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MeiosisMeiosis
The form ofcell divisioncell division by which gametes,gametes, with
halfhalfthe number ofchromosomes,chromosomes, are
produced.
Diploid (2n)Diploid (2n) pp haploid (n)haploid (n)
MeiosisMeiosis is required forsexual reproduction.sexual reproduction.
Two divisionsTwo divisions
meiosis Imeiosis I and meiosis IImeiosis II.
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MeiosisMeiosis
Sex cells (also known germ cells) divide toproduce gametes (sperm or egg).
Gametes have halfthe # ofchromosomes.
Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
Meiosis is similar to mitosis with some key
chromosomal differences.
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MeiosisMeiosis
2n=4
germ cell
diploid (2n)diploid (2n)
n=2
n=2
meiosis I
n=2
n=2
n=2
n=2
sex cells
(sperm,eggs, ovules,
pollen)
haploid (n)haploid (n)
meiosis II
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InterphaseInterphase II Similar to mitosismitosis interphase.
ChromosomesChromosomes replicate (S phase).(S phase).
Each duplicated chromosomechromosome consists of
two identical sisterchromatidschromatids attached at
theircentromerescentromeres.
CentrioleCentriole pairs also replicate.
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Interphase IInterphase I NucleusNucleus and nucleolusnucleolus visible.
nuclear
membrane
nucleolus
cell membrane
chromatin
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Homologous ChromosomesHomologous Chromosomes
Pair ofchromosomeschromosomes (maternalmaternal and paternalpaternal) that are
similar in shapeshape andand sizesize.
Homologous pairs (tetrads)(tetrads) carry genes controlling
the same inherited traitssame inherited traits.
Each locuslocus (position of a gene)(position of a gene) is in the same
position on homologues.
Humans have 23 pairs ofhomologous chromosomes.homologous chromosomes.a. 22 pairs ofautosomesautosomes
b. 1 pair ofsex chromosomessex chromosomes
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Homologous ChromosomesH
omologous Chromosomes
Paternal Maternal
eye color
locus
eye color
locus
hair color
locushair color
locus
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Sex ChromosomesSex Chromosomes
XX chromosome - female XY chromosome - male
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Meiosis I (four phases)Meiosis I (four phases) Cell divisionCell division that reduces the chromosomechromosome
number by oneone--half.half.
four phasesfour phases:
a.a. prophase Iprophase I
b.b. metaphase Imetaphase I
c.c. anaphase Ianaphase I
d.d. telophase Itelophase I
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Prophase IProphase I Longest and most complex phase (90%).Longest and most complex phase (90%).
ChromosomesChromosomes condense. SynapsisSynapsis occurs: homologous chromosomeshomologous chromosomes
come together to form a tetradtetrad.
TetradTetrad is two chromosomeschromosomes or fourchromatidschromatids (sister and nonsister chromatids).
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Prophase IProphase I -- SynapsisSynapsisHomologous chromosomes
sister chromatids sister chromatids
Tetrad
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Crossing OverCrossing Over
segments of nonsisterchromatidschromatids break and
reattach to the otherchromatidchromatid.
This is an important source ofgenetic variation.
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Crossing OverCrossing Over -- variationvariation
nonsister chromatids
chiasmata: siteof crossing over variation
Tetrad
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Crossing over
Now, chromatidsPaternal AA and
Maternal aa are
changed from their
original form.
This is a source of
genetic variationgenetic variation.
The DNA is a
mixture of genes
that has neverexisted before.
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Prophase IProphase I
centriolesspindle fiber
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Metaphase IMetaphase I
OR
Shortest phaseShortest phase
TetradsTetrads align on the metaphase platemetaphase plate.
Independent
assortment
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Anaphase IAnaphase I
Homologous chromosomesHomologous chromosomes separate andmove towards the poles.
SisterSister chromatidschromatids remain attached at their
centromerescentromeres.
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Telophase ITelophase I Each pole now has haploidhaploid set of
chromosomeschromosomes.
CytokinesisCytokinesis occurs and two haploid daughter
cells are formed.
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Telophase ITelophase I
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Meiosis IIMeiosis II
NoNo interphaseinterphase IIII(or very short - no more DNA replicationDNA replication)
Prophase IIProphase II
Metaphase IIMetaphase II
Anaphase IIAnaphase II
TelophaseTelophase IIIIRemember:Remember:
Meiosis IIMeiosis II is very similar to mitosismitosis
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Metaphase IIMetaphase II same as metaphasemetaphase in mitosismitosis
metaphase platemetaphase plate
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Anaphase IIAnaphase II same as anaphaseanaphase in mitosismitosis
sister chromatids separatesister chromatids separate
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Telophase IITelophase II Same as telophasetelophase in mitosismitosis.
Nuclei form. CytokinesisCytokinesis occurs.
Remember:Remember: four haploid daughter cellsfour haploid daughter cells
produced.produced.
gametes = sperm or egggametes = sperm or egg
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Telophase IITelophase II
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SpermatogenesisSpermatogenesis
2n=46
Male germ cell
diploid (2n)diploid (2n)
n=23
n=23
meiosis I
n=23
n=23
n=23
n=23
haploid (n)haploid (n)
meiosis II
sperm
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OogenesisOogenesis
2n=46
Female germ cell
diploid (2n)diploid (2n)
n=23
n=23
meiosis I
n=23
n=23 egg
haploid (n)haploid (n)
meiosis II
n=23
n=23polar
bodies
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FertilizationFertilization
The fusion of a spermsperm and eggegg to form a zygotezygote.
A zygote is a fertilized egg
n=23
egg
spermn=23
2n=46zygote
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Genetic VariationGenetic Variation Important to population as the rawImportant to population as the raw
material formaterial fornatural selectionnatural selection..
Question:Question:
What are the three sources ofWhat are the three sources of
genetic variation during sexualgenetic variation during sexualreproduction?reproduction?
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Answer:Answer:1. crossing over (prophase I)1. crossing over (prophase I)
2. independent assortment (metaphase I)2. independent assortment (metaphase I)
3. random fertilization3. random fertilization
Remember: variation is good!
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Question:Question:
A cell containing 20 chromosomes20 chromosomes(diploid)(diploid) at the beginning of meiosis
would, at its completion, produce
cells containing how many
chromosomeschromosomes?
Answer:Answer:10 chromosomes (haploid)10 chromosomes (haploid)
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KaryotypeKaryotype A method of organizing theA method of organizing the chromosomeschromosomes of aof a
cell in relation to number, size, and type.cell in relation to number, size, and type.
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KaryotypeKaryotype
Question
What is theWhat is the
sex of thissex of this
person?person?
Answer
MALEMALE
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Question:Question:
A cell containing 4040 chromatidschromatids atthe beginning of meiosis would, at its
completion, produce cells containinghow many chromosomeschromosomes?
Answer:Answer:
10 chromosomes10 chromosomes