homologous
chromosomes
Tetrad: Each pair of homologous
chromosomes
Crossing Over
Crossing Over: why we do not look exactly
like our parents.
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Portions of the chromatid breaks off and
attaches to adjacent chromatids on the
homologous chromosome
Permits the exchange of genetic material
between maternal and paternal chromosomes
Occurs during Prophase
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Genetic Recombination
Crossing over
produces a new
mixture of
genetic material
Causes of Variation
Chromosomes are assorted randomly
Crossing over may occur
Cells do not have identical genetic info as
each other or the parent
+ more chance of survival and evolution
- mistakes more likely
Metaphase I
Tetrads line up
randomly along the
mid-line
Spindle fibers
attach to
centromeres
Anaphase I
Homologous
chromosomes move
to the opposite poles
Random separation
or Independent
Assortment results in
separation of
maternal and paternal
chromosomes.
Telophase I
Chromosomes
reach opposite
ends of cell
Cytokinesis
begins
Cell is now
haploid
Meiosis II
Occurs in each
cell formed in
Meiosis I
Interphase does
not occur again
Prophase II
Spindle fibers
form and move
the
chromosomes to
the mid-line of
the dividing cell
Metaphase II
Chromosomes
move to the midline of the dividing
cell facing
opposite poles of
the dividing cell
Anaphase II
Chromatids
separate and
move to
opposite poles
of the cell
Telophase II
Nuclear membrane forms around the chromosomes
in each of 4 new cells
Cytokinesis II
Cytoplasm divides
Cell membrane closes off
End Result:
Four new cells that contain half of the
original cells number of chromosomes
Creates sex cells = gametes
What exactly is a gamete?
Formed by meiosis
Haploid reproductive cells
In humans, meiosis occurs in
the testes and ovaries
Meiosis Animation
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Meiosis Animation
Why do we need haploid gametes?
Female gamete
23
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EGG
Male gamete
23
SPERM
Fertilization restores the diploid number
1n
fertilization
1n
meiosis
2n
2n
Mitosis and
cell growth
2n
Mitosis vs. Meiosis
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Mitosis vs. Meiosis Animation
Nondisjunction
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Failure of homologous chromosomes to
separate properly during meiosis.
Both chromosomes of a homologous pair
move to the same pole of the cell.
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1 gamete has an extra chromosome
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Or
1 gamete is missing a chromosome
Nondisjunction Animation
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Nondisjunction Animation
Chromosome Pop Quiz!
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Chromosome:
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Rod-shaped coiled and compact DNA
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“Colored body”
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Found in the nucleus
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Looks like X
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Humans have 23 pairs of chromosomes
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Female = XX, Male = XY (23rd pair)
Trisomy: Extra Chromosomes
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Trisomy 13: Patau’s Syndrome
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Trisomy 21: Down’s Syndrome
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XXX: Trisomy X
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XYY: Jacob’s Syndrome
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XXY: Klinefelter’s Syndrome
Monosomy: Missing Chromosomes
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Most do not survive
Turner XO : develop as girls,
but infertile
OY: lethal