Meiosis &
Sexual Reproduction
Regents Biology
2006-2007
Cell division / Asexual reproduction
 Mitosis

produce cells with same information
 identical daughter cells

exact copies
 clones

same number of chromosomes
 same genetic information
Regents Biology
Aaaargh!
I’m seeing
double!
Asexual reproduction
 Single-celled eukaryotes
yeast
 Paramecium
 Amoeba

 Simple multicellular
eukaryotes

Hydra
 budding
What are the
disadvantages of
asexual reproduction?
Regents Biology
What are the
advantages?
 Budding in hydra
 http://youtu.be/a5oHMjGqjyo
 Planarians
 http://youtu.be/J9EuFuJF9N0
Regents Biology
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
Regents Biology
+
46
92
sperm
zygote
Doesn’t work!
diploid = 2 copies 2n
Human female karyotype
46 chromosomes
23 pairs
XX
Regents Biology
diploid = 2 copies 2n
Human male karyotype
46 chromosomes
23 pairs
XY
Regents Biology
How do we make sperm & eggs?
 Must reduce 46 chromosomes  23
must half the number of chromosomes
 haploid

23
46
meiosis
zygote
23
46
egg
23
46
Regents Biology
23
sperm
gametes
fertilization
Meiosis makes sperm & eggs
 46 chromosomes to 23 chromosomes

half the number of chromosomes
23
46
meiosis
46
diploid
Regents Biology
egg
23
sperm
haploid
Paired chromosomes
 Homologous chromosomes

both chromosomes of a pair carry “matching” genes
 control same inherited characters
 homologous = same information
diploid
2n
2n = 4
Regents Biology
eye color
(brown?)
eye color
(blue?)
homologous
chromosomes
double stranded
homologous chromosomes
double
stranded
Meiosis 1 overview
 1st division of meiosis
Copy DNA
Line Up 1
prophase 1
metaphase 1
 4 chromosomes
 diploid
 2n
Regents Biology
Divide 1
gamete
telophase 1
 2 chromosomes
 haploid
 1n
Bye Bye 2
Meiosis 2 overview
telophase 2
telophase 1
Line Up 2
 2nd division of meiosis

looks like mitosis
 2 chromosomes
 haploid
Regents Biology
 1n
metaphase 2
gametes
4
Meiosis = reduction division
 Meiosis
special cell division in sexually
reproducing organisms
 reduce number of chromosomes

 2n  1n
 diploid  haploid
 half

makes gametes
 sperm, eggs
Regents Biology
Meiosis & mitosis
 Meiosis to
make gametes

sperm & egg
 Mitosis to make
copies of cells
growth
 repair
 development

Regents Biology
Sexual reproduction lifecycle
 2 copies
 diploid
 2n
 1 copy
 haploid
 1n
fertilization
meiosis
We’re mixing
things up here!
A good thing?
Regents Biology
 1 copy
 haploid
 1n
Putting it all together…
meiosis  fertilization  mitosis + development
gametes
46
meiosis
23
23
egg
23
46
23
zygote
fertilization
sperm
Regents Biology
46
46 46
46 46 46
46
46
46
mitosis
&
mitosis
development
The value of meiosis 1
 Consistency over time

meiosis keeps chromosome number same
from generation to generation
from Mom
Mom
Dad
offspring
from Dad
Regents Biology
The value of meiosis 2
 Change over time

We’re
mixing things
up here!
meiosis introduces genetic variation
 gametes of offspring do not have
same genes as gametes from parents
 new combinations of traits
from Dad
variation
from Mom
offspring
Regents Biology
new gametes made by offspring
How does this explain:
family resemblance & differences!
Why are the kids so similar to
the parents but
not exact?
Michael & Kirk
Douglas
Regents Biology
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez
What are the
advantages of
asexual reproduction?
What are the
DISadvantages of
asexual reproduction?
Any Questions??
What are the
advantages of
sexual reproduction?
Regents Biology
What are the
DISadvantages of
sexual reproduction?
2006-2007