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
AP Biology
Aaaargh!
I’m seeing
double!
Asexual reproduction
 Single-celled eukaryotes
yeast (fungi)
 Protists

 Paramecium
 Amoeba
 Simple multicellular
eukaryotes

Hydra
What are the
disadvantages of
asexual reproduction?
AP Biology
What are the
advantages?
budding
budding
How about the rest of us?
 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
AP Biology
+
46
92
sperm
zygote
Doesn’t work!
Review of Mitosis
AP Biology
How is meiosis different?
AP Biology
Important Vocab Word
Homologous chromosomes
 Paired chromosomes

both chromosomes of a pair carry “matching” genes
 control same inherited characters
 Get one from mom and one from dad
 homologous = same information
diploid
2n
2n = 4
AP Biology
single stranded
homologous
chromosomes
double stranded
homologous chromosomes
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
AP Biology
diploid
Sexual reproduction lifecycle
 2 copies
 diploid
 2n
 1 copy
 haploid
 1n
gametes
AP Biology
fertilization
meiosis
In the next
generation…
We’re mixing
things up here!
A good thing?
 1 copy
 haploid
 1n
gametes
Double division
of meiosis
DNA replication
Repeat
I can’t
after
hear you!
me!
1st division of
meiosis separates
homologous pairs
Meiosis 1
Meiosis 2
AP Biology
2nd division of
meiosis separates
sister chromatids
Preparing for meiosis
 1st step of meiosis
Duplication of DNA
 Why bother?

 meiosis evolved after mitosis
2n = 6
single
stranded
 convenient to use
“machinery” of mitosis
 DNA replicated in
S phase of interphase
of MEIOSIS
(just like in mitosis)
AP Biology
M1 prophase
2n = 6
double
stranded
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
IRepeat
can’t
AP Biology after
hear you!
me!
1n = 2
double
stranded
STAGES of Meiosis I
AP Biology
Meiosis 2
 2nd division of

meiosis separates
sister
chromatids
Halfs the
chromosome
number
What does
this division
look like?
1n = 2
single
stranded
AP Biology
1n = 2
double
stranded
1n = 2
double
stranded
prophase 2
metaphase 2
4
telophase 2
STAGES of Meiosis II
AP Biology
Steps of meiosis
 Meiosis 1
interphase
 prophase 1
 metaphase 1
 anaphase 1
 telophase 1

 Meiosis 2
prophase 2
 metaphase 2
 anaphase 2
 telophase 2
AP Biology

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
AP Biology
prophase 1
Crossing over




Chromosomes form tetrad called synapsis
Crossing over: formation of chiasma
breakage of DNA
re-fusing of DNA
 New combinations of traits
AP Biology
Mitosis vs. Meiosis
 Mitosis






AP Biology
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
AP Biology
46
46 46
46 46 46
46
46
46
mitosis
development
The value of sexual reproduction
 Sexual reproduction introduces genetic variation
genetic recombination
1. independent assortment of chromosomes

 random alignment of homologous chromosomes in Metaphase 1
2. crossing over
 mixing of alleles across homologous chromosomes
3. random fertilization
 which sperm fertilizes which egg?
 Driving evolution

providing variation for natural selection
metaphase1
AP Biology
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
AP Biology
from
Mom
from Dad
offspring
new gametes
made by offspring
Variation from crossing over
 Crossing over creates completely new
combinations of traits on each chromosome

creates an infinite
variety in gametes
AP Biology
Variation from random fertilization
 Sperm + Egg = ?

AP Biology
any 2 parents will produce a zygote with
over 70 trillion (223 x 223) possible diploid
combinations
Sperm production
Epididymis
Testis
Coiled
seminiferous
tubules
germ cell
(diploid)
primary
spermatocyte
(diploid)
MEIOSIS I
secondary
spermatocytes
(haploid)
Vas deferens
spermatids
(haploid)
spermatozoa
Cross-section of
seminiferous tubule
AP Biology
MEIOSIS II
Sperm production
 Spermatogenesis



continuous & prolific
process
Occurs in testes
(within seminiferous
tubules) and
completed in the
epididymis
During ejaculation
sperm moves from
epididymus to vas
deferens to the
urethra and out the tip
of the penis. The
seminal vesicle adds
mucus that helps feed
and protect the sperm
AP Biology
each ejaculation =
100-600 million sperm

Egg production: Oogenesis
AP Biology
Egg production
 Oogenesis




Eggs in ovaries arrested in prophase I of meiosis
at birth. This is called the Primary oocyte
Meiosis 1 completed for one egg each month
upon female entrance into puberty. Called
Secondary oocyte. Note the unequal division and
the creation of a polar body
Meiosis 2 completed after fertilization by sperm
1 egg + 2 polar bodies
 Polar bodies are destroyed
What is the advantage of
this development system?
AP Biology