Meiosis for moodle
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MeiosisMeiosis
STS BiologySTS Biology
Unit 4 GeneticsUnit 4 Genetics
Chapter 11Chapter 11
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Chromosome Number Chromosome Number
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Chromosome NumberChromosome Number Eukaryotic cells have pairs of Eukaryotic cells have pairs of
homologous chromosomeshomologous chromosomes Cells with a Cells with a doubledouble set of chromosomes set of chromosomes
are are diploid (2n)diploid (2n) Cells with Cells with oneone set of chromosomes are set of chromosomes are
haploid (n)haploid (n)
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Humans Humans bodybody cells cells are diploid (2N)are diploid (2N) 2N = 462N = 46
GametesGametes - sex cells - sex cells Males – spermMales – sperm Females – eggsFemales – eggs
Gametes are Gametes are HAPLOIDHAPLOID N = 23N = 23
Fertilization restores Fertilization restores the diploid state (N + the diploid state (N + N = 2N)N = 2N)
ZygoteZygote - a fertilized egg (2N)
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Meiosis…bottom Meiosis…bottom lineline
Process to make haploid Process to make haploid gametesgametes
Chromosome # is cut in Chromosome # is cut in halfhalf 2N 2N N N
Occurs in ovaries & testesOccurs in ovaries & testes TWO cell divisionsTWO cell divisions Increases genetic Increases genetic
variationvariation
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Prior to Prior to Meiosis….INTERPHASEMeiosis….INTERPHASE
What happens during Interphase???What happens during Interphase??? DNA REPLICATION!!!DNA REPLICATION!!!
2N
2N
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Meiosis IMeiosis I• Prophase I – homologous chromosomes form tetrads & crossing over occurs• Metaphase I – tetrads line up along the middle
• Anaphase I – homologous chromosomes separate• Telophase I – two nuclei form
• Cytokinesis I – cells separate
2N
N N
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Crossing OverCrossing Over Occurs in Occurs in Prophase IProphase I Chromosomes exchange portions of their Chromosomes exchange portions of their
chromatids chromatids Increases genetic variationIncreases genetic variation
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Meiosis IIMeiosis II• Prophase II – spindle forms, prepares for division
• DNA replication DOES NOT occur between Meiosis I & II
• Metaphase II – chromosomes align along the center• Anaphase II – sister chromatids separate
• Telophase II – nuclei reform; cytokinesis
N N
N N NN Looks A LOT like MITOSIS!
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Meiosis in Men vs. Meiosis in Men vs. WomenWomen
Men: 4 sperm Men: 4 sperm are madeare made
Women: only Women: only 1 egg is made1 egg is made
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Meiosis vs. MitosisMeiosis vs. Mitosis
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Independent AssortmentIndependent Assortment Mendel’s Law of Independent AssortmentMendel’s Law of Independent Assortment Alleles (form of a gene) separate independently of Alleles (form of a gene) separate independently of
each othereach other Leads to the formation of MANY different gametesLeads to the formation of MANY different gametes
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Meiosis SummaryMeiosis Summary Chromosome number is cut in half!Chromosome number is cut in half! Produces haploid gamete cells – egg and spermProduces haploid gamete cells – egg and sperm Tetrads form during Prophase ITetrads form during Prophase I
Crossing over occurs in tetrad formation – Crossing over occurs in tetrad formation – leads to increased genetic variationleads to increased genetic variation
Two cell divisions, but DNA does not replicate Two cell divisions, but DNA does not replicate between divisionsbetween divisions
Results in 4 sperm cells in men and 1 egg in Results in 4 sperm cells in men and 1 egg in womenwomen
Which chromosomes wind up in which sex Which chromosomes wind up in which sex cells? It’s random…due to independent cells? It’s random…due to independent assortment!!assortment!!