Reduction-Division Genetic Recombination 1
Creation of GAMETESGAMETES, , with HALFHALF the number of CHROMOSOMESCHROMOSOMES, (, (HAPLOID)
MeiosisMeiosis is SEXUALSEXUAL reproduction.reproduction.
TWOTWO divisions divisions (MEIOSIS IMEIOSIS I and MEIOSIS IIMEIOSIS II).
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Similar to mitosismitosis interphase. CHROMOSOMESCHROMOSOMES (DNA)(DNA) replicate in the S S
phasephase
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Cell division Cell division that reduces the chromosome number chromosome number by one-half.one-half.
Four phasesFour phases:a.a. Prophase IProphase Ib.b. Metaphase IMetaphase Ic.c. Anaphase IAnaphase Id.d. Telophase ITelophase I
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Prophase IProphase I
• ChromosomesChromosomes condense.
• Nuclear membrane Nuclear membrane disappears
• SpindlesSpindles forms SynapsisSynapsis occurs -
Homologous Homologous chromosomeschromosomes come togetherto form a tetradtetrad.
• Crossing over Crossing over Occurs
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Crossing over Crossing over may occur between non-sister chromatidschromatids at sites called chiasmatachiasmata.
Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid.
Chiasmata (chiasma) Chiasmata (chiasma) are where chromosomes touch each other and exchange genes exchange genes (crossing overcrossing over.)
Causes Genetic RecombinationGenetic Recombination
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Tetrads align on the equator. Independent assortment occurs –
chromosomes separate randomly causing GENETIC RECOMBINATION
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In terms of In terms of Independent Independent AssortmentAssortment -how many -how many different combinations of different combinations of sperm could a sperm could a humanhuman malemale produce? produce?
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Formula: 2Formula: 2nn
Human chromosomes:Human chromosomes:2n = 462n = 46
n = 23n = 23 222323 = ~8 million = ~8 million combinationscombinations
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Homologous chromosomes Homologous chromosomes separate and move towards the poles.
Sister chromatids Sister chromatids remain attached at their centromerescentromeres.
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Interphase II Interphase II or very shortor very short No DNA ReplicationNo DNA Replication Remember: Meiosis II is Remember: Meiosis II is
similar to mitosissimilar to mitosis
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Same as Same as ProphaseProphase in in mitosismitosis
Nucleus & nucleolus disappearNucleus & nucleolus disappear Chromosomes condenseChromosomes condense Spindle formsSpindle forms
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Same as Same as TelophaseTelophase in mitosis. in mitosis. Nuclear membrane reformsNuclear membrane reforms, ,
spindle disappearsspindle disappears
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The cells splitThe cells split FOURFOUR HAPLOID HAPLOID
DAUGHTER cells are produced.DAUGHTER cells are produced. Called Called GAMETESGAMETES (eggs and sperm) (eggs and sperm)
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2n=46
humansex cell
diploid (2n)
n=46
n=46
Meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
Meiosis II25
2n=46
humansex cell
diploid (2n)
n=23
n=23
Meiosis I
n=23egg
Haploid (1n)
Meiosis II26
Polar Bodies (die)
Also known as Also known as GENETIC GENETIC RECOMBINATIONRECOMBINATION
Meiosis allows for much variation Meiosis allows for much variation due to:due to:
1. 1. Independent assortment Independent assortment 2. Crossing over2. Crossing over 3. Random fertilization3. Random fertilization
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A cell containing 20 20 chromosomeschromosomes (diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?
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AAn organized organized picture picture of the chromosomes of a human arranged in pairs arranged in pairs by size by size from largest to smallest.
PPairs 1-221-22 called AUTOSOMESAUTOSOMES
LLast pair are SEX SEX CHROMOSOMESCHROMOSOMES
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Male - XYMale - XY