Cell Division
Meiosis
Spring 2009
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Cell Division used for reproduction
Occurs in gonads (sex organs)
Male – testes
Female – ovaries
Goal: to produce 4 cells with ½ normal number
of chromosomes (Reduction division)
Why?
Chromosomes in pairs or singles?
Cells Diploid? Yes, for original
Goal : Haploid = ½ normal number of
chromosomes; chromosomes in singles
Also called monoploid
Meiosis
Male Meiosis
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Location: testes
Begins – puberty (11-13)
Ends – death
How often? Everyday to 100’s of
thousands of cells
• End product? Every one cell produces 4
sperm cells
Cell is normal cell in
Interphase
# Chromosomes?
46
Pairs or singles?
Diploid or Haploid?
During Synthesis phase
of cell cycle (S phase)
chromosomes duplicate
Number now?
92
Held together by?
Prophase I
Prophase Activities occur
Chromatin thickens and
sister chromatids appear
Centrioles duplicate and
move to poles
Spindle fibers grow
Nuclear membrane and
nucleolus disappear
92 chromosomes
Metaphase I
Sister chromatids line up on
equator
Same as mitosis?
NO……..
Line up 4 abreast - Tetrad
Number of chromosomes?
Still 92
Mitosis Chromosome Line Up
Meiosis Chromosome
Line Up
Anaphase I
Sister chromatids are pulled to
poles
Number of chromosomes?
Still 92
Telophase I
Cleavage furrow forms
Cell splits into two
Reverse of prophase
Nuclear membrane,
nucleolus return
Spindle fibers dissolve
Number of chromosomes in
each?
46
Diploid
• Two new cells skip G1 and S phases
• Do NOT duplicate the DNA
(chromosomes)
• Jump right back into a second division
cycle
Prophase II
Prophase Activities occur
Chromatin thickens and
sister chromatids appear
Centrioles duplicate and
move to poles
Spindle fibers grow
Nuclear membrane and
nucleolus disappear
Number of chromosomes?
46 in each
Diploid or Haploid?
Metaphase II
Sister chromatids line up on equator
In tetrads?
NO
Still 46 chromosomes in each
Still diploid
Anaphase II
Sister chromatids are pulled to poles
Still 46 chromosomes in each
Still diploid
Telophase II
Cleavage furrow forms
Cell splits into two
Reverse of prophase
Nuclear membrane, nucleolus return
Spindle fibers dissolve
Number of chromosomes in each?
23
Diploid?
No
Haploid = ½ normal number of
chromosomes
One centriole in each forms a flagellum
Female Meiosis
• Location: Ovaries
• Begins: 3 months before birth, goes to
metaphase I and stops
• Begins again: at puberty – Menarche (11-13)
• Ends: Menopause (average age of 52)
• How often? Occurs to one cell in one ovary once
every 28 days
• End product? One egg cell (ovum) and 3 polar
bodies that die
Cell is normal cell in
Interphase
# Chromosomes?
46
Pairs or singles?
Diploid or Haploid?
During Synthesis phase
of cell cycle (S phase)
chromosomes duplicate
Number now?
92
Held together by?
Prophase I
Prophase Activities occur
Chromatin thickens and
sister chromatids appear
Centrioles duplicate and
move to poles
Spindle fibers grow
Nuclear membrane and
nucleolus disappear
92 chromosomes
Metaphase I
• Sister chromatids
line up on equator
• Same as mitosis?
• NO……..
• Line up 4 abreast Tetrad
• Number of
chromosomes?
• Still 92
Meiosis Chromosome
Line Up
Anaphase I
Sister chromatids are pulled to
poles
Number of chromosomes?
Still 92
Telophase I
Cleavage furrow forms
Cell splits into two
Splitting is unequal
Reverse of prophase
Nuclear membrane, nucleolus return
Spindle fibers dissolve
Number of chromosomes in each?
46
Diploid
Two new cells skip G1 and S
phases
Do NOT duplicate the DNA
(chromosomes)
Jump right back into a second
division cycle
Prophase II
Prophase Activities occur
Chromatin thickens and sister
chromatids appear
Centrioles duplicate and move to poles
Spindle fibers grow
Nuclear membrane and nucleolus
disappear
Number of chromosomes?
46 in each
Diploid or Haploid?
Metaphase II
Sister chromatids line up on equator
In tetrads?
NO
Still 46 chromosomes in each
Still diploid
Anaphase II
Sister chromatids are pulled to poles
Still 46 chromosomes in each
Still diploid
Telophase II
Cleavage furrow forms
Cell splits into two
Splitting is again unequal
One large cell = ovum
Three polar bodies die
Reverse of prophase
Nuclear membrane, nucleolus return
Spindle fibers dissolve
Number of chromosomes in each?
23
Diploid?
No
Haploid = ½ normal number of
chromosomes
Meiosis
Fertilization
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Occurs in female body fallopian tube
Tube that leads from ovary to uterus
Egg and sperm are called gametes
Egg + sperm = zygote (fertilized egg)
Begins mitosis to form fetus
Takes only 266 days = gestation period
Female Reproductive Anatomy
23 Haploid
23 Haploid
46 Diploid
Twins
Fraternal Twins
Identical Twins
How are twins produced?
• Identical
• One sperm and one egg
• Cell mass splits early to form two identical individuals with the same
DNA
• Same sex?
• Yes! Two identical boys or two identical girls
• Fraternal
• Two separate sperm fertilize two separate eggs
• Two totally separate individuals (siblings) with different DNA
• Same sex?
• Do not have to be…….
• Can be two boys, two girls, one boy and one girl