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Meiosis & Sexual
Reproduction
Cell division & asexual reproduction
• Mitosis
• Produces cells with the same information
• Identical daughter cells
• Exact copies
• Clones
• Same amount of DNA
• Same number of chromosomes
• Same genetic information
Aaaargh!
I’m seeing
double!
Asexual reproduction
• Single-celled eukaryotes
• Yeast (fungi)
• Protists
• Ameoba
• Paramecium
• Simple multicellular eukaryotes
• Hydra
What are the
disadvantages of
asexual reproduction?
What are the
advantages?
How about the rest of us?
• What if a complex multicellular organism wants to
reproduce?
• Joining of sperm and egg
• Do we make sperm and egg by mitosis? No!
What if we
did, then….
46
egg
+
46
92
sperm
zygote
Doesn’t work!
Homologous chromosomes
• Paired chromosomes
• Both chromosomes of a pair carry “matching” genes
• Control same inherited characteristics
double stranded
• Homologous = same information
homologous chromosomes
diploid
2n
2n = 4
single stranded
homologous chromosomes
Human female karyotype
46 chromosomes
23 pairs
Human male karyotype
46 chromosomes
23 pairs
How do we make sperm and egg?
• Must reduce 46 chromosomes  23
• Must reduce the number of chromosomes by half
23
46
meiosis
egg
gametes
46
23
sperm
zygote
23
46
23
fertilization
Meiosis: production of gametes
• Alternating stages
haploid
• Chromosome number must be
reduced
• Diploid  haploid
• 2n  n
• Humans: 46  23
• Fertilization restores chromosome
number
• Haploid  diploid
• n  2n
diploid
Sexual reproduction
life cycle
In the next
generation…
We’re mixing
things up here!
A good thing?
 1 copy
 haploid
 1n
gametes
fertilization
 2 copies
 diploid
 2n
meiosis
 1 copy
 haploid
 1n
gametes
Meiosis
• Reduction division
• Special cell division for sexual
reproduction
• Reduce 2n  n
• Diploid  haploid
• “two”  “half”
• Makes gametes
• Sperm or egg
Warning: meiosis evolved from mitosis, so stages & “machinery” are
similar but the processes are radically different. Do not confuse the two!
I.P.M.A.T.P.M.A.T
Overview of
meiosis
2n = 4
interphase I
prophase I
metaphase I
anaphase I
n=2
n=2
prophase II metaphase II anaphase II telophase II
n=2
telophase I
Double division
of meiosis
DNA replication
1st division of
meiosis separates
homologous pairs
2nd division of
meiosis separates
sister chromatids
Meiosis 1
Meiosis 2
Preparing for meiosis
• 1st step of meiosis
2n = 6
single
stranded
• Replication of DNA
• Why bother?
• Meiosis evolved after mitosis
• Convenient to use “machinery” of
mitosis
• DNA replicated in S phase of interphase
of MEIOSIS
(just like in mitosis)
Prophase I
2n = 6
double
stranded
2n = 4
single
stranded
Meiosis I
• 1st division of meiosis
separates homologous
chromosomes
synapsis
prophase 1
2n = 4
double
stranded
metaphase 1
2n = 4
double
stranded
reduction
telophase 1
tetrad
1n = 2
double
stranded
Meiosis II
1n = 2
double
stranded
• 2nd division of meiosis
separates sister chromatids
prophase II
1n = 2
double
stranded
1n = 2
single
stranded
Metaphase II
4
telophase II
Steps of meiosis
• Meiosis I
1st division of
meiosis separates
homologous pairs
• Meiosis II
2nd division of
meiosis separates
sister chromatids
• Interphase
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
(2n  1n)
“reduction division”
(1n  1n)
* just like mitosis *
Trading pieces of DNA
Prophase I
• Crossing over
• During Prophase I, sister
chromatids intertwine
• Homologous pairs swap pieces
of chromosome
• DNA breaks & reattaches
synapsis
tetrad
Crossing over
• 3 steps
• Cross over
• Breakage of DNA
• Re-fusing of DNA
• New combination of traits
What are the
advantages of
crossing over in
sexual reproduction?
Mitosis vs Meiosis
Mitosis vs Meiosis
• 1 division
• Daughter cells genetically
identical to parent cell
• Produces 2 cells
• 2n  2n
• Produces cells for growth
& repair
• No crossing over
• 2 divisions
• Daughter cells genetically
different from parent
• Produces 4 cells
• 2n  n
• Produces gametes
• Crossing over
Putting it all together …
meiosis  fertilization  mitosis + development
gametes
46
23
meiosis
46
egg
23
23
23
sperm
fertilization
46
46 46
46 46 46
46
46
46
zygote
mitosis
development
Metaphase I
The value of sexual reproduction
• Sexual reproduction introduces genetic variation
• Genetic recombination
• Independent assortment of chromosomes
• Random alignment of homologous chromosomes during
Metaphase I
• Crossing over
• Mixing of alleles across homologous chromosomes
• Random fertilization
• Which sperm fertilizes which egg?
• Drives evolution
• Provides variation for natural selection
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 the parents
• Random assortment in humans produces 223 (8,388,608)
different combinations in gametes
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
Variation from random fertilization
• Sperm + egg = ?
• Any 2 parents will produce a zygote with over 70 trillion
(223 x 223) possible diploid combinations
• Spermatogenesis
• continuous & prolific process
• each ejaculation =
100-600 million sperm
Sperm production
Testis
Epididymis
Coiled
seminiferous
tubules
germ cell
(diploid)
primary
spermatocyte
(diploid)
MEIOSIS I
secondary
spermatocytes
(haploid)
Vas deferens
spermatids
(haploid)
spermatozoa
Cross-section of
seminiferous tubule
MEIOSIS II
Egg production
• Oogenesis
What is the advantage of
this development system?
• Eggs in ovaries halted before
Anaphase I
• Meiosis I completed during
maturation
• Meiosis II
Meiosis 1 completed
completed after during egg maturation
fertilization
• 1 egg + 3 polar bodies
Meiosis 2 completed
triggered by fertilization
unequal divisions
ovulation
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
Differences across kingdoms
• Not all organisms use haploid & diploid stages in the
same way
• Dominant stage differs (2n or n)
• But still alternate between haploid and diploid
• Must for sexual reproduction
Any Questions??
What are the
DISadvantages of
sexual reproduction?