Name: ________________________________
Genetics 314 - Spring, 2006
Exam 3 – 100 points
1. You are working in an agro-chemical company in the industrial safety division.
Here you are in charge of determining the potential harmful effects of new
chemicals before they are released on the market.
a) The chemical division comes up with a chemical that targets basic proteins found
in the nucleus, specifically Histones 2A and 2B. What are the roles of these
proteins and would this chemical affect chromatin organization in both interphase
and during cell division (mitosis and meiosis). Briefly explain your answer.
The two histone proteins are part of a histone octomer that makes-up the
nucleosome. The nucleosome is required for the initial condensing of the
chromatin. This condensing is essential for both the proper chromatin
organization during interphase and for chromatin condensation to form
chromosomes for both mitosis and meiosis.
b) The chemical division brings you a new compound that now targets acid proteins
associated with chromatin organization in the nucleus. Would this chemical have
an impact on both interphase and cell division? Briefly explain your answer.
This chemical would impact both interphase and cell division because the acidic
proteins in the nucleus are associated with the formation of chromatin looped
domains during interphase and the formation of the protein scaffolding
necessary for chromosome formation for mitosis and meiosis.
c) The chemical division is given an assignment to stop plant growth and sexual
reproduction of weeds by preventing cell division. They first come up with a
chemical that prevents formation of the proteins associated with formation of the
synaptonemal complex.
1) Will this have the intended effect of halting mitosis? Briefly explain your answer.
Targeting the proteins associated with the synaptonemal complex would have no
impact on halting mitosis. The synaptonemal complex is associated with pairing
of the homologous chromosomes and chromosome pairing does not occur in
mitosis.
2) Would this chemical be effective in interfering with sexual reproduction of the
weed? Briefly explain your answer.
Yes it would. For proper separation of the homologous chromosomes in meiosis
it is essential that the homologous chromosomes pair. If no pairing occurs
proper distribution of the chromosomes may not occur in the reduction division
resulting in non-viable gametes. In addition the lack of bivalent formation may
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Name: ________________________________
prevent the completion of the reduction division because the tension created by
microtubules pulling on the homologous chromosomes involved in the bivalent
can act as a signal that the cell is ready to proceed from metaphase I to anaphase
I.
d) Trying to be proactive the chemical division asks you what proteins or protein
products you would target if you wanted to prevent mitosis from occurring?
Name one protein or protein product that you could target that would prevent
mitosis from starting and explain how briefly explain how your system would
work.
One could target either cyclin, cdc2, MPF formation or activation of MPF to
prevent the initiation of mitosis. The signal for mitosis is the build-up of
activated MPF. Once a critical level is achieved the nuclear membrane will
dissolve and mitosis will start. Preventing the formation of MPF by preventing
synthesis of cyclin or reducing the availability of cdc2 in the cell would prevent
the start of mitosis by keeping the level of MPF below the critical level.
Preventing the phosphorylation of MPF would also serve the same function
because the initiation of mitosis is dependent not on the level of MPF put on
activated MPF.
2. What are the three major differences between mitosis and meiosis in terms of
recombination in eukaryotes?
1. The number of chromosomes is reduced to half the somatic chromosome
number in meiosis while there is no reduction in the number of chromosomes
in cells after mitosis.
2. Homologous chromosomes pair in meiosis while they do not pair in mitosis.
3. There is the potential for crossing-over and recombination to occur between
non-sister chromatids in meiosis but not in mitosis.
3. The chemical division calls you to tell you they have found a chemical that
induces transposable element movement producing deletions, duplications and
inversions during post-meiotic mitotic divisions. They want to know if the impact
of this chemical would be the same in plants and animals. In your answer
explain:
1)
2)
3)
4)
What are post-meiotic mitotic divisions?
Why they are important?
What would be the impact of the chemical?
Would the effect be the same in plants and animals (Briefly explain your
answer)?
Post-meiotic mitotic divisions are a series of mitotic cell divisions that occur in
plant gametes after meiosis to create the extra nuclei necessary for double
fertilization. Double fertilization allows for the production of a food source for
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the plant embryo (endosperm) to provide energy and nutrients to the embryo
upon germination before the seedling can produce its own energy by
photosynthesis. The impact of this chemical would be negligible in animals
since animals do not require post-meiotic mitotic divisions to produce a mature
gamete but in plants it would result in non-functional gametes due to the
increased level of duplications and deletions resulting in reduced or no seed
production.
4. You become intrigued by the concept of transposable elements. In studying them
you find that they can modify the expression of genes and that some increase in
number over time in an organism while others never increase in number in an
organism.
a) How do transposable elements modify gene expression in an organism?
Transposable elements can modify gene expression by inserting into a gene’s
promoter sequence or with in the gene sequence itself. In the first case insertion
could result in halting initiation of transcription preventing expression of the
gene. Inserting into the gene sequence itself would allow for transcription and
translation but would result in a non-functional protein product by modifying
the codon sequence similar to a large frame-shift mutation.
b) What is the reason for the difference in number of copies of transposable elements
over time depending on the type of transposable element? (Briefly explain your
answer).
There are several types of transposable elements. One type, the conservative
transposable element, does not make copies of itself when it moves resulting in
no net increase in the number of copies of transposable elements in the cell.
There are two other types of transposable elements, replicative and retro-, that
do not move from their original insertion point but instead make a DNA or RNA
copy which then is converted to dsDNA that can insert in another region of
chromatin. This results in a net increase in the number of transposable elements
over time.
5. Using genetic engineering you create a gene that produces a fluorescent protein
allowing tissues to glow green under U.V. light. You transform a male cat and
recover a glow in the dark kitty. Thinking you have a financial gold mine you
start breeding you cat and determine the presence of the gene in the kittens using
PCR and expression of the gene in the fur by exposing 6 week old kittens to U.V.
light. You recover kittens that carry the gene based on PCR but observe
differences in expression depending on the sex of the kitten. If the kitten is male
and the gene is present 100% of the fur will glow green, if the kitten is female and
carries the gene expression is variable with some patches of fur glowing green
while other patches of fur showing no expression of the fluorescent protein.
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Name: ________________________________
a) You suspect your gene inserted into the X sex chromosome in the original male
cat. If this were true why would it explain the results you observed in the kittens?
If the gene was on the X sex chromosome in the male it would explain the
pattern of inheritance in the kittens if it had been originally transferred to a
female because in the male whatever gene is on the X chromosome will be
expressed in every cell resulting in 100% of the fur glowing. In the female
kittens, if they were heterozygous for the gene then depending on which X
chromosome was condensed in different cell lines you would have some cells
expressing the trait and some cells not expressing the trait even it the gene is
present based on PCR results. This would result in kittens that show a patch
work pattern of glowing and non-glowing fur.
b) If the gene had inserted into the Y sex chromosome would you expect the same
results or different results? Briefly explain your answer.
No, in this case all males would show the trait because they carry the Y
chromosome but females would never show the trait because they are the
homogametic sex (XX).
6. You are given some seed for a leguminous tree (a tree that fixes nitrogen so does
not need any fertilizer) and are told it is an autotetraploid with 60 chromosomes.
a) What is the somatic (2N) chromosome number of this tree?
2N = 60
b) What is the gametic (N) chromosome number of this tree?
N = 30
c) What is the monoploid (X) chromosome number of this tree?
X = 60 /4 = 15
d) Would you be surprised to find out that the tree was partially sterile? Briefly
explain your answer.
No, since it is an autotetraploid, all the chromosome sets originate from one
species allowing for the formation of tetravalents during meiosis. The
presence of tetravalents would allow for an unequal distribution of
chromosomes during the reduction division resulting in non-viable gametes.
7. In doing chromosome analysis you discover that the plant is an autotriploid
instead of an autotetraploid.
a) How could an autotriploid be produced?
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Cross a 4X (tetraploid) with a 2X (diploid). The product will be an autotriploid.
b) What are the advantages and disadvantages of an autotriploid?
The advantages of an autotriploid are:
-
more vigorous growth
larger fruit
no seeds
The disadvantage of an autotriploid is no seed are produced so need to produce
plants each year through vegetative reproduction or by crossing an
autotetraploid by a diploid (see a).
8. Returning to your study of transposable elements, you find that in some
organisms the presence of transposable elements leads to an increase in
chromosome deletions.
a) What is a chromosome deletion?
When a section of the chromosome is missing, it can be a terminal deletion or an
interstitial deletion.
b) How could a transposable element lead to a chromosome deletion?
When a conservative transposable element moves the DNA stands do not
reconnect resulting in the loss of a piece of the chromosome. There could also be
recombination between transposable elements resulting in the loss of a section of
chromosome through formation of an acentric piece of chromatin.
c) Why would a chromosome deletion be potentially deleterious to an organism?
A deletion could be deleterious if an essential gene is lost or proper expression of
a trait requires a specific dose which would be cut in half due to a deletion.
9. You look at your family history and see that a genetic disorder occurs periodically
in your family. This disorder is related to either aneuploidy (trisomic for
chromosome 20) or a translocation involving chromosome 20. What difference
would you expect to see in the family history if the disorder was due to
Aneuploidy compared to the disorder being due to a translocation? Briefly
explain your answer.
If the trait was due to aneuploidy which is a product of random non-disjunction
you would expect it to appear at a very low frequency with no specific pattern of
inheritance. If the trait was due to a translocation you would expect it to appear
at a higher frequency in the family history because a translocation is a
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permanent change in the chromosomes and would have a 25 – 33% change of
producing the genetic disorder every time a child was conceived.
10. You are studying two related species using molecular markers and discover that
the gene organization is similar but different between the two species. In some
cases genes are on the same chromosome but differ in order between the two
species and in some cases the genes are found on different chromosomes. What
could explain the differences in gene order and gene location and could it be the
reason why hybrids between the two species are sterile.
The differences in gene order and location could be due to inversions and
translocations. With both, hybrids from parents that differ for the inversions
and translocations (one having normal chromosomes and one having modified
chromosomes) would be heterozygous for the chromosome changes would be
partially sterile due to problems in chromosome pairing during meiosis resulting
in gametes carrying duplications and deletions. These gametes would be nonviable and if the modifications are extensive enough result in complete sterility of
the hybrid between the two species.
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