meiosis notes. meiosis process of reduction division in which the number of chromosomes per cell is...
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Meiosis NotesMeiosis Notes
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Meiosis• Process of reduction division in which the
number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell; happens in gametes (sex cells) – sperm & egg
• Homologous chromosomes – chromosomes that have a corresponding chromosome from the opposite-sex parent (2 sets of chromosomes, one from each parent)
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Diploid vs. Haploid• Diploid – a cell that contains both sets of
homologous chromosomes (two sets); represented by the symbol 2N – Found in somatic or body cells (ex. Skin,
digestive tract)– Example : Humans – 2N = 46
• Haploid – a cell that contains only a single set of chromosomes (one set); represented by the symbol N or 1N– Found in gametes or sex cells – sperm & egg– Example: Humans – N = 23
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Steps of Meiosis
• Meiosis usually involves two distinct divisions, called Meiosis I and Meiosis II
• By the end of Meiosis II, the one diploid cell that entered meiosis has become 4 haploid cells
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Meiosis IMeiosis I• Interphase I
–Cell undergoes a round of DNA replication forming duplicate chromosomes
ChromatinNuclear Envelope
Centrioles
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Meiosis IMeiosis I• Prophase I
–Each chromosome pairs with its corresponding homologous chromosome to form a tetrad
TETRAD
CROSSING OVERHomologous chromosomes
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Meiosis IMeiosis I• As the homologous
chromosomes pair up and form tetrads in Meiosis I, they exchange portions of their chromatids in a process called crossing-over crossing-over
• This produces chromosomes with different combinations than the parents
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Meiosis IMeiosis I• Metaphase I
–Chromosomes attach to the spindle fibers and move to the Middle of the cell
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Meiosis IMeiosis I• Anaphase I
–The spindle fibers pull the homologous chromosomes toward opposite ends of the cell
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Meiosis IMeiosis I• Telophase I &
Cytokinesis
–Nuclear envelopes form & the cell separates into two cells
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Meiosis II• Meiosis I results in two haploid (N)
daughter cells, each with half the number of chromosomes as the original cell
• The two cells produced by meiosis I now enter a second meiotic division, but unlike the first meiotic division, neither cell goes through a round of DNA replication before entering Meiosis II
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Meiosis IIMeiosis II• Prophase II
–Spindle fiber forms from the centrioles and the chromosomes attach themselves
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Meiosis IIMeiosis II• Metaphase II
–Chromosomes line up in the center of each cell (similar to mitosis)
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Meiosis IIMeiosis II• Anaphase II
–The sister chromatids separate and move toward opposite ends of the cell
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Meiosis IIMeiosis II• Telophase II &
Cytokinesis–Nuclear
membranes form and the cells separate into 4 haploid (N) daughter cells
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Gamete (Sex Cell) Formation• In male animals
(including humans), the haploid gametes produced by meiosis are called sperm
• 4 sperm are produced from one meiotic division
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Gamete (Sex Cell) Formation• In female animals
(including humans), the haploid gametes produced by meiosis are called eggs
• The cell divisions at the end of meiosis I & II are uneven, so that 1 large egg is produced and other 3 cells produced, called polar bodies, are not involved in reproduction
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Meiosis Animation
Meiosis I Animation
http://wps.prenhall.com/wps/media/objects/487/498728/CDA9_1/CDA9_1b/CDA9_1b.htm
Meiosis II Animation
http://wps.prenhall.com/wps/media/objects/487/498728/CDA9_1/CDA9_1c/CDA9_1c.htm