biología, meiosis
TRANSCRIPT
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MEIOSISMEIOSIS
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Meiosis
• The form of cell divisioncell division by which gametes, gametes, with halfhalf the number of chromosomes,chromosomes, are produced.
• Diploid (2n) Diploid (2n) haploid (n)haploid (n)
• MeiosisMeiosis is sexual reproduction.sexual reproduction.
• Two divisionsTwo divisions (meiosis I meiosis I and meiosis IImeiosis II).
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MeiosisMeiosis
• Sex cells Sex cells divide to produce gametesgametes (sperm (sperm or egg)or egg).
• GametesGametes have halfhalf the # of chromosomeschromosomes.
• Occurs only in gonads (testes or ovaries).Occurs only in gonads (testes or ovaries).
Male: spermatogenesisMale: spermatogenesis
Female: oogenesisFemale: oogenesis
• MeiosisMeiosis is similar to mitosismitosis with some chromosomal differences.
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SpermatogenesisSpermatogenesis
2n=46
humansex cell
diploid (2n)
n=23
n=23
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II
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Interphase IInterphase I
• Similar to mitosismitosis interphase.
• ChromosomesChromosomes replicate (S phase).(S phase).
• Each duplicated chromosomechromosome consist of two identical sister chromatidschromatids attached at their centromerescentromeres.
• 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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Meiosis I (four phases)Meiosis I (four phases)
• Cell division Cell division that reduces the chromosomechromosome number by one-half.one-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 chromosomes homologous chromosomes come together to form a tetradtetrad.
• TetradTetrad is two chromosomeschromosomes or four chromatidschromatids (sister and nonsister chromatids).
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Prophase IProphase I - - SynapsisSynapsis
Homologous chromosomes
sister chromatids sister chromatidsTetrad
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Homologous ChromosomesHomologous Chromosomes
• Pair of chromosomeschromosomes (maternalmaternal and paternalpaternal) that are similar in shape and size.
• Homologous pairs (tetrads) (tetrads) carry genes controlling the same inherited traits.
• Each locuslocus (position of a gene) (position of a gene) is in the same position on homologues.
• Humans have 23 pairs of homologous chromosomes.homologous chromosomes.
a. 22 pairs of autosomesautosomes
b. 01 pair of sex chromosomessex chromosomes
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Homologous ChromosomesHomologous Chromosomes
Paternal Maternal
eye color locus
eye color locus
hair color locus
hair color locus
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Crossing OverCrossing Over
• Crossing over (variation) Crossing over (variation) may occur between nonsister chromatidschromatids at the chiasmatachiasmata.
• Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid.
• Chiasmata (chiasma) Chiasmata (chiasma) are the sites of crossing crossing overover.
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Crossing Over - variation Crossing Over - variation
nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad
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Sex ChromosomesSex Chromosomes
XX chromosome - female XY chromosome - male
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Prophase IProphase I
centriolesspindle fiber
asterfibers
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Metaphase IMetaphase I• Shortest phaseShortest phase
• TetradsTetrads align on the metaphase platemetaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n
Example:Example: 2n = 42n = 4then then n = 2 n = 2
thusthus 2 22 2 = 4 combinations= 4 combinations
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Metaphase IMetaphase I
metaphase plate
OR
metaphase plate
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Question:Question:
• In terms of In terms of Independent AssortmentIndependent Assortment - -how many different combinations of how many different combinations of sperm could a sperm could a humanhuman malemale produce? produce?
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AnswerAnswer
• Formula: 2Formula: 2nn
• Human chromosomes:Human chromosomes: 2n = 462n = 46
n = 23n = 23
• 222323 = ~8 million combinations = ~8 million combinations
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Anaphase IAnaphase I
• Homologous chromosomes Homologous chromosomes separate and move towards the poles.
• Sister chromatids Sister chromatids remain attached at their centromerescentromeres.
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Anaphase IAnaphase I
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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
• No interphase II No interphase II
(or very short - no more DNA replicationDNA replication)
• Remember:Remember: Meiosis II Meiosis II is similar to mitosismitosis
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Prophase IIProphase II
• same as prophaseprophase in 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 cells four haploid daughter cells produced.produced.
gametes = sperm or egggametes = sperm or egg
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Telophase IITelophase II
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MeiosisMeiosis
2n=4
sex cell
diploid (2n)
n=2
n=2
meiosis I
n=2
n=2
n=2
n=2
sperm
haploid (n)
meiosis II
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VariationVariation
• Important to population as the raw Important to population as the raw material for material for natural selectionnatural selection..
• Question:Question:
What are the three sexual sources ofWhat are the three sexual sources of
genetic variation? genetic variation?
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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?
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Answer:Answer:
• 10 chromosomes (haploid)10 chromosomes (haploid)
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KaryotypeKaryotype• A method of organizing the A method of organizing the chromosomes chromosomes of a of a
cell in relation to number, size, and type.cell in relation to number, size, and type.
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FertilizationFertilization
• The fusion of a spermsperm and eggegg to form a zygotezygote.• A zygote is a fertilized egg
n=23egg
sperm n=23
2n=46zygote
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Question:Question:
• A cell containing 40 chromatids40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?
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Answer:Answer:
• 10 chromosomes10 chromosomes