Division Ave. High School
Ms. Foglia
AP Biology
Meiosis &
Sexual Reproduction
AP Biology
2007-2008
Cell division / Asexual reproduction
§ Mitosis
u
produce cells with same information
u
exact copies
u
same amount of DNA
§ identical daughter cells
§ clones
§ same number of chromosomes
§ same genetic information
Aaaargh!
I’m seeing
double!
AP Biology
Asexual reproduction
§ Single-celled eukaryotes
u
u
yeast (fungi)
Protists
§ Paramecium
§ Amoeba
§ Simple multicellular
eukaryotes
u Hydra
budding
budding
What are the
disadvantages of
asexual reproduction?
AP Biology
What are the
advantages?
1
Division Ave. High School
Ms. Foglia
AP Biology
How about the rest of us?
§ What if a complex multicellular organism
§
(like us) wants to reproduce?
u joining of egg + sperm
Do we make egg & sperm by mitosis? No!
What if we did, then….
46
egg
+
46
92
sperm
zygote
Doesn’t work!
AP Biology
Human female karyotype
46 chromosomes
23 pairs
AP Biology
Human male karyotype
46 chromosomes
23 pairs
AP Biology
2
Division Ave. High School
Ms. Foglia
AP Biology
Homologous chromosomes
§
Paired chromosomes
u both chromosomes of a pair carry “matching” genes
§ control same inherited characters
§ homologous = same information
diploid
2n
2n = 4
single stranded
homologous
chromosomes
double stranded
homologous chromosomes
AP Biology
How do we make sperm & eggs?
§
Must reduce 46 chromosomes ® 23
u must reduce the number of chromosomes by half
23
46
meiosis
46
AP Biology
zygote
23
egg
46
23
23
fertilization
sperm
gametes
Meiosis: production of gametes
§ Alternating stages
u
chromosome number
must be reduced
haploid
§ diploid ® haploid
§ 2n ® n
w humans: 46 ® 23
§ meiosis reduces
chromosome number
§ makes gametes
u
AP Biology
fertilization restores
chromosome number
§ haploid ® diploid
§ n ® 2n
diploid
3
Division Ave. High School
Ms. Foglia
AP Biology
Sexual reproduction lifecycle
§ 2 copies
§ diploid
§ 2n
§ 1 copy
§ haploid
§ 1n
fertilization
meiosis
In the next
generation…
We’re mixing
things up here!
A good thing?
gametes
§ 1 copy
§ haploid
§ 1n
gametes
AP Biology
Meiosis
§
Reduction Division
u special cell division for
sexual reproduction
u reduce 2n ® 1n
u diploid ® haploid
§ “two” ® “half”
u
makes gametes
§ sperm, eggs
Warning: meiosis evolved from mitosis, so stages
& “machinery” are similar but the processes are
AP radically
Biology
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
prophase 2 metaphase 2 anaphase 2 telophase 2
n=2
telophase
AP Biology
1
4
Division Ave. High School
Ms. Foglia
AP Biology
Double division
of meiosis
DNA replication
Repeat
I can’t
after
hear you
me!
Meiosis 1
1st division of
meiosis separates
homologous pairs
Meiosis 2
2nd division of
meiosis separates
sister chromatids
AP Biology
Preparing for meiosis
§ 1st step of meiosis
u
u
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)
M1 prophase
AP Biology
Meiosis 1
§ 1st division of meiosis
2n = 6
double
stranded
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
hear you
me!
AP Biology after
1n = 2
double
stranded
5
Division Ave. High School
Ms. Foglia
AP Biology
Meiosis 2
§ 2nd division of meiosis
1n = 2
double
stranded
separates sister
chromatids
prophase 2
What does
this division
look like?
1n = 2
double
stranded
1n = 2
single
stranded
AP Biology
metaphase 2
4
telophase 2
Steps of meiosis
§ Meiosis 1
1st division of
interphase
prophase 1
u metaphase 1
u anaphase 1
u telophase 1
§ Meiosis 2
u prophase 2
u metaphase 2
u anaphase 2
AP Biologyu telophase 2
u
u
meiosis separates
homologous pairs
(2n ® 1n)
“reduction division”
2nd division of
meiosis separates
sister chromatids
(1n ® 1n)
* just like mitosis *
Meiosis 1 & 2
AP Biology
6
Division Ave. High School
Ms. Foglia
AP Biology
Trading pieces of DNA
§ Crossing over
u
prophase 1
during Prophase 1, sister
chromatids intertwine
§ homologous pairs swap
pieces of chromosome
w DNA breaks & re-attaches
synapsis
tetrad
AP Biology
Crossing over
§ 3 steps
What are the
advantages of
crossing over in
sexual reproduction?
cross over
breakage of DNA
u re-fusing of DNA
u
u
§ New combinations of traits
AP Biology
Mitosis vs. Meiosis
AP Biology
7
Division Ave. High School
Ms. Foglia
AP Biology
Mitosis vs. Meiosis
§
Mitosis
u 1 division
u daughter cells
genetically identical
to parent cell
u produces 2 cells
u 2n ® 2n
u produces cells for
growth & repair
u no crossing over
§
Meiosis
u 2 divisions
u daughter cells
genetically different
from parent
u produces 4 cells
u 2n ® 1n
u produces gametes
u
crossing over
AP Biology
Putting it all together…
meiosis ® fertilization ® mitosis + development
gametes
46
23
meiosis
46
egg
23
23
46
46 46
46 46 46
46
4646
23
zygote
fertilization
AP Biology
sperm
mitosis
development
The value of sexual reproduction
§
Sexual reproduction introduces genetic variation
u genetic recombination
§ independent assortment of chromosomes
w random alignment of homologous chromosomes in Metaphase 1
u
crossing over
u
random fertilization
§ mixing of alleles across homologous chromosomes
§
§ which sperm fertilizes which egg?
Driving evolution
u providing variation for natural selection
metaphase1
AP Biology
8
Division Ave. High School
Ms. Foglia
AP Biology
Variation from genetic recombination
§ Independent assortment of chromosomes
u
u
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
u creates an infinite
variety in gametes
AP Biology
Variation from random fertilization
§ Sperm + Egg = ?
u
any 2 parents will produce a zygote with
over 70 trillion (223 x 223) possible diploid
combinations
AP Biology
9
Division Ave. High School
Ms. Foglia
AP Biology
Sexual reproduction creates variability
Sexual reproduction allows us to maintain both
genetic similarity & differences.
Jonas
Brothers
AP Biology
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez
Sperm production
Epididymis
Testis
germ cell
(diploid)
Coiled
seminiferous
tubules
primary
spermatocyte
(diploid)
MEIOSIS I
secondary
spermatocytes
(haploid)
Vas deferens
spermatids
(haploid)
MEIOSIS II
spermatozoa
§ Spermatogenesis
continuous
& prolific process
Cross-section of
seminiferous tubule
each ejaculation
=
AP Biology
100-600 million sperm
u
u
Egg production
§ Oogenesis
eggs in ovaries halted
before Anaphase 1
u Meiosis 1 completed
during maturation
u Meiosis 2 completed
unequal divisions
after fertilization
u 1 egg + 2 polar bodies
u
Meiosis 1 completed
during egg maturation
ovulation
What is the advantage of
this development system?
Meiosis 2 completed
AP Biology
triggered by fertilization
10
Division Ave. High School
Ms. Foglia
AP Biology
Putting all
your egg
in one basket!
Oogenesis
germinal cell
(diploid)
primary follicles
fallopian tube
fertilization
primary
oocyte
(diploid)
MEIOSIS I
secondary
oocyte
(haploid)
first polar body
MEIOSIS II
after fertilization
second
polar body
ovum
(haploid)
developing
follicle
mature follicle with
secondary oocyte
ruptured follicle
(ovulation)
corpus luteum
AP Biology
Differences across kingdoms
§
Not all organisms use haploid & diploid
stages in same way
u which one is dominant (2n or n) differs
u but still alternate between haploid & diploid
§ must for sexual reproduction
AP Biology
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