Name:______________________________________
Genetics 314 – Spring, 2008
Exam 3 – 100 points
1. You have been hired by a molecular genetics company and are working in a division that uses
siRNA technology to prevent expression of specific genes in eukaryotes to determine the
function of specific gene products (proteins) in a cell.
a. The first protein you target is histone 1. If you prevented histone 1 from being produced in a
cell what impact would this have on chromatin formation in interphase and chromosome
formation during mitosis?
If histone 1 was not present then condensation of chromatin could not occur because hitone
1 is needed to connect the nucleosomes to form the solenoid structure, which is the second
step of chromatin condensation. With no solenoid structure the chromatin could not form
loop domains which is needed during interphase for proper chromatin organization and
would prevent chromosome formation since the chromatin could not be properly attached
to the acidic protein scaffolding.
b. The next protein you target is the primary protein component of microtubules. What impact
would the loss of this protein have on cell division?
With no microtubules the spindle would not form during mitosis or meiosis and the
chromosomes would not be properly aligned on the metaphase plate. Since alignment is
necessary for mitosis or meiosis to proceed beyond metaphase cell division would be
prevented from being completed.
c. The last protein you target is cyclin. How would limiting the production of cyclin impact a
cell’s growth cycle?
Cyclin is a protein that helps regulate the cell cycle. For cell division (mitosis) to be
intiated, cycline levels need to increase so it can combine with the cdc2 protein to form
MPF. If cycline is not present MPF will not be produced so the cell would remain in
interphase and mitosis would not be signaled to begin.
Name:______________________________________
2. You are asked if there are any proteins that could be targeted that could prevent chromosome
recombination.
a. What set of proteins could you target and how would they prevent chromosome
recombination?
The proteins that make up the synaptonemal complex. Without these proteins the
homologous chromosomes would not be brought together close enough to allow
recombination to occur during prophase 1 of meiosis.
b. A newly hired employee from a southern Idaho university looks at mitosis and declares the
system worked to prevent recombination. You tell him it is too early to celebrate, why?
Because formation of the synaptonemal complex, synapsis and/or homologous chromosome
pairing and recombination only occur in meiosis so observing mitosis would not be of any
use in determining if chromosome pairing had been prevented.
3. What are the three differences between mitosis and meiosis that relate to genetic
recombination?
1. Chromosome number is reduced in meiosis with the make-up of maternal and paternal
chromosomes varying from gamete to gamete because of random assortment of the
non-homologous chromosomes when the homologous chromosome pairs separate
during the reduction (1st division) of meiosis. Chromosome number remains the same
in mitois.
2. Homologous chromosomes pair during meiosis while they do not pair in mitosis.
3. Crossing-over between non-sister chromatids occurs in meiosis while it does not occur
in mitosis.
Name:______________________________________
4. You discover in one of the plants that you had successfully turned off expression of a protein
by inserting a gene for siRNA silencing that the protein is again being expressed in some of
the progeny of that plant. In the subsequent generation you observe in some of the progeny
that the expression of the gene has been turned off again. What could be happening in the
plants that could cause the siRNA silencing gene to turn on and off again from generation to
generation? Briefly explain your answer.
There could be a transposable element present in your plant that has inserted into your
siRNA silencing gene causing it to be not expressed. The gene turns back on (expressed) in
the next generation because the transposable element moved out of the gene and inserted
somewhere else in the genome. This insertion and then disassociation can occur repeatedly
over the generations turning the gene on and off causing the gene silencing to turn on and
off.
5. You are moved to the animal cloning division and are asked to select traits in an animal to
clone that would increase at least the appearance of successful cloning for a client. You say
you want to use a female animal that is heterozygous for a coat color gene on the X
chromosome like a calico cat. Why would cloning such an animal allow you to claim success
even if the clone did not look like the ‘parent’ animal?
Because with a gene on the X or sex chromosome like in a female cat you will get
differential expression of the gene in the cat due to random condensing of one or the other
X chromosome causing a patchwork phenotypic pattern (i.e. calico). Since which X
chromosome condenses is random, in a clone the pattern would be expected to be random
and not like the source animal’s pattern. Hence the cloned animal could look completely
different from the source animal and you would still be a success.
6. You switch from the molecular biology section of the company to the plant breeding section.
You are assigned to the project to produce triploid watermelons.
a. What are some of the advantages of producing a triploid crop?
With a triploid you can have more vigorous growth, larger fruit size and no seeds
b. To produce the triploid watermelon your boss first doubles the number of chromosome sets in
a diploid watermelon to produce an autotetraploid watermelon. She then crosses the
tetraploid with a diploid to produce the triploid watermelon. She is disappointed with the
amount of seed produced on the tetraploid watermelon plant but you are not surprised, why?
The problem in seed production is that an autotetraploid may have reduced fertility
because of problems in chromosome segregation during the reduction division of meiosis.
Since the chromosomes came from one source more than two homologous chromosomes
may pair during prophase 1 (get trivalents and tetravalents) which may not separate
equally in anaphase 1 resulting in gametes that have an unequal number of chromosomes
making them non-viable.
Name:______________________________________
7. You are given an allohexaploid plant with 66 chromosomes to study.
a. What is the 2N somatic chromosome number of the plant?
2N = 66
b. What is the N gametic chromosome number of the plant?
N = 33
c. What is the X monoploid chromosome number of the plant?
X = 66/6 = 11
d. What is meant by the term ‘allo’ and would you expect this plant to be fertile?
Allo indicates that the chromosome sets originated from two or more species. With the
multiple origins of the sets it is more likely that only bivalents will form during the
reduction division of meiosis resulting in viable gametes and normal fertility.
8. You are moved to the plant cytogenetic section and are asked to study aneuploidy. Your boss
is interested in developing monosomic plants but finds that they can only be successfully
produced in polyploid plant species and not in diploid plant species.
a. What is a monosomic plant and why would you find them in polyploid species and not in
diploid species?
A monosomic plant is one that is missing one chromosome (2N-1). The reason you see them
more often in polyploids is that polyploids will have extra copies of chromosomes/genes so
the loss of a chromosome is not as detrimental. In diploids there are no additional
chromosomes to compensate for the lost chromosome creating a dose effect that is
deleterious to the survival of the plant.
b. You suggest that if he wants to study aneuploidy in diploid plant species he should look for
trisomic plants instead of monosomic plants. What is a trisomic plant and why would it be
possible to find them in diploid plant species?
A trisomic plant has an extra chromosome making it 2N+1. The presence of an extra
chromosome (and extra doses of genes) is not as deleterious as missing a chromosome so
diploid trisomics can survive and reproduce.
Name:______________________________________
9. Tired of working on plants you switch to human cytogenetics and are asked to help out in the
genetic counseling program. A young couple comes in concerned because their first child was
born with a genetic disorder caused by having three copies of the genes on the long arm of
chromosome 20.
a. What are the two ways the first child could have three copies of the long arm of chromosome
20?
A child could have three copies of a chromosome through aneuploidy (trisomic) or by a
translocation.
b. Since the couple want to have another child, why is important to know the source of the extra
copy of the long are of chromosome 20 in the first child? Briefly explain your answer.
Because with one it was due to random chance (trisomy) and the chances of another child
with the same condition are quite low while the other due to a translocation is a permanent
change in one of the parents chromomsomes and would have a 25% chance of occurring
again.
10. You are approached by a couple who are having trouble conceiving a child because they both
have reduced fertility and are wondering if there could be a genetic reason. You check their
chromosomes and find that they are both heterozygous for a paracentric inversion.
a. What does it mean to have a paracentric inversion?
There is a section of chromosome that is reversed and it does not involve the centromere
b. What would you expect to see in meiosis that would confirm that the people were
heterozygous for the inversion?
In meiosis during prophase 1 you could see a looped structure and during anaphase 1 you
would see an acentric fragment and a dicentric bridge.
c. Could this be the reason for the reduced fertility? Briefly explain your answer.
Yes, if dicentric bridges are visible it indicates that some gametes will have duplications
and deletions for genes on the chromosome with the inversion. The presence of
duplications and deletions will result in a non-viable gamete which would be observed as a
reduction in fertility.
Name:______________________________________
11. Tired of working with people you move to working on animal speciation. Looking at the
chromosomes of two very similar appearing animal species that can mate but produce sterile
off-spring you find that while they have the same number of chromosomes the two species’
chromosomes differ due to several reciprocal translocations.
a. What is a reciprocal translocation?
A reciprocal translocation is when two non-homologous chromosomes have exchanged
pieces. In this exchange not genes are lost but they are now located on different
chromosomes.
b. Would the off-spring of the two species being heterozygous for the translocations be the
reason for the sterility? Briefly explain your answer
Yes, if an individual is heterozygous for reciprocal translocations then there will be a
problem in chromosome pairing during meiosis. Four chromosomes will pair and
depending on how they separate (adjacent or alternate) you will get duplications and
deletions leading to non-viable gametes. Have enough translocations and the individual
will most likely be sterile because the probability of fertility will be only 33% for each
translocation in a heterozygous individual.