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Meiosis
MEiosis
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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
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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
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Female sex cell
 Egg
 Made in ovaries
Gametes contain ½ the number of
original chromosomes.
Human Chromosome Numbers
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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
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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.com/what-is-the-difference-between-mitosis-and-meiosis/
Metaphase I
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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
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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
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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
Let’s Review!
Prophase II
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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
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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
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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
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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
Let’s Review
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Meiosis produces 4 new cells from one.
The new cells each contain ½ number of chromosomes.
Notice that no two sex cells are alike!
http://www.wereyouwondering.com/what-is-the-difference-between-mitosis-and-meiosis/
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Meiosis forms 4 male gametes
(sperm)
Meiosis forms 1 egg and 3 smaller
cells in female.
Why Meiosis?
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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?
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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
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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!
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Sex cells
Gametes
Egg
Sperm
Meiosis happens in ovaries and testes
Let’s Look at Meiosis
2n
n
n
http://www.phschool.com/science/biology_place/biocoach/meiosis/geneseg1.html
n
n
n
n
Why is meiosis important?
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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
http://onepregnancy.files.wordpress.com/2011/02/2cell-embryo.jpg
Zygote
(2n)
http://www.americanpregnancy.org/infertility/zift.html
Let’s Review
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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
Growth by mitosis
23 chromosomes
Formed by meiosis
Egg (n)
haploid
23 chromosomes
Zygote (2n) diploid
Fertilization
46 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