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AP Biology
Lecture #23Meiosis
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2n = 6 doublestranded
2n = 6 singlestranded
Preparing for meiosis
• 1st step of meiosis– Duplication of DNA– Why bother?
• meiosis evolved after mitosis • convenient to use
“machinery” of mitosis• DNA replicated in
S phase of interphase of MEIOSIS(just like in mitosis)
M1 prophase
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Repeatafter me!
2n = 4 singlestranded
Meiosis 1
2n = 4doublestranded
prophase 1
• 1st division of meiosis separates homologous pairs
tetrad
synapsis
1n = 2doublestranded
telophase 1
2n = 4doublestranded
metaphase 1
I can’t hear you!
reduction
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Fig. 13-8b
Prophase I Metaphase I
Centrosome(with centriole pair)
Sisterchromatids Chiasmata
Spindle
Centromere(with kinetochore)
Metaphaseplate
Homologouschromosomes
Fragmentsof nuclearenvelope
Microtubuleattached tokinetochore
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During metaphase, chromosomes line up in PAIRS, but they line up randomly. This picture shows all the different possible arrangements for an organism with 6 chromosomes.
This is called INDEPENDENT ASSORTMENT
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Fig. 13-8c
Anaphase I Telophase I andCytokinesis
Sister chromatidsremain attached
Homologouschromosomesseparate
Cleavagefurrow
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Meiosis 2
1n = 2doublestranded
metaphase 2
1n = 2singlestranded
telophase 2
prophase 2
1n = 2doublestranded
• 2nd division of meiosis separates sisterchromatids
What doesthis divisionlook like?
4
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Fig. 13-8d
Prophase II Metaphase II Anaphase II Telophase II andCytokinesis
Sister chromatidsseparate Haploid daughter cells
forming
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Steps of meiosis
• Meiosis 1– interphase– prophase 1– metaphase 1– anaphase 1– telophase 1
• Meiosis 2– prophase 2– metaphase 2– anaphase 2– telophase 2
2nd division of meiosis separates sister chromatids
(1n 1n)* just like mitosis *
1st division of meiosis separates homologous pairs
(2n 1n)“reduction division”
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Meiosis 1 & 2
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