Name: ____________________________
Genetics 314 – Spring 2004
Second Exam – 100 points
1. Neighbors of a chemical plant that produces fluorocarbons believe that emissions
from the plant have resulted in a localized hole in the ozone allowing for an
increase in UV light exposure leading to an increase incidence of mutations that
have lead to an increase in skin cancer.
a) The company claims that the mutations are not due to the U.V. light. Is it
possible to have mutations without an external cause such as U.V. light?
Briefly explain your answer.
Yes it is possible to have mutations without an external cause.
Spontaneous or natural mutations can occur from errors in proof reading
by DNA polymerase to changes in base form (ex. keto to enol) that result
in mispairing.
b) You decide to test the theory on increased U.V. light by exposing several
strains of bacteria to the light around the chemical plant. In two strains you
find a low level of mutation (lower than you expected for the level of U.V.
light exposure) but in one strain you observe a high level of mutation.
1. What could keep the mutation rate lower than expected in the first two
bacterial strains? Briefly explain your answer.
Bacteria possess two forms of thymine dimer repair systems, light
activated and dark (excision – repair), that could minimize the
damage caused by U.V. light. In the case of the first two bacterial
strains the repair systems are working properly reducing the number
of mutations induced by the U.V. light.
2. What is different about the third strain to make it show such a high rate of
mutation?
In the case of the third strain it appears that it does not have
functional repair systems so the damage caused by the U.V. light is
not repaired leading to base pairing errors resulting in a high
mutation rate.
2. You are asked to make modifications in plants using a chemical mutagen. You
want to make mutations that modify the function of enzymes but not make them
non-functional. You have a choice between a chemical that causes base deletions
and a chemical that causes base changes. Which chemical would you pick and
why?
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Name: ____________________________
I would choose the chemical that causes base changes over the chemical that
causes base deletions. A base change may lead to the changing of a single
amino acid that may or may not have a major affect on the functionality of
the protein. A base deletion on the other hand would lead to a frameshift
mutation changing all codons and resulting amino acids downstream of the
deletion. This would have a high probability of producing a non-functional
protein product.
3. Your neighbor is afraid of getting bacterial infections but also is afraid of
contributing to the problem of creating antibiotic resistant bacteria by using
antibiotic soaps so he buys a new cleaning solution that claims it kills bacteria
from the inside out by soaking the bacteria in a base analog of adenine.
a) Could this cleaning solution kill bacteria? Briefly explain your reasoning.
In theory yes, a chemical that is a base analog could induce base change
mutations by replacing a base but at the next replication pair with a different
base leading to a base change mutation. If the chemical induces a high
enough level of mutation, critical protein products will be affected leading to
the death of the bacteria.
b) You ask your neighbor if he wears gloves when he uses the cleaning solution,
why?
The base analog could cause mutations in his DNA as easily as it could cause
mutations in the bacterial DNA.
4. You are asked to create a regulatory system for a three gene/enzyme biochemical
pathway in bacteria that you can turn on and off by adding an ingredient to the
media that that you use to grow the bacteria. You have learned about two
pathways in bacteria for lactose utilization and tryptophan synthesis.
a) What are the differences between the two gene/enzyme systems in terms of
regulation of gene expression?
Lactose utilization is an inducible system where a repressor protein will bind
to the operator site preventing transcription unless lactose is present.
Lactose will act as an inducer and bind to the repressor making it unable to
bind to the operator site and transcription can occur.
Tryptophan is a repressible system where the repressor protein can not bind
to the operator site allowing transcription unless the end-product tryptophan
is present. Tryptophan will act as a co-repressor and when it binds to the
repressor it becomes active and can bind to the operator site preventing
transcription.
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Name: ____________________________
b) Which of the regulatory systems would you use for your three gene/enzyme
system? Briefly explain your answer.
I would use the inducible system since I could then control when
transcription would start by adding the inducer. The inducer would then
bind with the repressor protein preventing it from blocking the operator site
allowing transcription to begin. As long as I keep adding the inducer
transcription of my gene would be occurring.
5. You are asked to move your three gene/enzyme system into a eukaryotic
organism.
a) How would you change your system to allow for simultaneous expression of
the three genes in the eukaryote?
Since operons do not exist in eukaryotes I would use a combination of
enhancer sequences and activator proteins to turn on the genes at the same
time. By having the same enhancer sequences upstream from the promoter
regions for all three genes I could ‘turn on’ all three genes simultaneously by
adding the transcription activator proteins that correspond to the enhancer
sequences.
b) You want to add a second level of regulation on to your system. What could
you regulate in eukaryotes that can not be regulated in prokaryotes and how
could it affect expression of your gene?
Eukaryotes require an mRNA processing step before translation can occur.
Regulation of the processing such as intron removal, addition of the poly A
tail, or movement of the mRNA out of the nucleus would affect the rate of
protein synthesis by affecting when translation could start and in the case of
the poly A tail, how long translation could occur. The length of the poly A
tail appears to affect how long the mRNA will be available for translation.
The greater the number of adenines the longer the life of the mRNA and the
longer it can be translated.
6. Viruses are a challenge to control but it is proposed that RNA viruses could
possibly be controlled. What makes them different and how could you limit their
ability to replicate?
RNA virus requires special ‘non-host’ enzymes for replication. In the case of
an RNA to RNA virus this enzyme is replicase and for retrovirus (RNA to
DNA to RNA) virus the enzyme is reverse transcriptase. Since these enzymes
are not essential for normal functioning of the host cell, elimination or
deactivation of these enzymes would not hurt the host cell but would
effectively prevent the RNA virus from replicate.
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Name: ____________________________
7. There was talk recently about the flu virus and how an avian flu virus could
recombine with another virus to create a virulent flu virus that could attack
people. How could genetic recombination between two viruses occur?
Key for recombination of two viruses is the simultaneous infection of the host
cell by two viruses. After infection there could be recombination between the
genetic material of the two viruses, copy-choice errors during replication
and/or errors in packaging that could lead to genetic material from both
viruses being packaged into one viral capsule.
8. Virus and prokaryotic DNA can replicate by two methods. What are the two
methods and why are two methods needed for replication of circular DNA?
The two methods are bi-directional ((Theta) replication and rolling circle
replication. Bi-directional replication will produce two circular pieces of
DNA. Rolling circle replication has the potential to produce single or double
stranded linear pieces of DNA. Linear DNA is needed by virus when it is
packaged into the viral capsule and by bacteria for conjugation to occur.
9. You are asked to put a piece of DNA into bacteria.
a) What are the three ways for recombination to occur in bacteria?
Transformation
Conjugation
Transduction
b) Which would be the easiest to use and how would it work?
Transformation, all that would be needed is competent cells, and pieces of
DNA. The competent bacterial cells will take up the DNA from the
environment and some of the DNA will recombine with the bacteria’s
chromosome. In the case of plasmid DNA the bacteria will take up the
plasmid and it will be functional without recombination with the bacterial
chromosome.
10. You are given the following data on recombination using conjugation and Hfr
strains for gene mapping in bacteria
Strain
A34
B22
D15
F55
-------------gene order-------------trp
ala
cys
pro
met
ala
trp
bio
gal
lac
pro
met glu
leu
lac
bio
gal
lac
leu
glu
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Name: ____________________________
Diagram a gene map showing gene order, F factor insertion site and F factor
orientation.
F55
trp
bio
ala
gal
A34
B22
cys
lac
pro
D15
leu
glu
met
11. You discover bacteria recombination is occurring whenever you infect bacteria
with a bacterial phage. In some cases it is only recombination involving two
genes, bio and gal, and in other cases a low level of recombination involving
many bacterial genes. What form of recombination is occurring and how do the
two types of cases differ?
Transduction is occurring where a virus is serving as a vector to move the
DNA from one bacterium to other bacteria. In the first case specialized or
specific transduction is occurring where a temperate phage disassociates
from the bacteria chromosome improperly taking a gene adjacent to the
insertion site with it. In the case of the temperate lambda phage this
insertion site is in between the bio and gal genes so one or the other of these
genes would be transferred. In the second case generalized transduction is
occurring where pieces of bacterial chromosome are accidentally packaged
into the viral capsule. This is a random event so any genes could be
transferred.
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Name: ____________________________
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