Biology – Premed
Windsor University School of Medicine
and Health Sciences
DR. UCHE AMAEFUNA.
CHAPTER 21
MEIOSIS
There is more to lectures than the
power point slides!
Engage your mind
Meiosis &
Sexual Reproduction
2006-2007
Cell division / Asexual reproduction
Mitosis
• produce cells with same information
o identical daughter cells
• exact copies
o clones
• same number of chromosomes
o same genetic information
Aaaargh!
I’m seeing
double!
Asexual reproduction
• Budding
• Single-celled eukaryotes
o Yeast, Paramecium
o Amoeba
• Simple multicellular
eukaryotes
o Hydra
What are the
disadvantages of
asexual reproduction?
What are the
advantages?
How about the rest of us?
• What if a complex multicellular organism
(like us) wants to reproduce?
o joining
of egg + sperm
• Do we make egg & sperm by mitosis?
What if we did, then….
46
egg
+
46
92
sperm
zygote
Doesn’t work!
No!
diploid = 2 copies 2n
Human female karyotype
46 chromosomes
23 pairs
XX
diploid = 2 copies 2n
Human male karyotype
46 chromosomes
23 pairs
XY
How do we make sperm & eggs?
• Must reduce 46 chromosomes
23
o must half the number of chromosomes
o haploid
23
46
meiosis
46
zygote
23
egg
46
23
23
sperm
gametes
fertilization
Meiosis makes sperm & eggs
• 46 chromosomes to 23 chromosomes
o half the number of chromosomes
23
46
meiosis
46
diploid
egg
23
sperm
haploid
Paired chromosomes
• Homologous chromosomes
o
both chromosomes of a pair carry“matching” genes
 One from mom, one from dad
 control same inherited characters (ie. eye color)
 homologous = same information
diploid
2n
2n = 4
eye color
(brown?)
eye color
(blue?)
homologous
chromosomes
double stranded
homologous chromosomes
Crossing Over -> variation
nonsister chromatids
chiasmata: site of
crossing over
Prophase 1
Tetrad
variation
Meiosis 1 overview
• 1st division of meiosis
Copy DNA
prophase 1
• 4 chromosomes
• diploid
• 2n
double
stranded
Divide 1
Line Up 1
metaphase 1
telophase 1
• 2 chromosomes
• haploid
• 1n
Bye Bye 2
Meiosis 2 overview
telophase 2
telophase 1
Line Up 2
• 2nd division of meiosis
o looks like mitosis
metaphase 2
• 2 chromosomes
• haploid
• 1n
gametes
4
Meiosis = reduction division
• Meiosis
o special
cell division in sexually
reproducing organisms
o reduce number of chromosomes by
half
 2n
1n
 diploid
o makes
haploid
gametes (sperm, eggs)
Meiosis & mitosis
• Meiosis to
make gametes
o sperm
& egg
• Mitosis to make
copies of ALL
OTHER cells
for:
o growth
o repair
o development
Sexual reproduction lifecycle
• 2 copies
• diploid
• 2n
• 1 copy
• haploid
• 1n
fertilization
meiosis
We’re mixing
things up here!
A good thing?
• 1 copy
• haploid
• 1n
Putting it all together…
meiosis
fertilization
mitosis + development
gametes
46
23
meiosis
46
23
egg
23
23
46 46
46 46 46
4646 46
zygote
fertilization
sperm
46
mitosis &
mitosis
development
The value of meiosis 1
• meiosis keeps chromosome number
same from generation to generation
from Mom
Mom
Dad
offspring
from Dad
The value of meiosis 2
We’re
mixing things
up here!
• Change over time
o meiosis
introduces genetic variation
 new combinations of traits from
generation to generation
from Dad
variation
from Mom
offspring
new gametes made by offspring
How does this explain why kids are
similar to their parents but not the
same?
Michael & Kirk
Douglas
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez
 Meiosis I
 Purpose:
Meiosis I has two main purposes:
 It is the reduction division, so it reduces the number of
chromosomes in half, making the daughter cells haploid (when
the parent cell was diploid).
 It is during meiosis I that most of the genetic recombination
occurs.

 Phases:

Keep in mind that before meiosis begins at all, the DNA
undergoes replication, just like it did before mitosis started. So,
when you first see chromosomes in meiosis I, they have sister
chromatids, just like in mitosis. It is just that in meiosis I, we
will be talking about tetrads becoming visible, lining up,
separating, and decondensing (rather than chromosomes, like in
mitosis). Finally, cytokinesis occurs, too, any time after the
tetrads have moved out of the equator (just like in mitosis).
 Prophase I
Just like in mitosis, during prophase, DNA condensation
occurs, the nuclear envelope and nucleoli disappear, and the
spindle starts to form. The big difference is what is going on
with the chromosomes themselves.
 As DNA condensation proceeds and the chromosomes first
become visible, they are visible as tetrads. So, tetrads become
visible during prophase.

 Metaphase I
Tetrads line up at the equator. The spindle has completely
formed.
 It is during prophase I and metaphase I that genetic
recombination is occurring. Take a look at the genetic
recombination page to find out about how that happens
here. Keep in mind that it only happens when there are tetrads,
so as soon as anaphase I gets going, genetic recombination is over.

 Anaphase I

Tetrads pull apart and chromosomes with two chromatids
move toward the poles.
 Telophase I
Chromosomes with two chromatids decondense and a nuclear
envelope reforms around them. Each nucleus is now haploid.
 Keep in mind that it is not the number of chromatids per
chromosome that determine whether a cell is diploid or haploid,
but, it is the number of chromosomes and whether they are
paired that determines this.

 Meiosis II
 Purpose:

At the end of meiosis I, each chromosome still had two
chromatids. That is double the amount of DNA that a cell
should have. So, the entire reason to go through meiosis II is to
reduce the amount of DNA back to normal-- basically, to split
the chromosomes so that each daughter cell has only one
chromatid per chromosome (the normal genetic content).
 Phases:

As you read through the phases of meiosis II, you will see that
it looks just like mitosis. It is really similar to mitosis-- so keep
that in mind. The only difference is that the two chromatids per
chromosome are not necessarily identical due to genetic
recombination occuring in meiosis I.
 Prophase II

Chromosomes with two chromatids become visible as they
condense (and the nuclear envelope and nucleoli disappear, and
the spindle is forming).
 Metaphase II
Chromosomes with two chromatids line up at the
equator. The spindle is fully formed.
 Although genetic recombination primarily occurs during
meiosis I, the way the chromosomes line up during metaphase II
can also help to make unique daughter cells. I mention this on
the genetic recombination page.

 Anaphase II

Chromosomes split, so that a chromosome with only one
chromatid heads toward each pole.
 Telophase II

Chromosomes with only one chromatid decondense and get
surrounded by new nuclear envelopes. The four daughter cells
are now all haploid and have the right amount of DNA. They are
ready to develop into sperm or eggs now.