Meiosis &
Sexual Reproduction
AP Biology
2007-2008
 What if a complex multicellular organism
(like us) wants to reproduce?

joining of egg + sperm
 Do we make egg & sperm by mitosis? No!
What if we did, then….
46
egg
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+
46
92
sperm
zygote
Doesn’t work!
Human female karyotype
46 chromosomes
23 pairs
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Human male karyotype
46 chromosomes
23 pairs
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Homologous chromosomes
 Paired chromosomes- 1 from Mom, 1 from Dad

both chromosomes of a pair carry “matching” genes
 control same inherited characters
 homologous = same information
diploid
2n
2n = 4
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single stranded
homologous
chromosomes
double stranded
homologous chromosomes
How do we make sperm & eggs?
 Must reduce 46 chromosomes  23

must reduce the number of chromosomes by half
23
46
meiosis
46
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zygote
23
46
egg
23
23
sperm
gametes
fertilization
Meiosis: production of gametes
 Alternating stages

chromosome number
must be reduced
haploid
 diploid  haploid
 2n  n
 humans: 46  23
 meiosis reduces
chromosome number
 makes gametes

fertilization restores
chromosome number
 haploid  diploid
 n  2n
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diploid
Meiosis
 reduction division


reduce 2n  1n
diploid  haploid
 makes gametes
 sperm, eggs
Warning: meiosis evolved from mitosis, so stages
& “machinery” are similar but the processes are
AP
Biology
radically
different. Do not confuse the two!
Overview of meiosis
I.P.M.A.T.P.M.A.T
2n = 4
interphase 1
prophase 1
metaphase 1
anaphase 1
n=2
n=2
telophase 1
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prophase 2 metaphase 2 anaphase 2 telophase 2
n=2
Double division
of meiosis
DNA replication
1st division of
meiosis separates
homologous pairs
Meiosis 1
Meiosis 2
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2nd division of
meiosis separates
sister chromatids
Meiosis 1
 1st division of meiosis
2n = 4
single
stranded
separates homologous pairs
prophase 1
2n = 4
double
stranded
metaphase 1
2n = 4
double
stranded
synapsis
tetrad
reduction
telophase 1
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1n = 2
double
stranded
Meiosis 2
 2nd division of meiosis
1n = 2
double
stranded
separates sister
chromatids
prophase 2
1n = 2
double
stranded
1n = 2
single
stranded
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metaphase 2
4
telophase 2
Steps of meiosis
 Meiosis 1
interphase
 prophase 1
 metaphase 1
 anaphase 1
 telophase 1

 Meiosis 2
prophase 2
 metaphase 2
 anaphase 2
 telophase 2
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
1st division of
meiosis separates
homologous pairs
(2n  1n)
“reduction division”
2nd division of
meiosis separates
sister chromatids
(1n  1n)
* just like mitosis *
Trading pieces of DNA
 Crossing over

-during Prophase 1, sister
chromatids intertwine
 homologous pairs swap
pieces of chromosome
 DNA breaks & re-attaches
synapsis
tetrad
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prophase 1
Mitosis vs. Meiosis
 Mitosis






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1 division
-daughter cells
genetically identical
to parent cell
-produces 2 cells
2n  2n
-produces cells for
growth & repair
no crossing over
 Meiosis

2 divisions
-daughter cells
genetically different
from parent
-produces 4 cells
2n  1n
-produces gametes

-crossing over




Putting it all together…
meiosis  fertilization  mitosis + development
gametes
46
meiosis
23
23
egg
23
46
23
zygote
fertilization
sperm
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46
46 46
46 46 46
46
46
46
mitosis
development
Mendel’s Contribution
 segregation (Alleles separate.)
 independent assortment (random

separation)
for genes on different chromosomes
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Linked Genes
 Found on same chromosome
 Probability of segregation dependent

on distance between genes
Greater distance apart = more likely to
separate during crossing-over
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Chromosome Mapping
 Ex. Consider the following recombination
frequencies:
genes Recombination Frequency
A-B
8%
A-C
28%
A-D
25%
B-C
20%
B-D
33%
What order are these genes in the chromosome
map?
A) A-B-C-D
E) E-D-B-A
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B) A-C-D-B C) B-A-C-D D) D-A-B-C
The value of sexual reproduction
 Sexual reproduction introduces genetic variation

genetic recombination
 independent assortment of chromosomes
 random alignment of homologous chromosomes in Metaphase 1

crossing over
 mixing of alleles across homologous chromosomes

random fertilization
 Which sperm fertilizes which egg?
 Driving evolution

providing variation for natural selection
metaphase1
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Variation from genetic recombination
 Independent assortment of chromosomes
meiosis introduces genetic variation
 gametes of offspring do not have same
combination of genes as gametes from
parents

 random assortment in humans produces
223 (8,388,608) different combinations in gametes
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from
Mom
from Dad
offspring
new gametes
made by offspring
Figure 15.11 Meiotic nondisjunction
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Human Chromosomal Disorders
 Trisomy 21 (Down syndrome)
 Klinefelter syndrome
 Trisomy 13 (Patau syndrome)
 Trisomy 18 (Edwards syndrome)
 Turner syndrome
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Credits
 Based on the efforts of Kim B. Foglia,
ExploreBiology.com.
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