Chapter 13 - Meiosis and Sexual Life Cycles
Review: Describe the functions of mitosis.
High frequency quiz mistakes:
1. Incorrect number of chromosomes used in
depiction of mitotic cell division. Question
stated that the cell was diploid and the
haploid number (n) was 2.
Diploid (2n)
2 sets
Haploid number (n)
(number of chromosomes in a set)
X
2
=
4 chromosomes
in the cell
Chapter 13 - Meiosis and Sexual Life Cycles
Review: Describe the functions of mitosis.
High frequency quiz mistakes:
Practice
A. How many chromosomes in a triploid cell
with a haploid number of 7? 21
B. A diploid cell has 24 chromosomes. What is
the value of n? 12
C. A 4n cell has 40 chromosomes. What is the
ploidy of this cell and how many chromosomes
would you expect to find in its gametes? Tetraploid, 20
Chapter 13 - Meiosis and Sexual Life Cycles
Review: Describe the functions of mitosis.
High frequency quiz mistakes:
Practice
D. Draw a triploid nucleus of a cell with n = 2.
Chapter 13 - Meiosis and Sexual Life Cycles
Review: Describe the functions of mitosis.
High frequency quiz mistakes:
2. What are the two possible chromosome
combinations found in human male sperm?
- Are the gametes haploid or diploid?
Ans: One of each homologous pair
1 through 22 and X
1 through 22 and Y
Chapter 13 - Meiosis and Sexual Life Cycles
Review: Describe the functions of mitosis.
This brings us to our next adventure…how are
the gametes made?
Meiotic Cell Division (the other cell division)
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Compare asexual to sexual reproduction
Reproduction
A. Asexual Reproduction
B. Sexual Reproduction
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Compare asexual to sexual reproduction
A. Asexual Reproduction
1. One parent
2. Genetically identical offspring (called clones) if we ignore
mutations, which they rely on to evolve.
Made possible by begin able to have numerous offspring very quickly as positive mutations
are rare, but if you have millions of offspring in a few days one likely has such a mutation.
What is a positive mutation in general?
One that give the organism (vehicle) a better ability to survive and reproduce in current environment.
3. Single-cell organisms (certain protists [ex. amoeba] and fungi
[ex. yeast] and all prokaryotes [bacteria])
A. Prokaryotic cell cycle (binary fission)
B. eukaryotic cell cycle (“mitosis”)
4. Plants (vegetative propagation) and animals
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Compare asexual to sexual reproduction
B. Sexual Reproduction
- Two parents
- Gametes produced (fertilization)
- Highly variable offspring due to
mixing of DNA of two parents
- Most eukaryotes
Tend to have fewer offspring over longer periods of
time making mutations not a reliable means of
generating diversity…
…better to shuffle the DNA between two organisms.
Chapter 13 - Meiosis and Sexual Life Cycles
NEW AIM: Describe how gametes are formed.
Gametes
Somatic cells
Fig. 8.13
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Meiotic cell division
1. Formation of gametes in animals
- Therefore essential for sexual reproduction
2. Formation of spores in plants and fungi
3. Called reduction division
- The number of chromosomes is cut in half
(typically from diploid [2n] to haploid [n])
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Compare gametes to somatic cells
in terms of chromosome number.
(Soma- means body; somatic cell = body cell)
Gametes are haploid (have one set of 23 chromosomes in humans),
while somatic cells are diploid (have two sets or 46 chromosomes in
humans)
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Gametes
Somatic cells
Fig. 8.13
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
Review the DNA/chromosomes in a human nucleus…
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
Humans have 23 pairs of chromosomes for a
total of 46.
This image shows the 46
chromosomes from the
nucleus of a single human
male cell.
You can see that each chromosome
has a very similar (homologous)
matching pair with the exception of
the sex chromosomes (X and Y).
Females would have a homologous
pair of X’s. Males have an X and a
Y (not homologous).
This display of chromosomes is called a Karyotype.
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
Humans have 23 pairs of chromosomes for a
total of 46.
Therefore we have how many of each gene?
Ans: at least two of every gene, except for the
genes on the X and Y chromosomes in males
since these chromosomes are not homologous.
These are homologous (similar) chromosomes
If the gene for hemoglobin were on one of these (green bar), then
it is on the other as well in the same location (locus).
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
Humans have 23 pairs of chromosomes for a
total of 46.
Every nucleus has 22 pairs of
autosomes (chromsomes 1
through 22)
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
Humans have 23 pairs of chromosomes for a
total of 46.
…and one pair of sex
chromosomes
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
Humans have 23 pairs of chromosomes for a
total of 46.
What chromosomes
would have been found
in the sperm that
fertilized the ovum that
was to be this person?
*One of each pair
comes from the mother,
and the other comes
from the father.
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
Humans have 23 pairs of chromosomes for a
total of 46.
*Therefore, this is the DNA
that would have been
packaged in the nucleus of
the… (sperm or egg?)
…father’s sperm that
penetrated and fertilized the
ovum to form a zygote because
only males have Y
chromosomes.
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
Humans have 23 pairs of chromosomes for a
total of 46.
*And this is the DNA that
would have been packaged
in the nucleus of the
mother’s ovum (has an X).
This could also be in a sperm,
but the previous slide could not
be in an ovum. Explain.
Ans: Males are XY. Therefore sperm can have an
X or Y (they determine sex. Females are XX and
therefore the ovum can only have an X.
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
Humans have 23 pairs of chromosomes for a
total of 46.
- One of each of
chromosomes 1 through 22
and a sex chromosome as
shown to the right is
considered to be one
complete set or n.
The word for this is haploid.
hap- = one
-ploid = set
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
Humans have 23 pairs of chromosomes for a
total of 46.
If I said that human cells
are 2n that means each cell
has how many sets? 2
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
Humans have 23 pairs of chromosomes for a
total of 46.
If I said that human cells
are 2n that means each cell
has how many sets? 2
The word for this is diploid.
di- = 2
-ploid = sets
Humans are diploid organisms. What
are our gametes, ha- or diploid?
Ans: haploid. When they fuse during fertilization
the resulting zygote is diploid.
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
What is “n”?
n is a complete set of
chromosomes (1 of each
chromosome)
n is called the haploid number
…or number of chromosomes
in a complete set.
Chapter 12/13 - Cell Cycle, Meiosis, and Sexual cycles
AIM: Describe the eukaryotic cell cycle.
What is the value of “n” in
humans?
23, because we have 23
chromosomes in one set
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
How and where are the gametes formed in humans?
Gametogenesis - formation of gametes (two types)
1. spermatogenesis
- Formation of sperm, occurs in testes
(male gonads)
2. Oogenesis
- Formation of ovum, occurs in ovaries
(female gonads)
This is all meiosis (gametogenesis, spermatogenesis, Oogenesis)
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
The variety of sexual life cycles
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Meiotic cell division
Ploidy? n?
This figure shows a simple
diploid cell with one
homologous chromosome pair
(n=1).
Red one is maternal (coming
from mother) and blue one is
paternal (coming from
father).
What will happen first?
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Meiotic cell division
DNA will be replicated.
Then what?
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Meiotic cell division
Homologous pairs will be
pulled apart.
And lastly?
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Haploid (n)
Meiotic cell division
Sister chromatids will be
pulled apart.
Haploid (n)
Diploid (2n)
Each resulting cell is haploid
with one of each homologous
pair!
Diploid (2n)
Meiosis I
(1st division)
Meiosis II
(2nd division)
If you want to add a second,
third or even 23 pairs, they
all behave the same way.
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Meiotic cell division
Overview
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
MEIOSIS (meiotic cell division)
(Gamete/spore formation)
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
MEIOSIS (meiotic cell division)
(Gamete/spore formation)
Mitotic cell division (asexual reproduction) would have evolved first
before sexually reproducing organisms, which require meiosis, came
onto the scene…
Reminder: Evolution builds on old ideas (Ex. RNA world hypothesis)
You might hypothesize then that meiotic cell division should be very
similar to… Mitotic Cell Division.
General overview to start…
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
MEIOSIS (meiotic cell division)
(Gamete/spore formation, reduction division)
Why is it logical to pair up homologous chromosomes?
The proteins of the cell are simply programmed to pull paired up
chromosomes apart (life is simple, just a lot of simple things
happening at once making it appear overly complex)…
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Meiotic cell division (details):
1. Interphase (G1, S, G2)
- Similar to cell cycle (mitotic cell division)
interphase. Use those notes.
2. Meiosis – two rounds of cell division
a. Meiosis I – first round (PI, MI, AI, TI)
b. cytokinesis
c. Interaphase II (interkinesis) - only in certain species (do not
need to include this on test)
d. Meiosis II – second round (PII, MII, AII, TII)
e. cytokinesis
IPMATPMAT
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Interphase (G1, S, G2)
- Similar to cell cycle (mitotic cell division)
interphase. Use those notes.
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
MEIOSIS I: Separate the homologous pairs
Prophase I (longest phase, up to 90% of meiosis)
-Most complex phase of meiosis
-Occupies 90% of meiotic cell division
- Chromosomes condense
- Synapsis occurs = homologous
chromosomes come together in pairs
resulting in the formation of tetrads.
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
MEIOSIS I: Separate the homologous pairs
Prophase I
- Crossing over occurs – homologous
chromosomes exchange equivalent
segements
i. This “shuffles” the genes so that the
same genes are not always together on
the chromosome
ii. This site of crossing over is called
the chiasma
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
MEIOSIS I: Separate the homologous pairs
Prophase I
-nucleoli disappear
-centrosomes move to poles
- Spindle begins to form
- Nuclear envelope breaks down
- MTs attach to kinetochores at centromeres
-one pole is attached to one
homologous pair while the other pole
attaches to the other homologous pair
- Free MTs interact with each other to
elongate cell like in mitosis
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
MEIOSIS I: Separate the homologous pairs
Metaphase I
-tetrads align on metaphase plate (brought
there by kinetochore motor proteins using
ATP for fuel)
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
MEIOSIS I: Separate the homologous pairs
Anaphase I
- Homologous pairs (homologs) separate
as kinetochore proteins walk along
spindle fibers toward opposite poles
- sisters stay together attached by
centromere
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
MEIOSIS I: Separate the homologous pairs
Telophase I
- Homologs arrive at poles
- Each pole now has a haploid (n) set:
remember that sister chromatids are
considered a single chromosome
Cytokinesis
- Overlaps telophase I
- Similar to cytokinesis in mitotic phase
- Results in two haploid (n) cells although
the amount of DNA is similar to the starting
diploid cell
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
diploid
diploid
diploid
diploid
Haploid after
cytokinesis –
each cell has
one of each
homologous pair
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Meiosis II
separate the sisters
haploid
- Similar to the mitotic phase – separate the sisters
- Meiosis II starts with two haploid cells and forms four haploid cells with
half the amount of DNA
**** Chromosomes do not replicate. Only the centrosomes
replicate during interphase II/prophase II
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Why go through all of this trouble (use lots of energy) to
makes gametes, cross-over, find a mate, try and fertilize an
egg to reproduce sexually?
Genetic variation/diversity of offspring
(sexually reproducing organisms do this to mix/shuffle their DNA resulting in
very different offspring genetically so that as the environment changes, there
will inevitably be certain gene combinations that will survive it.)
Ex. The flu pandemic of 1918 may have killed 50 million people, but it
didn’t kill everyone. Different gene combinations means different
biochemistry and therefore different susceptibility to disease or other
environmental changes. Another example is HIV…
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Why go through all of this trouble (use lots of energy) to
makes gametes, find a mate, try and fertilize an egg…
basically to reproduce sexually?
Genetic variation/diversity of offspring
Evolutionary Trade-offs: realize that it is a trade-off since it does
require more energy to do this and time to find a mate, etc…
There is almost always a trade-off. Ex) We stand upright on two legs
using skeletons naturally selected to walk on all fours. Some of the
trade offs are back problems, foot problems, hemorrhoids, and many
others…
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Irish Potato Famine
(another example of the importance of genetic diversity)
- Period of mass starvation and disease in
Ireland between 1845 and 1852
~1,000,000 died and another 1,000,000 fled
1/3rd of Irish population depended on potatoes
as major source of food. Potatoes do not
reproduce well from seed (sexual reproduction)
and were asexually propagated (vegetative
propagation). Therefore, all the potato plants in
Ireland were essentially genetically identical…
clones.
Never depend on one crop, especially one
crop of genetically identical plants.
- Potato blight, a disease that destroys potato
plants caused by the fungus Phytophthora
infestans, ripped through and destroyed all the
Irish crops because all the plants were identical
and therefore if one is susceptible to the
disease, they all are…
Chapter 13 - Meiosis and Sexual Life Cycles
NEW AIM: How do sexually reproducing organisms generate diversity?
When a human male and female conceive a child, there are
greater than 64 trillion possible outcomes (>64 trillion
different possible genetic combinations in the offspring)
How do sexually reproducing organisms
generate this kind of diversity ?
1. INDEPENDENT ORIENTATION OF CHROMOSOMES
1. INDEPENDENT ORIENTATION OF CHROMOSOMES
Because the tetrads independently and randomly orient themselves on
the metaphase plate during metaphase I, many different chromosomal
combinations can arise:
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How do sexually reproducing organisms generate diversity?
How many different possible gametes can be
generated by a human?
In the example to the left, the cell
is diploid with n=2. The outcome
of meiotic cell division is 4
different possible combinations.
DIPLOIDS
n
Possible
combinations
2
4
3
8
4
16
5
32
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How do sexually reproducing organisms generate diversity?
How many different possible gametes can be
generated by a human?
How many chromosomal combinations can be
made in the gametes of a diploid cell (2n= 6),
or n = 3?
Let’s call the chromosomes 1m, 1d, 2m, 2d,
3m and 3d. Just make all the possible gamete
combinations (remember that each gamete
will get on of each homologous pair):
1m, 2m, 3m
1m, 2m, 3d
1m, 2d, 3d
1m, 2d, 3m
1d, 2m, 3m
1d, 2m, 3d
1d, 2d, 3d
1d, 2d, 3m
8 possible
combinations
in the gametes
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How do sexually reproducing organisms generate diversity?
How many different possible gametes can be
generated by a human?
We can keep playing this game for n=4, n=5,
etc… until we get to humans, n=23:
DIPLOIDS
n
n
Possible
combinations
Possible combinations (2n)
2
4
2
2n = 4
3
8
3
2n = 8
4
16
4
2n = 16
5
32
5
2n = 32
23
8,388,608
23
2n = 8,388,608
The number of POSSIBLE chromosomal combinations in the gametes of a diploid
organism is 2n where n = haploid number.
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How do sexually reproducing organisms generate diversity?
2. Random fertilization
How many possible sperm and
ovum combinations?
There are 223 different sperm combinations
and 223 different ovum combinations.
That makes 223 x 223 = 246 different combinations
246 = ~64 trillion possible combinations
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
You are 1 in 64 trillion
(well…not exactly)
What are we ignoring?
Crossing-over
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
3. Crossing Over
Alleles
A Gene coding for a specific RNA/polypeptide can
have different versions (slightly different DNA
sequences) possibly resulting in an RNA/polypeptide/
protein with a very similar functionality, but not
identical. These different gene versions are called
alleles.
Alleles
C and c are different versions of the same gene. They are alleles. C codes
for a protein that will somehow be involved in making mouse fur black
while c will make almost the same protein, but will be involved in making
mouse fur white. If I showed you a picture of C and c you would not be
able to tell them apart without looking VERY closely.
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
3. Crossing Over
Without crossing-over, C and E would always travel together in the gametes just as c and
e would always be together. If genes are located on the same chromosome we say they
are LINKED.
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
3. Crossing Over
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
3. Crossing Over
How can these different genes (C and E; c and e) be
unlinked so that C is with e and E is with c?
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
3. Crossing Over
During prophase I, just after synapsis,
crossing over will occur.
During crossing-over there is a reciprocal
(equal both ways) chromosomal exchange
between homologous chromosomes
Why must it be an equal exchange?
Each gamete must get one set of chromosomes
will all the necessary genes. An unequal
exchange would result in some gametes getting
too many genes and some getting too few
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
3. Crossing Over
Reminder: Crossing over occurs at sites
called chiasma and there can be more
than one crossing-over per homologous
pair…
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
3. Crossing Over
This figure shows three
crossing over events:
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
3. Crossing Over
Crossing over begins after synapsis (formation of the tetrad) during prophase I.
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
3. Crossing Over
Enzyme will cut the DNA at the same locus of each homologous pair…
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
3. Crossing Over
Other proteins will reconnect the chromosome fragments to the opposite homolog…
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
3. Crossing Over
and meiosis continues as normal...
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
3. Crossing Over
Parental type chromosome (looks like parent)
Recombinant chromosome
Recombinant chromosome
Parental type chromosome (looks like parent)
The resulting chromosomes have special names (doesn’t everything?)
*A recombinant chromosome is one that has been broken and recombined with
another chromosome hence recombinant. Crossing over is also known as
homologous recombination – recombining homologous chromosomes.
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: How are homologous chromosomes related?
Identify the sources of genetic variability in
sexually reproducing organisms:
1. Independent orientation
2. Random fertilization
3. Crossing-Over
Is that it? Are we finally finished? (not quite…)
4. Mutations
Mutations are changes in the DNA that can be caused by electromagnetic radiation (light: UV, xrays, gamma rays), mutagenic chemicals, viruses, nuclear radiation, mistakes during replication,
and others. However, such changes are only relevant to ones offspring if they occur where?
In the gametes
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
I guess you aren’t 1 in 64 trillion
possible combinations…
You are 1 in some ridiculously huge number
that I can’t calculate possible combinations!
(The real question is, would you still be you if you were any other combination? And how
many changes in your DNA would it take for it not to be you anymore?...)
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Review Question:
A particular diploid species of annelid has a
chromosome number of 10. How many
chromosomal combinations are possible in the
gametes assuming no crossing over/mutations?
n=5
2n = 25 = 32
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: Describe how gametes are formed.
Review Question:
A fruit fly (Drosophila melanogaster) somatic
cell (diploid) contains 8 chromosomes. How
many possible offspring can be generated if
crossing over/mutations is ignored?
n=4
24 = 16 possible sperm
16 x 16 = 256
24 = 16 possible ovum
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
Nothing is perfect, including meiosis. A number of
diseases can be caused by errors in meiosis
resulting in abnormal numbers of chromosomes.
How can one determine if a disease is caused by or if
someone has an abnormal chromosome arrangement?
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
How can one determine if a disease is caused by or if
someone has an abnormal chromosome arrangement?
1. Isolate lymphocytes (type of white blood cell) from a simple blood
sample, or fetal cells from amniotic fluid / placenta
2. Stimulate cell division in a tissue culture flask
3. Arrest (stop) the cell in metaphase. Why?
- Chromosomes are condensed and visible, and it is a checkpoint, which allows us to stop it here by
inhibiting internal signals.
4. Put cells on a cover slide and lyse them (break them open with
certain chemicals) and wash away cell fragments leaving behind the
chromosomes.
5. Stain the chromosomes with special dye
(Process takes about a week)
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
A second method:
Fig. 8.19
1. Blood sample taken and spun in a centrifuge.
- Centrifuges can spin samples at very high speeds (10,000’s of rpms
resulting in forces as high as 100,000 times the force of gravity)
- The more dense material ends up on the bottom (red blood cells in this
case) and less dense material on above this (white blood cells and then
fluid or plasma).
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
A second method:
2. A hypotonic solution is added to the sample, which will lyse the red blood
cells (weaker), leaving the white ones, some of which are in prophase/
metaphase swollen, but not popped.
Fig. 8.19
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
A second method:
3. A drop of the sample containing WBCs is placed on a cover slide, dried and
then stained.
Fig. 8.19
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
For both methods, the result is a
smear of condensed chromosomes
from a single cell called a
karyotype.
What is the first thing you would
do?
A digital picture is taken and the
chromosomes are first counted.
There should be (from a human
cell): 46
Then what?
karyotyping
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
The chromosomes are paired up
and organized according to size
and banding patterns (the dye
sticks better to A-T rich regions –
segments of the DNA with many
A-T base pairs):
Is this smear (karyotype) from a
male or a female cell?
Female, it is XX
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
Is this smear (karyotype) from a
male or a female cell?
Male, it has a Y chromosome
Analyze the karyotype:
There are 3 number 21
chromosomes. This is
called Trisomy 21
(tri:3, somy:body)
Down Syndrome
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
What causes Down Syndrome (Trisomy 21)?
1. First hypothesize what the chromosome combination present
in the gametes of the parents of an individual with Down
Syndrome would be.
- One gamete could have an extra chromosome 21 (two number
21’s), while the other gamete is normal.
2. How could an extra chromosome end up in one of the gametes?
CHROMOSOMAL NONDISJUNCTION in Meiosis II
CHROMOSOMAL NONDISJUNCTION in Meiosis II
CHROMOSOMAL NONDISJUNCTION in Meiosis II
CHROMOSOMAL NONDISJUNCTION in Meiosis I
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
Viable Autosomal nondisjunctions:
1) Trisomy 21 (Down syndrome)- most common in viable (survivable) births
2) Trisomy 18 (Edwards syndrome)
3) Trisomy 13 (Patau syndrome)
4) Trisomy 12 (A indicator of Chronic Lymphocytic Leukemia)
5) Trisomy 9
6) Trisomy 8 (Warkany syndrome 2)
Non-Viable Autosomal nondisjunctions:
Trisomy 16 - most common trisomy in humans:
- occurs in more than 1% of pregnancies.
- Usually results in spontaneous miscarriage in the first trimester.
(Do not memorize these other then trisomy 16 info and Down Syndrome)
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
What about if non-disjunction occurs in the
sex chromosomes?
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
Klinefelter’s Syndrome (XXY)
- Sterile male typically
- Female characteristics
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
XXXXY
Klinefelter’s syndrome:
can be any multiple of
X with a single Y.
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
Turner Syndrome
- Shorter stature (height)
- Enlarged hands and feet
- Underdeveloped ovaries (infertile)
- Other problems (high blood pressure, heart
problems, etc…)
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
(normal for the most part)
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
What is the total number of chromosomes
you would expect to find in a woman with
Turner syndrome?
45, missing one X
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What happens when meiosis goes wrong?
Other mishaps can occur during meiosis resulting in
chromosomal abnormalities that may cause disease.
These abnormalities include…
1. Deletions
2. Duplications
3. Inversions
4. Translocations
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What are some other ways chromosomes can be altered?
There is a “mistake” made during meiosis resulting in the
loss of a chromosome segment.
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What are some other ways chromosomes can be altered?
Diseases caused by deletions include:
1. Cri du chat “cry of the cat”
- Cat-like cry
- Low birth weight, cognitive (information
processing) delays, motor (movement) and
speech problems
- Chromosome 5 deletion
- Life span is normal (your book is incorrect)
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What are some other ways chromosomes can be altered?
Diseases caused by deletions include:
2. Williams syndrome
- Deletion of 26 genes in chromosome 7
- Elf like facial appearance
- Unusually cheerful
- Unpredictable negative outbursts
- Mental retardation
- Shortened life span due to narrowed arteries
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What are some other ways chromosomes can be altered?
Diseases caused by deletions include:
3. Duchenne’s muscular dystrophy
i. Dystrophin gene
- Largest gene in genome
- 2.4 million base pairs!!!!!!!!!
- Takes 16 hours to transcribe (to
make a mRNA)!!!!!!!
- Protein is 3500 amino acids!
- X linked gene (means it is on the X-chromosome)
- Function: connects muscle cell cytoskeleton to ECM (anchors cell to ECM)
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What are some other ways chromosomes can be altered?
Diseases caused by deletions include:
3. Duchenne’s muscular dystrophy
ii. Symptoms begin at 2 to 6 years old
iii. Develop muscle weakness and
eventually failure
iv. Wheelchair by age 12
v. Survival beyond 20 years old is rare
vi. Affects 1 in 3500 MALES
Why are carrier females prevalent, but afflicted females rare?
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What are some other ways chromosomes can be altered?
Duplications can accompany deletions if the deleted
section is inserted into the homologous chromosome
during crossing over.
This can have a major role in evolution. Can you predict
why?
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What are some other ways chromosomes can be altered?
The duplicated genes are now free to mutate (change) without
causing a loss of critical proteins since the original genes are
there. The genes can eventually produce proteins that might
become new tools for the cell…giving it new functions.
**Many of our genes arose by duplication events followed by mutations resulting in
many different proteins having similar structure (divergent evolution).
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What are some other ways chromosomes can be altered?
Above are three enzymes (proteins) that catalyze three different reactions coded for by
three different genes. Do you think those genes arose independent of each other?
Highly, highly, highly unlikely. These three genes likely arose by gene duplication from a single original gene.
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What are some other ways chromosomes can be altered?
Inversion results when a segment of a chromosome gets flipped or inverted.
- Less likely to be harmful compared to deletion and duplications
- All genes still present
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What are some other ways chromosomes can be altered?
Reciprocal translocations can occur between NONHOMOLOGOUS chromosomes.
Normal if it happens in somatic cells – all genes still present.
If occurs in meiosis it can lead to Down syndrome if a piece of chromosome 21 is
tranlocated onto another chromosome.
Reciprocal translocation
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What are some other ways chromosomes can be altered?
Congenital disorders
- Disorders present at birth (all the ones we have
spoken about thus far)
- Caused by genetic changes in sperm and/or ovum
or environment in uterus.
Non-Congenital disorders
- Disorders that occur or arise after birth
- Ex) cancer, heart disease, etc…
Chapter 13 - Meiosis and Sexual Life Cycles
AIM: What are some other ways chromosomes can be altered?
Cancer (non-congenital) caused by a reciprocal translocation
between chromosomes 9 and 22.
The translocation activates a cell division gene.
Chapter 13 - Meiosis and Sexual Life Cycles
Review Question
Reminder Genetic changes are ONLY passed to offspring (inherited)
if they occur…
IN THE GAMETES