Meiosis &
Sexual Reproduction
AP Biology
2007-2008
Cell division / Asexual reproduction
 Mitosis

produce cells with same information
 identical daughter cells

exact copies
 clones

same amount of DNA
 same number of chromosomes
 same genetic information
 What is Binary Fission?
- Answer this for Wednesday!
AP Biology
Aaaargh!
I’m seeing
double!
Okay then….?
 Why produce sperm and eggs (and even go through
the hassle of dating)?
 Why can’t we make eggs & sperm by mitosis?
Answer these!! Before continuing!!
+
egg
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sperm
zygote
Sexual reproduction creates variability
Sexual reproduction allows us to maintain both
genetic similarity & differences.
Jonas
Brothers
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Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez
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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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

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
Meiosis: production of gametes

chromosome number
must be reduced
 diploid  haploid
haploid
 2n  n
 humans: 46  23
 meiosis reduces
chromosome number
 makes gametes

fertilization restores
chromosome number
 haploid  diploid
 n  2n
diploid
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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
AP Biology
1n = 2
double
stranded
Trading pieces of DNA
 Crossing over

during Prophase 1, sister
chromatids intertwine
 homologous pairs swap
pieces of chromosome
 DNA breaks & re-attaches
synapsis
tetrad
AP Biology
prophase 1
Meiosis 2
 2nd division of meiosis
1n = 2
double
stranded
separates sister
chromatids
prophase 2
What does
this division
look like?
1n = 2
single
stranded
AP Biology
1n = 2
double
stranded
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 *
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
Sperm production
Epididymis
Testis
Coiled
seminiferous
tubules
germ cell
(diploid)
primary
spermatocyte
(diploid)
MEIOSIS I
secondary
spermatocytes
(haploid)
Vas deferens
spermatids
(haploid)
spermatozoa
 Spermatogenesis
continuous
& prolific process
Cross-section of
seminiferous tubule
 each ejaculation =
100-600 million sperm
AP Biology

MEIOSIS II
Egg production: Oogenesis
 Begins in the ovaries of the


female fetus before birth
 Pauses during first meiotic
division
Final development occurs in
the ovaries of the adult female
 Each month one egg
matures as cued by
hormones
 Completes the first meiotic
division and starts the
second meiotic division
Last bit of meiosis is finalized
at time of fertilization
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Putting all
your egg
in one basket!
Oogenesis
primary follicles
germinal cell
(diploid)
fallopian tube
fertilization
primary
oocyte
(diploid)
MEIOSIS I
secondary
oocyte
(haploid)
first polar body
MEIOSIS II
developing
follicle
mature follicle with
secondary oocyte
ruptured follicle
(ovulation)
after fertilization
second
polar body
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ovum
(haploid)
corpus luteum
sperm vs. egg production
 Similarities
Both produce haploid cells by meiosis
 Both take place in the gonads
 both are controlled by hormones
 Differences
 Spermatogenesis produces 4 sperm each time
while oogenesis produces only 1 egg
 Formation of mature sperm continually occurs
while eggs only mature once a month (on
average)
 Sperm formation never stops, egg formation
ends at menopause
 Sperm can be released at anytime while eggs
AP Biology are released only once a month

Differences across kingdoms
 Not all organisms use haploid & diploid
stages in same way


which one is dominant (2n or n) differs
but still alternate between haploid & diploid
 must for sexual reproduction
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