Meiosis

MEiosis Look at the first two letters of the word meiosis. It spells the word me. Meiosis is how ME got here! “Me” got here from sexual reproduction. Two gametes (one egg and one sperm) came together to make an organism (me!) Meiosis is the cell division that makes sex cells or gametes.

Meiosis Cells divide into sex cells (gametes) Gametes are haploid, or n (1/2 the correct number of chromosomes for organism) Male sex cell Sperm Made in testes

Meiosis Female sex cell Egg Made in ovaries Gametes contain ½ the number of original chromosomes.

Human Chromosome Numbers Human body cells have 46 chromosomes. Human sex cells formed from meiosis have 23 chromosomes. Human sex cells or gametes have ½ the number of chromosomes found in the body cells of an organism.

Prophase I Chromosome pairs (homologous) pair up Spindle fibers form Crossing over, the exchange of genetic information between chromosomes occurs. (increases variation) Pictture: http://www.educarchile.cl/UserFiles/P0001/Image/Mod_2_contenidos_estudiantes_ciencias_biologia/Dibujo%2009.JPG

http://www. wereyouwondering http://www.wereyouwondering.com/what-is-the-difference-between-mitosis-and-meiosis/

Metaphase I Paired chromosomes line up side by side across the center of the cell Pictture: http://www.educarchile.cl/UserFiles/P0001/Image/Mod_2_contenidos_estudiantes_ciencias_biologia/Dibujo%2009.JPG

Anaphase I Paired chromosomes move apart Pulled to opposite ends of the cell Pictture: http://www.educarchile.cl/UserFiles/P0001/Image/Mod_2_contenidos_estudiantes_ciencias_biologia/Dibujo%2009.JPG

Telophase I Center pinches cell into two cells Each new cell contains half of the genetic information. Pictture: http://www.educarchile.cl/UserFiles/P0001/Image/Mod_2_contenidos_estudiantes_ciencias_biologia/Dibujo%2009.JPG

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Prophase II New cells divide again Spindle fibers form and attach to chromosomes in each cell Pictture: http://www.educarchile.cl/UserFiles/P0001/Image/Mod_2_contenidos_estudiantes_ciencias_biologia/Dibujo%2009.JPG

Metaphase II Chromosomes line up single file across center of the cell. Pictture: http://www.educarchile.cl/UserFiles/P0001/Image/Mod_2_contenidos_estudiantes_ciencias_biologia/Dibujo%2009.JPG

Anaphase II Sister chromatids separate and move to opposite sides Double stranded chromosomes are separated into single stranded chromosomes Pictture: http://www.educarchile.cl/UserFiles/P0001/Image/Mod_2_contenidos_estudiantes_ciencias_biologia/Dibujo%2009.JPG

Telophase II Nuclei reform Spindle fibers break down Cytoplasm divides (cytokinesis) Pictture: http://www.educarchile.cl/UserFiles/P0001/Image/Mod_2_contenidos_estudiantes_ciencias_biologia/Dibujo%2009.JPG

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Meiosis produces 4 new cells from one. http://www.wereyouwondering.com/what-is-the-difference-between-mitosis-and-meiosis/ Meiosis produces 4 new cells from one. The new cells each contain ½ number of chromosomes. Notice that no two sex cells are alike!

Meiosis forms 4 male gametes (sperm) Meiosis forms 1 egg and 3 smaller cells in female.

Why Meiosis? Results in sex cells (gametes) for sexual reproduction Cells made by meiosis contain ½ the number of chromosomes for the organism (23 for humans) Sex cells or gametes are haploid or have an “n” number of chromosomes

Why Meiosis? The beginning of the word haploid sounds like the beginning of the word half. Source of variation in organisms (as well as sexual reproduction and mutations) Half the number of chromosomes is important so that fertilization can take place

Meiosis What is the second letter in the word meiosis? It is the letter e. All of the cells that are formed from meiosis have an e in them! Sex cells Gametes Egg Sperm Meiosis happens in ovaries and testes

Let’s Look at Meiosis 2n n n n n n n http://www.phschool.com/science/biology_place/biocoach/meiosis/geneseg1.html

Why is meiosis important? Meiosis must reduce the number of chromosomes by half for fertilization In fertilization the egg and sperm come together to make one cell called a zygote. Fertilization creates a single-celled zygote with the correct number of chromosomes for the organism (46 for humans). Zygote then grows by mitosis, making body cells.

Fertilization Fertilization egg (n) + sperm (n) http://www.dnalc.org/view/16240-Gallery-8-Sperm-fertilzing-an-egg.html Growth of cells by mitosis Zygote (2n) http://www.americanpregnancy.org/infertility/zift.html http://onepregnancy.files.wordpress.com/2011/02/2cell-embryo.jpg

Let’s Review Father makes sperm by meiosis Mother makes eggs by meiosis Sperm and eggs have ½ the number of chromosomes for an organism Sperm (n) haploid 23 chromosomes Growth by mitosis Zygote (2n) diploid 46 chromosomes Formed by meiosis Fertilization Egg (n) haploid 23 chromosomes

Meiosis I http://terra.dadeschools.net/books/Biology/BiologyExploringLife04/0-13-115075-8/text/chapter9/concept9.5.html

Meiosis II http://terra.dadeschools.net/books/Biology/BiologyExploringLife04/0-13-115075-8/text/chapter9/concept9.5.html