Transcript Meiosis

Meiosis
meiosis
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Nuclear division
Reduces the chromosome number
Diploid-2n, haploid-n, 46, 23
Gamete – sex cell, haploid
Zygote – fused gametes, diploid
Homologous chromosomes
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Look alike, have similar banding pattern,
same length and centromere position
One from mom, one from dad
Alleles – alternate forms of a gene, have
genes for same traits, just code for
differences.
meiosis
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2 nuclear divisions
Produce 4 haploid cells
Not genetically identical
Differences from mitosis
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Homologous chromosomes line up together
during meiosis 1
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Called synapsis, results in a bivalent (aka tetrad)
No interphase between phases
Genetic Variation
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Crossing over – exchange of genetic
material between non-sister
chromatids of a bivalent (2-3 times)
Nucleoprotein lattice forms
Chiasmata – regions where non-sister
chromatids are held together due to
crossing over.
Results in different sequence of alleles
and are different from their parents.
Independent assortment
Homologous chromosomes separate in a random
order. (8,388,608 possible chromosomal
combinations)
Fertilization
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Random…
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Significance of genetic variation:
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In changing environments, sexual reproduction is
an advantage
Phases of meiosis
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Prophase 1
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Spindle fibers form, centrosomes migrate
Nuclear envelopes fragments, nucleolus gone
Chromosomes condense
Homologous chromosomes line up, synapsis
forming bivalents
Crossing over occurs
Metaphase 1
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Homologous chromosomes move to equator
Kinetochores are seen outside of centromere and
connected to spindles
Bivalents independently align in middle
Anaphase 1
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Homologous chromosomes separate from
each other and move to opposite poles
Telophase 1
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Spindle disappears
Nuclear envelope appears (not necessary by
end of telophase to move on)
Cytokinesis may or may not happen during
this phase
interkinesis
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Between divisions
Like interphase with no “s” phase
Meiosis II
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Similar to Mitosis in division except ends with
4 haploid daughter cells
Haploid cells become gametes, in plants =
spores
Comparison of Mitosis and Meiosis
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1 nuclear division
Results in 2 daughter
cells
Daughter cells
genetically identical to
parent
Daughter cells have
same # of
chromosomes as
parent (diploid)
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2 nuclear divisions
Results in 4 daughter
cells
Daughter cells not
genetically identical to
parent
Daughter cells have
half the # of
chromosomes as
parent (haploid)
Comparison of Mitosis and Meiosis
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Occurs in all tissues for
growth and repair,
common
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Occurs only in
reproductive organs
and produces gametes
Human life cycle
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Gametogenesis – formation of gametes
Spermatogenesis – formation of sperm in
testes, male meiosis
Oogenesis – egg production in ovary, female
meiosis
spermatogenesis
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Testes contain stem cells – spermatogonia which
keep the testes supplied with primary spermatocytes
(2n - 46) that undergo spermatogenesis
Primary spermatocytes form secondary
spermatocytes (n- 23), which form spermatids (n23), which differentiate into 4 viable sperm
oogenesis
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Ovaries have stem cells called oogonia
Produce primary oocytes (2n-46) during fetal development
and undergoes oogenesis during puberty
Forms 1 secondary oocyte (n-23), which receives the most
cytoplasm, and a polar body
Secondary oocyte begin meiosis II but stops at metaphase II.
It leaves ovary and moves to oviduct where it awaits sperm.
If no sperm enters, it disintergrates.
If sperm enter oocyte, Meiosis II continues and another polar
body is formed.