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MICROBIOLOGY EXAM 3

Exam three practice problems!
STUDY
PLAY
Which microorganism is the source of the antibiotic neomycin?
Streptomyces
If penicillin G is chosen as the best treatment of a given infection, what microorganism is the cause of the infection?
Gram-positive bacteria
Which of the following drugs does NOT act by inhibiting protein synthesis?
Sulfanilamide
Which of the following natural penicillins has the longest retention time?
Benzathine penicillin
What is meant by selective toxicity?
Chemotherapeutic agents should act against the pathogen and not the host.
Why are chemotherapeutic agents that work on the peptidoglycan cell wall of bacteria a good choice of drug?
Humans and other animal hosts lack peptidoglycan cell walls.
Why is polymyxin only used on the skin?
It can also damage living human cell membranes, but the drug is safely used on the skin, where the outer layers of cells are dead.
Quinolones and fluoroquinolones act against what bacterial target?
DNA gyrase
Why is it difficult to find good chemotherapeutic agents against viruses?
Viruses depend on the host cell's machinery, so it is hard to find a viral target that would leave the host cell unaffected.
Which mode of action of antimicrobial drugs involves competitive inhibition between sulfanilamide and para-aminobenzoic acid (PABA)?
Inhibition of essential metabolite synthesis
Why is chloramphenicol effective against prokaryotes but not eukaryotes?
Due to the 70S prokaryotic structure, chloramphenicol inhibits protein synthesis selectively in prokaryotes.
Which of the following statements regarding the drug acyclovir is FALSE?
Acyclovir structurally resembles the nucleoside deoxycytosine.
Which microorganism is the source of the antibiotic neomycin?
Cephalosporium
Why is potassium clavulanate combined with penicillins?
Because potassium clavulanate is a noncompetitive enzyme inhibitor.
In this example, which drug resulted in no growth in any well?
Streptomycin
Which statement regarding tests for microbial susceptibility to chemotherapeutic agents is FALSE?
The diffusion method can determine both whether a drug is bactericidal as well as bacteriostatic.
Why are Gram-negative bacteria relatively more resistant to antibiotics?
Due to prevention of penetration to the target site.
An enlarged zone of inhibition around two adjacent antibiotic disks, containing penicillin and streptomycin, is an example of which of the following?
Synergism
Which statement regarding antibiotic safety is FALSE?
Fortunately, the effects of one antimicrobial drug cannot neutralize the intended effects of another drug.
Which antibiotic is overcome by beta-lactamases?
Penicillin
How might efflux pumps increase antibiotic resistance in bacteria?
Resistant bacteria can have more efflux pumps, and can have less specific efflux pumps.
Bacteria that are resistant to sulfonamide have enzymes that have a greater affinity for what?
PABA
Why would an efflux pump for penicillin located on a bacterial cell membrane not be effective at providing resistance to the drug?
Penicillin disrupts the cell wall, which is located outside of the cell membrane.
Membrane transport proteins are required for which mode(s) of antibiotic resistance?
Efflux pumps, beta-lactamases, and modification of porins all utilize membrane transport proteins.
A drug that inhibits mitosis, such as griseofulvin, would be more effective against
Fungi
Most of the available antimicrobial agents are effective against
Bacteria.
Antimicrobial peptides work by
Disrupting the plasma membrane.
In what way are semisynthetic penicillins and natural penicillins alike?
Both are based on β-lactam.
Which of the following antibiotics is NOT bactericidal?
Polyenes
Which one of the following does NOT belong with the others?
Streptomycin
The antibiotic tetracycline binds to the 30S subunit of the ribosome as shown in the figure. The effect is to
Prevent attachment of tRNA.
Which of the following drugs does NOT act by competitive inhibition?
Streptomycin
Which of the following antibiotics is recommended for use against gram-negative bacteria?
Polymyxin
Which of the following antimicrobial agents is recommended for use against fungal infections?
Amphotericin B
The antibiotic streptomycin binds to the 30S subunit of the ribosome as shown in the figure. The effect is to
Cause misreading of mRNA in 70S ribosomes.
Which compound would be the most useful to treat candidiasis?
Flucytosine
Which of the following antibiotics are used to treat fungal infections?

1. Aminoglycosides
2. Cephalosporins
3. Griseofulvin
4. Polyenes
5. Bacitracin
3 and 4
Which of the following statements about drug resistance is false?
It is found only in gram-negative bacteria.
Drug resistance occurs
When antibiotics are used indiscriminately.
The following results were obtained from a disk-diffusion test for microbial susceptibility to antibiotics (Table 2). Staphylococcus aureus was the test organism, and the interpretive standards table for the different antibiotics are shown in Table 1.

Table 1. Antibiotic Resistant Intermediate Susceptible
Antibiotic
A <9mm 10-11mm >12 mm
B <14mm15-18mm >19mm
C <19mm 20-22mm >23mm
D <11mm 12-14mm > 15mm

Table 2. Antibiotic Zone of Inhibition
Antibiotic
A 11mm
B 13mm
C 19mm
D 16mm

In the table, the most effective antibiotic tested was
D; this is the only antibiotic tested that gave a zone of inhibition value within the susceptible range for that antibiotic.
Mebendazole is used to treat cestode infections. It interferes with microtubule formation; therefore, it would NOT affect
Bacteria.
Use of antibiotics in animal feed leads to antibiotic-resistant bacteria because
The antibiotics kill susceptible bacteria, but the few that are naturally resistant live and reproduce, and their progeny repopulate the host animal
Which of the following is the correct sequence of events that occur at the DNA replication fork?
A. Proteins stabilize the unwound parental DNA.
B. DNA polymerase continuously synthesizes the leading strand and the lagging strand is synthesized discontinuously.
C. Enzymes unwind the parental double helix.
D. DNA polymerase digests RNA primer.
C-A-B-D
Which of the following enzymes functions to relax supercoiling ahead of the replication fork, and also separates DNA circles?
Topoisomerase
Which of the following is a DNA strand complementary to CGA ATC AGC?
GCT TAG TCG
Which of the following describes the correct steps in transcription? A. RNA is synthesized by complementary base pairing of free nucleotides with the nucleotide bases on the template strand of DNA. B. Transcription reaches the terminator. C. RNA polymerase binds to the promoter, and DNA unwinds at the beginning of a gene. D. The site of synthesis moves along DNA; DNA that has been transcribed rewinds. E. RNA and RNA polymerase are released and the DNA helix re-forms.
C-A-D-B-E
What are the products of semiconservative replication for a double-stranded DNA molecule?
Two double-stranded DNA molecules, each consisting of one parental strand and one daughter strand.
Why is DNA replication essential for a cell?
An organism must copy its DNA to pass genetic information to its offspring
What is the initial target of RNA polymerase?
The promoter
RNA polymerase is guided by the
template strand of DNA.
How would one increase the concentration of a particular polypeptide in a cell?
Increase the level of transcription
During elongation, how is the RNA synthesized?
5' to 3'
Transcription produces which of the following?
mRNA, tRNA, and rRNA are all produced by transcription
According to the animation, which of the following makes mRNA from the information stored in a DNA template?
RNA polymerase
Ribosomes contain which of the following?
rRNA
Which of the following is involved in bringing amino acids to the ribosomes?
tRNA
Which of the following can be translated into protein?
mRNA
What results from the process of translation?
A polypeptide
How is bacterial translation different from eukaryotic translation?
Bacteria can begin translation before transcription has terminated
According to the animation, ribosomes move along the mRNA in which direction?
5' to 3'
As shown in the animation, the start codon also codes for which amino acid?
Methionine
How many codons code for the amino acid arginine?
Six
GAU codes for
aspartic acid
Which of the following amino acids is coded by only one codon?
Trp
What would happen if the DNA sequence that coded for Cys was mutated in the third position from a U to an A?
A stop codon would be introduced prematurely.
During the initiation step of translation, the fMet charged tRNA assembles in which site of the ribosome?
P site
How does the ribosome know if the entering charged tRNA is correct?
The anticodon on the tRNA base pairs to the codon on the mRNA.
What kind of bond is formed when two amino acids join together?
A peptide bond
How is translation terminated?
When a protein called a release factor enters and binds to the A site
Which of the following genetic elements is transcribed into a single mRNA?
The structural genes
Which operons are always transcribed unless deactivated?
Repressible operons
Which operons are never transcribed unless activated?
Inducible operons
According to the animation, where on the DNA strand does a repressor bind?
The operator
What is the inducer molecule in the lac operon?
Allolactose
With which genetic region does the repressor protein interact?
The operator region
When the cell is not in the presence of lactose
the repressor proteins bind to the operator.
What is the basic function of the lac operon?
To code for enzymes involved in catabolizing lactose.
According to the animation, to what genetic element does the RNA polymerase bind?
The promoter
When the cell is NOT in the presence of tryptophan,
RNA polymerase can transcribe mRNA
When is the repressor protein transcribed?
It is always transcribed
Why is the tryptophan operon turned off in the presence of tryptophan?
Tryptophan binds to and activates the repressor proteins; the repressor proteins, in turn, bind to the operator, preventing transcription.
What is the overall function of the trp operon?
To ensure that the cell has a supply of tryptophan at all times
According to the animation, how do the repressor proteins block the transcription of the structural genes?
It binds to the operator when activated.
Thymine dimers result from
nonionizing radiation.
Which of the following describes how 5-bromouracil might create a mutation?
It can replace the base thymine, and can base pair with guanine rather than adenine.
Mutagens
increase the likelihood of mutations in DNA
A nucleotide-altering chemical
can alter nitrogenous bases of DNA, resulting in incorrect base pairing.
What is considered to be the average natural mutation rate that occurs during DNA replication?
One in every billion nucleotides replicated.
A mutation that affects the genotype of the organism but not the phenotype is called a
silent mutation.
A base insertion or deletion in the translated region of the gene may lead to
frameshift mutation
A base substitution that changes a codon coding for an amino acid to a stop codon is called a
nonsense mutation
How frequently do silent mutations occur?
One out of every three mutations
Which of the following is a method of vertical gene transmission?
Cellular division
A recombinant cell
is a cell that receives DNA from an outside source and incorporates it into its own
What is the hallmark of all horizontal gene transfers?
Transfer of DNA between organisms of the same generation
Which of the following is an example of horizontal gene transfer in bacteria?
Transduction, conjugation, and transformation
Competent cells are cells that
can take up DNA from their surrounding environment and integrate it into their own chromosomes by recombination.
Mice that are injected with only the R strain of Streptococcus pneumoniae
stay healthy, because their immune systems can kill this strain easily.
What characteristic of the S strain allows it to evade the immune system of the mice?
The cells have a capsule.
What most likely explains the recovery of live S strain cells from a mouse injected with heat-killed S strain mixed with live R strain cells?
The R strain picked up the S strain DNA, enabling it to produce a capsule
Which finding is most surprising from Griffith's experiments?
S strain cells are isolated from the blood of mice infected with heat-killed S strains and live R strains
What is the function of the conjugation pilus?
It pulls the F+ and F- cells together.
What is required by an F- cell to become an F+ cell?
F plasmid
What is the key difference between donor cells and recipient cells?
An F plasmid
What cellular macromolecule is the fertility factor comprised of?
Nucleic acid
At which point does a recipient cell become an F+ cell?
Fusion of the cell membranes
Attachment of the sex pilus
Transfer of the single stranded F factor
Pulling of donor and recipient cells together
Formation of the complementary strand of the F factor
5
Bacterial conjugation is often referred to as bacterial sex. Why is this term inaccurate?
Conjugation does not result in the formation of new offspring
What must occur for bacterial conjugation to take place?
The cells must come into contact with each other.
Which statement about conjugation is false?
Conjugation is a process of bacterial reproduction.
Based on the animation, what is transferred during bacterial conjugation?
A bacterial plasmid
What is unique about transduction compared to normal bacteriophage infection?
Transduction transfers DNA from the chromosome of one cell to another.
How is generalized transduction different from specialized transduction?
Generalized transduction is initiated during lytic cycle of a virulent bacteriophage; specialized transduction is initiated during the lysogenic cycle of a temperate bacteriophage.
A transducing phage
contains fragments of the host chromosome instead of the viral genome.
When a transducing phage interacts with a new host cell,
the DNA from the previous host can recombine with the new host chromosome.
Activity
56, 57, 58
REVIEW
Which of the following events occurs during transcription?
A molecule of RNA is formed based on the sequence of nucleotides in DNA.
Which of the following is a correct statement about mRNA?
mRNA moves from the nucleus to the cytoplasm following RNA processing.
The site of translation is
ribosomes in the cell cytoplasm
Which one of the following does not play a role in translation?
DNA
Which of the following does not occur during RNA processing?
mRNA attaches to the small subunit of a ribosome
A gene is best defined as
A sequence of nucleotides in DNA that codes for a functional product.
DNA is constructed of
Two strands of nucleotides running antiparallel.
Which of the following is NOT a product of transcription?
polypeptide
An enzyme produced in response to the presence of a substrate is called
An inducible enzyme.
Transformation is the transfer of DNA from a donor to a recipient cell
As naked DNA in solution.
Synthesis of a repressible enzyme is stopped by
The corepressor-repressor binding to the operator
The damage caused by ultraviolet radiation is
Cut out and replaced.
According to the operon model, for the synthesis of an inducible enzyme to occur, the
Substrate must bind to the repressor.
Which of the following statements is false regarding a bacterium that is R+?
It is F+
In the figure, if base 4 is thymine, what is base 4'?
Adenine
Codon on mRNA and corresponding amino acid
UUA leucine UAA nonsense
GCA alanine AAU asparagine
AAG lysine UGC cysteine
GUU valine UCG serine
(Use the table.) The anticodon on tRNA for alanine is
CGU
Codon on mRNA and corresponding amino acid
UCG serine UAA stop
CGC arginine CAU histidine
AAG lysine CCA proline
UGC cysteine ACC threonine
(Use the table.) What is the sequence of amino acids encoded by the following sequence of bases in a coding strand of DNA?

5' AAGTGCACCTAACAU 3'
lysine-cysteine-threonine
Conjugation differs from reproduction because conjugation
Transfers DNA horizontally, to cells in the same generation.
An enzyme that copies DNA to make a molecule of RNA is
RNA polymerase.
An enzyme that makes covalent bonds between nucleotide sequences in DNA is
DNA ligase.
In transcription
DNA is copied to RNA.
List the events in the cycle of the ascomycete in the sequence in which they occur, from early to late stages or events.

Plasmogamy
Conidium germinates to produce hyphae
Vegetative mycelium grows
Hypha produces conidiophore
Karyogamy
Ascus opens to release ascospores
d-b-c-a-e-f
Regarding the life cycle of fungi, which of the following is the sexual spore phase that occurs when (+) and (-) nuclei fuse to form a diploid zygote nucleus?
Karyogamy phase
Which of the following statements about fungi is false?
All fungi are unicellular.
1-Arthroconidium 5-Chlamydoconidium
2-Ascospore 6-Conidiospore
3-Basidiospore 7-Sporangiospore
4-Blastoconidium 8-Zygospore
In the table, which spore is in a sac and results from the fusion of two nuclei from different strains of the same fungi?
2
1-Arthroconidium 5-Chlamydoconidium
2-Ascospore 6-Conidiospore
3-Basidiospore 7-Sporangiospore
4-Blastoconidium 8-Zygospore
In the table, which spore is found externally on a pedestal?
3
1-Arthroconidium 5-Chlamydoconidium
2-Ascospore 6-Conidiospore
3-Basidiospore 7-Sporangiospore
4-Blastoconidium 8-Zygospore
In the table, which is a thick-walled spore formed as a segment within a hypha?
5
1-Arthroconidium 5-Chlamydoconidium
2-Ascospore 6-Conidiospore
3-Basidiospore 7-Sporangiospore
4-Blastoconidium 8-Zygospore
In the table, which of these spores are asexual spores?
1, 4, 5, 6, 7