meiosis advanced animal science. meiosis cell division type of cell division one germ cell makes 4...
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Meiosis
Advanced Animal Science
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Meiosis Type of cell divisioncell division
One germ cell makes 4 gametes with ½ the # of chrom.
Occurs only in germ cells in gonads ○ testes /ovaries
Occurs in flowers ○ ovary and anther
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Sperm
surrounding an egg
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This shows how only one single sperm gets to penetrate the egg, releasing its nucleus of 23 chromosomes to merge with the nucleus of the egg and its 23 chromosomes.
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Homologous Homologous ChromosomesChromosomes
Pair of chrom. similar in shape , size, and types of genes. Each locus (location of the gene) in same position on chrom.
Humans have 23 pairs of homologueshomologues Housefly – 6 prs Housefly – 6 prs Chicken – 39 prs Chicken – 39 prs Apple – 17 prsApple – 17 prs Dog – 39 prsDog – 39 prs Cat – 19 prs Cat – 19 prs
This is a karyotype (an image of an organism’s chromosomes)
This is a karyotype of a normal human male
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Homologous Homologous ChromosomesChromosomes
Paternal(from Dad)
Maternal (from Mom)
eye color locus
eye color locus
hair color locus
hair color locus
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Prophase IProphase I
Longest and most complex phase (90%). Chromosomes condense. SynapsisSynapsis - - a process: when hom. chrom. hom. chrom.
come together, pair up, form a tetradtetrad.
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Prophase IProphase I - - SynapsisSynapsis
Nonsister chromatids
sister chromatids sister chromatidsTetrad
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Prophase I -Prophase I - Crossing Crossing OverOver
Crossing over may occur in the tetrad:between nonsister chromatids, ends break and reattach
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Crossing Over - Provides Crossing Over - Provides
VariationVariation
nonsister chromatids
Chiasma: site of crossing over
variation
Tetrad
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Metaphase IMetaphase I
Shortest phase; paired homologues align. INDEPENDENT ASSORTMENTINDEPENDENT ASSORTMENT occurs
pairs of homologues line up independently of other pairs’ orientation toward the poles -- random. Adds variation.
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Anaphase IAnaphase I Homologous chromosomes separate towards
the poles (Tetrads separate)
Sister chromatids Sister chromatids remain attached
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Telophase ITelophase I Each pole now has haploid set of chromosomes Each pole now has haploid set of chromosomes
(however – still doubled).(however – still doubled). CytokinesisCytokinesis occurs: two haploid daughter cells
formed.
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Meiosis IIMeiosis II No interphase II No interphase II
( no more DNA replicationDNA replication) Remember: Meiosis II Remember: Meiosis II is similar to mitosismitosis
Prophase II / Metaphase II
Anaphase II
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Telophase IITelophase II Same as telophasetelophase in mitosismitosis. Nuclei form. CytokinesisCytokinesis occurs (2nd time). Four haploid daughter cells produced (chromosomes now
back to single condition).gametes ~ sperm or egg; ovule or pollen grain
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Gamete Formation in Animals Diff. bet. male and female gametes.
Male: spermatogenesisMale: spermatogenesis all 4 develop into sperm cells.
Female: oogenesisFemale: oogenesiscytokinesis in meiosis is uneven. most of cytoplasm goes into 1 of the 4 meiotic products
(forms large egg cell) 3 other cells are small “polar bodies”, break down (extra
chrom. lost).
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SpermatogenesiSpermatogenesiss
2n = 46
humangerm cell in testes
diploid (2n)
n=23Still doubled chromosomes
n=23Still doubled chromosomes
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II
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Oogenesis
2n = 46
human germ cellin ovary
diploid (2n)
n=23Still doubled
n=23Still doubled
meiosis I
23
23
23
Ovumn=23
Polar Bodies
meiosis II
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Mitosis vs Meiosis Mitosis
Body (somatic cells) 2 daughter cells made
(identical) Each w/ same # & kind
of chrom. as parent cell 1 division process 1 cytokinesis No synapsis or
crossing over Are diploid (2n)
Meiosis Germ cells of gonads 4 gamete cells made
(all different) Each w/ ½ chrom. # as
parent cell 2 divisions 2 cytokineses events Synapsis &
crossing over occurs in Prophase 1
Are haploid (n)
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Chromosome numbers: However many
“types” of chromosomes an organism has, that number is the “n” number of chromosomes it has.
Body cells (somatic cells) therefore have a“2n” # (diploid)
Gametes have “n” # (haploid)
For humans, 2n = 46n = 23
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Nondisjunction
When the tetrad (in Anaphase I) or the sister chromatids (in Anaphase II) do not separate, creating an abnormal # of chrom. to occur in the gametes.
Lethal most of the time
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Karyotype… male
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Karyotype… female
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