Transcript Meiosis

MEIOSIS
Production of Gametes
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Meiosis
Facts
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Meiosis
• The form of cell division by which
gametes, with half the number of
chromosomes, are produced
• Diploid (2n)  haploid (n)
• Called a reduction division (because
# of chromosomes is reduced)
• Meiosis is sexual reproduction
• Two divisions (meiosis I and meiosis
II)
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Meiosis
• Germ Line cells (aka sex
cells) divide to produce
gametes (sperm or egg)
• Gametes have half the
number of chromosomes
• Meiosis is similar to
mitosis with some
chromosomal differences
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Meiosis
• Occurs only in gonads
(testes or ovaries)
• Male: Spermatogenesis
• Female: Oogenesis
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Spermatogenesis
n=23
Human
sex cell
Sperm
n=23
n=23
2n=46
n=23
Diploid (2n)
Haploid
(n)
n=23
n=23
Meiosis I
Meiosis II
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Interphase I
• Similar to mitosis interphase
• Chromosomes replicate (S
phase)
• Each duplicated chromosome
consist of two identical sister
chromatids attached at their
centromeres
• Centriole pairs also replicate.
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Interphase I
• Nucleus and nucleolus visible.
Chromatin
Nuclear
membrane
Cell membrane
Nucleolus
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Meiosis I
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Stages of Meiosis I
• Cell division that reduces
the chromosome number
by one-half
• Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I
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Prophase I
• Longest and most complex
phase (90%)
• Chromosomes condense
• Synapsis occurs: homologous
chromosomes come together
to form a tetrad
• Tetrad is two chromosomes or
four chromatids (sister and
nonsister chromatids)
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Prophase I - Synapsis
Homologous chromosomes
Sister chromatids
Tetrad
Sister chromatids
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Homologous
Chromosomes
• Pair of chromosomes (maternal
and paternal) that are similar in
shape and size
• Homologous pairs (tetrads)
carry genes controlling the
same inherited traits
• Each locus (position of a gene)
is in the same position on
homologues
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Homologous
Chromosomes
• Humans have 23 pairs of
homologous chromosomes
• First 22 pairs of
chromosomes called
autosomes
• Last pair called sex
chromosomes
• XX female or XY male
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Homologous
Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
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Gene Loci
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Crossing Over
• Crossing over (variation) may
occur between nonsister
chromatids at the chiasmata
• Crossing over: segments of
nonsister chromatids break and
reattach to the other chromatid
• Chiasmata (chiasma) are the
sites of crossing over
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Crossing Over - Variation
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
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variation
Sex Chromosomes
XX chromosome - female
XY chromosome - male
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Prophase I
Spindle fiber
Centrioles
Aster
fibers
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Metaphase I
• Shortest phase
• Tetrads align on the
metaphase plate
• Independent assortment
occurs
• Chromosomes separate
randomly to the poles of the
cells
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Metaphase I
• Independent assortment causes
variation in the forming cells
• Orientation of homologous pair to
poles is random
• Formula for determining variation:
Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations
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Metaphase I
OR
Metaphase plate
Metaphase plate
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Question:
• In terms of Independent
Assortment, how many
different combinations of
sperm could a human male
produce?
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Answer
• Formula: 2n
• Human chromosomes:
2n = 46
n = 23
223 = ~8 million combinations
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Anaphase I
• Homologous chromosomes
separate and move
towards the poles
• Sister chromatids remain
attached at their
centromeres
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Anaphase I
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Telophase I
• Each pole now has haploid
set of chromosomes
• 1n = 23 (human)
• Cytokinesis occurs and two
haploid daughter cells are
formed
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Telophase I
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Meiosis II
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Meiosis II
• Short or No interphase II
• DNA NOT replicated
again
• Remember: Meiosis II is
similar to mitosis
• Prophase I, Metaphase
II, Anaphase II, and
Telophase II
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Prophase II
• Same as prophase in mitosis
• Nuclear envelope breaks down and
the spindle apparatus forms
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Metaphase II
• Same as metaphase in mitosis
• Chromatids lined up at equator
Metaphase plate
Metaphase plate
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Anaphase II
• Same as anaphase in mitosis
• Sister chromatids separate
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Telophase II
• Same as telophase in
mitosis
• Nuclei reform
• Cytokinesis occurs
• Remember: Four haploid
daughter cells produced
• Gametes = sperm or egg
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Telophase II
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Meiosis
sex cell
n=2
Sperm
n=2
n=2
2n=4
Haploid (n)
n=2
diploid (2n)
n=2
n=2
Meiosis I
Meiosis II
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Genetic
Variation
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Variation
Important to the survival
of populations
Aids in natural selection
Strongest individuals are
able to survive and
reproduce
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Question
• What are the three
sources of genetic
variation in sexual
reproduction?
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***Answer:
1. Crossing Over
(Prophase
I)
2. Independent Assortment
(Metaphase I)
3. Random Fertilization
(sperm joins with egg)
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Remember:
Variation is helpful
to the survival of a
species!
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Question:
• A diploid cell containing 20
chromosomes (2n = 20) at
the beginning of meiosis
would, at its completion,
produce cells containing
how many chromosomes?
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Answer:
• 10 chromosomes
(haploid)
• 1n = 10
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Karyotype
• A method of organizing the chromosomes of a
cell in relation to number, size, and type.
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Trisomy
• Genetic disorders such as
Down’s Syndrome (Trisomy 21)
are the result of
nondisjunction
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Fertilization
• The fusion of a sperm and egg to form
a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
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Question:
• A cell containing 40
chromatids at the beginning
of meiosis would, at its
completion, produce cells
containing how many
chromosomes?
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Answer:
• Four cells with 10
chromosomes each
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