meiosis
DESCRIPTION
Meiosis. Making Reproductive Cells. Chromosomes. Meiosis. Produce 4 haploid gametes through Interphase – replication of DNA Meiosis I – separation of homologous chromosomes (in 4-5 steps: prophase I, metaphase I, anaphase I, and telophase I and cytokinesis I) - PowerPoint PPT PresentationTRANSCRIPT
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Meiosis
Making Reproductive Cells
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Chromosomes
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Meiosis Produce 4 haploid gametes through
- Interphase – replication of DNA
- Meiosis I – separation of homologous chromosomes (in 4-5 steps: prophase I, metaphase I, anaphase I, and telophase I and cytokinesis I)
- Meiosis II – separation of sister chromatids (prophase II, metaphase II, anaphase II, telophase II and cytokinesis II)
Only occurs in gonads
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MEIOSIS
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Interphase
• Growth and sythesis of DNA (and copy organelles)
• Same as mitosis
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Meiosis I
• division of nucleus (and cell) to form 2 haploid cells by splitting homologous pairs
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Prophase I
Synapsis – pairing of the homologous chromosomes
Tetrad – homologous pair
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Prophase I• During prophase I, the homologous
pairs meet and join together to form a tetrad (four chromatids all together)
• This process is called synapsis• While a tetrad is present, crossing
over occurs which involves the genes of one chromatid are exchanged for the genes of another. This gives rise to genetic variability.
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Crossing over
exchange
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pairs line up in middle
• Homologous pairs split
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Meiosis II
• Now the chromosome number has been reduced to haploid
• The function:• separate the chromosomes into four
chromatids to produce four genetically unique gametes
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• http://www.tokyo-med.ac.jp/genet/mfi-e.htm
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Twins
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Sex determination• The sex of a child is always determined by
the sperm of the father. Any presence of a Y chromosome makes the person a male.
• The mother’s eggs always carry an X chromosome.
• If the sperm that fertilizes the egg carries an X chromosome, the child will be a girl and if it carries a Y chromosome the child will be a boy.
• Girl=XX• Boy=XY
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Mutations
• There are many types of mutations that can occur in DNA.
• If the mutation involves one or only a few base pairs (only one gene), it is called a point mutation. • Substitution-effects only one amino
acid• Frame shift-can effect the entire
protein
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Chromosomal mutations•Duplication occurs when a
piece of a chromosome is duplicated and becomes too long.
•Example• Rett Syndrome
• is X-linked dominant, affecting almost exclusively girls
• Similar symptoms to autism and cerebral palsy
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Chromosomal mutations•Deletion occurs when a piece
of chromosome is entirely lost or broken off.
•Example•Cri du Chat ( Cry of the Cat)
•Distinctive cry •Severe Mental Retardation•Short Life Span
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Chromosomal mutations
• Inversion occurs when a chromosome piece breaks off and is reinserted in reverse orientation.
•There are typically no abnormalities
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Chromosomal mutations
• Translocation occurs when a piece of a chromosome breaks off and is attached to a different chromosome.
• Some diseases caused by translocations are:• Cancer• Inferility• Downs Syndrome (5% or less)
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Chromosome mutations
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Chromosomal mutations
• Nondisjunction occurs when an entire chromosome fails to separate in meiosis. One gamete will receive an extra chromosome and the other will lack the chromosome entirely.
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Nondisjunction
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Nondisjunction