10.1Meiosis

Traits- characteristics you are born with !

ex: hair color, eye color, attached earlobes!!

The instructions are coded in DNA, located in your chromosomes!!

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Each chromosome contains hundreds of genes !

(humans have about 30,000 genes)

Genes- segments of DNA that control the production of a protein.!!

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Human cells have 46 chromosomes, !or 23 pairs (one from each parent).!!homologous chromosomes- !pairs of chromosomes !

-same length!-same centromere position!-carry genes for the same traits!

Karyotype

Chromosomenumbers:•  Ciliatedprotozoa-15,000•  Butterfly-268•  Fern-184•  Hedgehog-90•  Sheep-54•  Human-46•  Tiger-38•  Cabbage-18•  Koala-16•  FruitFly-8

gametes – sex cells with half the number of chromosomes !

Ex: egg and sperm!!To maintain the same number of chromosomes from generation to generation, organisms produce gametes.!

Haploid- a cell with n chromosomes !Diploid- a cell with 2n chromosomes!

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n = number of chromosomes in a gamete !!

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Skincell- haircell- spermcell-

Meiosis - cell division that reduces the number of chromosomes - produces gametes !

Requires two cell divisions, !meiosis I and meiosis II!!

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Meiosis!Copyright © McGraw-Hill Education

Meiosis I

Interphase-DNA replicates.!-Chromatin condenses.!

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Prophase I-Pairing of homologous chromosomes (one from mom, one from dad)!-Each chromosome consists of two sister chromatids (exact copies)!!

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Synapsis- a process during prophase/condensation which allows for crossing over. !Crossing over – DNA is exchanged between homologous chromosomes.!

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Homologouschromosomes-sisterchromatids

Metaphase Icentromeres attach to spindle fibers!Homologous chromosomes line up as a pair in the middle (equator)!

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Anaphase IHomologous chromosomes move apart.!chromosome number reduced from 2n to n!

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Meiosis!Copyright © McGraw-Hill Education

Telophase IChromosomes reach opposite poles.!

!Cytokinesis.!

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Meiosis II - second set of phases !!!**no second interphase**

Prophase IIspindle apparatus forms and chromosomes condense!

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Metaphase IIChromosomes line up in the middle (equator)!haploid number of chromosomes!

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Anaphase IISister chromatids pulled apart and move toward poles!

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Telophase IIchromosomes reach the poles!nuclear membranes reform!!! !

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Cytokinesis results in four haploid cells (n chromosomes)!!

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Each nucleus has !slightly different DNA!

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Mitosis !one cell division !two diploid (2n) cells!identical!

Meiosis !two cell divisions !four haploid (n) daughter cells !not genetically identical.!

• Meiosis = genetic variation.!

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The Importance of Meiosis- variation• During prophase I, chromosomes

line up randomly.!• Gametes end up with different

combinations of chromosomes.!• Genetic variation also comes from

crossing over and randomness of fertilization!

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Asexual reproduction (ex: mitosis)!• organism inherits all chromosomes from one parent!• offspring is genetically identical to its parent.!

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Sexual reproduction (ex: meiosis)!Rate of beneficial mutations is faster. !Beneficial genes multiply faster than for asexual organisms. !!

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