reduction-division genetic...
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Reduction-DivisionGenetic Recombination
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Creation of GAMETES, with HALF the number of CHROMOSOMES, (HAPLOID)
Meiosis is SEXUAL reproduction.
TWO divisions (MEIOSIS I and MEIOSIS II).
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Similar to mitosis interphase.
CHROMOSOMES (DNA)replicate in the S phase
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Cell division that reduces the chromosome number by one-half.
Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I4
Prophase I
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• Chromosomescondense.
• Nuclear membrane disappears
• Spindles forms
Synapsis occurs -Homologous chromosomescome togetherto form a tetrad.
• Crossing over Occurs
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Homologous chromosomes
sister chromatids sister chromatidsTetrad
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Crossing over may occur between non-sister chromatids at sites called chiasmata.
Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.
Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.)
Causes Genetic Recombination
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nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad
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Tetrads align on the equator. Independent assortment occurs – chromosomes
separate randomly causing GENETIC RECOMBINATION
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Homologs line up at equator or metaphase plate
OR
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In terms of Independent Assortment -how many different combinations of sperm could a human maleproduce?
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Formula: 2n
Human chromosomes: 2n = 46
n = 23
223 = ~8 million combinations
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Homologous chromosomes separate and move towards the poles.
Sister chromatids remain attached at their centromeres.
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Nuclear membrane reforms around each group of chromosomes
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Cytokinesis occurs and two haploid daughter cells are formed.
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cytokinesis
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Interphase II or very short
No DNA Replication
Remember: Meiosis II is similar to mitosis
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Same as Prophase in mitosisNucleus & nucleolus disappear Chromosomes condense
Spindle forms
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Same as Metaphase in mitosis
Chromosomes line up at equator
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Same as Anaphase in mitosis
SISTER CHROMATIDS separate
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Same as Telophase in mitosis.
Nuclear membrane reforms, spindle disappears
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The cells split
FOUR HAPLOID DAUGHTER cells are produced.
Called GAMETES (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 II
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2n=46
humansex cell
diploid (2n)
n=23
n=23
Meiosis I
n=23
egg
Haploid(1n)
Meiosis II
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Polar Bodies (die)
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Also known as GENETIC RECOMBINATION
Meiosis allows for much variation due to:
1. Independent assortment
2. Crossing over
3. Random fertilization
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A cell containing 20 chromosomes(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?
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10 chromosomes (haploid or 1n)
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An organized picture of the chromosomes of a human arranged in pairs by size from largest to smallest.
Pairs 1-22 called AUTOSOMES
Last pair are SEX CHROMOSOMES
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Male - XY
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Female - XX
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Female - XXDown Syndrome – Trisomy 21
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The fusion of a sperm and egg to form a zygote.
A zygote is a FERTILIZED EGG 64 trillion combinations for the zygote
n=23egg
spermn=23
2n=46zygote
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