Transcript What do these 3 people have in common?

Start with 4 chromosomes
The word "meiosis" comes from the Greek meioun, meaning  "to make small,"
since it results in a  reduction in chromosome number in the gamete cell.
End with 2
chromosomes
Consider these Questions …
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1. How many chromosomes would a human sperm or an egg
contain if either one resulted from the process of mitosis?
a. 46 chromosomes
2. If a sperm containing 46 chromosomes fused with an egg
containing 46 chromosomes, how many chromosomes would
the resulting fertilized egg contain? Do you think this would
create any problems in the developing embryo?
b. 46 + 46 = 92; a developing embryo would not survive if
it contained 92 chromosomes.
3. In order to produce a fertilized egg with the appropriate
number of chromosomes (46), how many chromosomes
should each sperm and egg have?
 c. Sperm and egg should each have 23 chromosomes.
*Meiosis  how you make sperm and egg cells with only 23 chromosomes*
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1.  Number varies by organism
› a. Ex: turkey  82, fruit fly  6, human 46
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2. Organisms that reproduce sexually
have pairs of similar chromosomes
› a. Called  HOMOLOGOUS CHROMOSOMES
› b. Similar in size, structure, and carry genes for
the  same traits (but may be different forms of
the gene)
› c. Each member of a pair comes from  each
parent
› d. Ex: humans have  23 pairs of homologous
chromosomes, or  46 total
 i.  23 in sperm,  23 in egg, pair up
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3. Diploid vs. Haploid
a. Diploid
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i.  Double set of chromosomes
ii.  “2n”
iii. Humans – 23 sets/pairs,  2n = 46
b. Haploid
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i..  Single set of chromosomes
ii.  “n”
iii. Humans – one chromo. from each
set,  n = 23
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4. Chromosome numbers & cell types
a. Somatic cells
› i. “Regular” body cells
› ii.  Always diploid
› iii. Humans  46 chromo.
b. Gametes
› i. “sex cells”
› ii. Usually egg & sperm (or
pollen/spores)
› iii.  Always haploid
› iv. Humans  23 chromo.
Meiosis I
 1. Interphase I
› a. Cells undergo a round of  DNA replication,
forming duplicate chromosomes
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2. Prophase I
› a. Each chromosome pairs with its
corresponding homologous chromosome to
form a  tetrad
 i. tetrad  contains 4 chromatids
› b. Homologous chromosomes exchange
portions of their chromatids
 ii.  Called crossing over
tetrad
crossing-over
Crossing-Over
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3. Metaphase I
› a.  Spindle fibers attach to the chromosomes
› b. Tetrads line up in the  middle of the cell
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4. Anaphase I
› a. Fibers pull homologous chromosomes toward
 opposite ends of the cell
› b. Centromere  stays intact
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5. Telophase I and Cytokinesis
› a.  Nuclear membranes form
› b. Cell separates into  2 cells
Meiosis II
 1. Prophase II
› a. Meiosis I results in  2 haploid (N)
daughter cells each with  half the
number of homologous chromosomes as
the original cell
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2. Metaphase II
› a. Chromosomes line up in the  center of the
cell
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3. Anaphase II
› a. Paired  sister chromatids separate and
move towards  opposite ends of the cell
› b. Centromere  is broken
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4. Telophase II and Cytokinesis
› a. Meiosis II results in  4 haploid (N) daughter
cells
› b. Each cell contains  half the number of
chromosomes as the original cell
Tetrad = 4 chromatids
Homologous
chromosome
Meiosis II
Prophase II
Metaphase II
Meiosis I results in two
The chromosomes line up in a
haploid (N) daughter cells, similar way to the metaphase
each with half the number of stage of mitosis.
chromosomes as the original.
Anaphase II
The sister chromatids
separate and move toward
opposite ends of the cell.
Telophase II
Meiosis II results in
four haploid (N)
daughter cells.
Unequal
division of
cytoplasm
1. Males  4 sperm formed
2. Females  1 egg formed (larger)
and 2 polar bodies (smaller)
a.  only the egg is used in reproduction
Mitosis
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1. Known as  cell division
2. Consists of  1 division
3. Forms  2 genetically
identical daughter cells
4. Daughter cells have 
same amount of
chromosomes as parent
cell (humans = 46)
5. Occurs only in  body
cells
› somatic cells
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6. Purpose =  make more
body cells for growth and
repair
Meiosis
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1. Known as  cell reduction
division
2. Consists of  2 divisions
3. Forms  4 genetically different
daughter cells
4. Daughter cells have  half the
amount of chromosomes as the
parent cell (humans = 23)
5. Occurs only in  sex cells
› gametes
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6. Purpose =  form egg and
sperm cells for fertilization