meiosis& - weebly · 2019-11-06 · •eachsynapsisis madeupof2pairsof sisterchromads •...
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Meiosis
Varia%on -‐ The art of crossing over
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Vocabulary You Need to Know!
• Chroma%d • Chroma%n • Sister Chroma%ds • Chromosomes • Homologous Chromosomes • Tetrad • Centromere
• Daughter Cell • Parent Cell • Haploid • Diploid • Crossing Over • Gene%c Recombina%on • Independent Assortment
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Diploid vs Haploid • Diploid – 2 copies of each chromosome – All our body cells are diploid – 2n
• Haploid – one copy of each chromosome – Our sex cells (sperm, egg) are haploid – 1n *n= number of copies of chromosome*
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• Each synapsis is made up of 2 pairs of sister chroma8ds
• This matched set of 4 chroma8ds is called a tetrad
MEIOSIS AND CROSSING OVER
Chromosomes are matched in homologous pairs
Chromosomes
Centromere
Sister chromatids
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• Crossing over is the exchange of corresponding segments between two non-‐sister chroma8ds of homologous chromosomes
• Gene8c recombina8on results from crossing over during prophase I of meiosis – This increases varia8on further
Crossing over further increases gene%c variability
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• How crossing over leads to gene8c recombina8on
• Nonsister chroma8ds break in two at the same spot
• The 2 broken chroma8ds join together in a new way
Tetrad (homologous pair of chromosomes in synapsis)
Breakage of homologous chromatids
Joining of homologous chromatids
Chiasma
Separation of homologous chromosomes at anaphase I
Separation of chromatids at anaphase II and completion of meiosis
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
1
2
3
4
Coat-color genes
Eye-color genes
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• A segment of one chroma8d has changed places with the equivalent segment of its nonsister homologue
• If there were no crossing over meiosis could only produce 2 types of gametes
Tetrad (homologous pair of chromosomes in synapsis)
Breakage of homologous chromatids
Joining of homologous chromatids
Chiasma
Separation of homologous chromosomes at anaphase I
Separation of chromatids at anaphase II and completion of meiosis
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
1
2
3
4
Coat-color genes
Eye-color genes
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Increasing Gene%c Diversity
• During Meiosis I two steps are key in increasing gene%c diversity
1. Crossing over: During Prophase I – Homologous chromosomes exchange pieces of DNA
2. Independent assortment: During Anaphase II – Homologous chromosomes and/or sister chroma%ds are randomly assigned to each daughter cell
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Crossing Over
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Independent Assortment
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Why Meiosis? • Humans are DIPLOID: 23 pairs of chromosomes
• Sex cells are HAPLOID: contain 1 set of 23 chromosomes
• Fer%liza%on of two SEX CELLS (sperm and egg) create one ZYGOTE that is DIPLOID
• Sexual reproduc%on creates a zygote that is gene%cally different than both its parents
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Why is Meiosis Important?
• First and foremost it allows for gene8c variability – This is the most important part of Meiosis. You get chromosomes from both parents. This is what allows you to be different from your parents or siblings.
• Gene%c variability means that everything isn’t a clone of the next
• It helps maintain the chromosome number in species • Meiosis can play a part in evolu%on (via natural selec%on)
• THE MULE
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Stages • The stages look very similar to the stages of mitosis-‐ a few differences:
• Meiosis I – Homologous chromosomes pair (tetrad) and line up at equator
– Sister chroma%ds move to the SAME pole of the cell “Tetrad”
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Stages • Meiosis II – Each daughter cell con%nues to divide to produce two haploid cells
– No replica%on of DNA between Meiosis I and Meiosis II
Move me to reveal important informa%on!