Micro ch 8

Strong chemical mutagens may be used to treat cancer cells. Is this a good or bad idea?

Good and bad-they're very good at killing cancer cells, but depending on mode of administration, they could also be dangerous to non-cancerous cells.

Every 24 hours, every genome in every cell of the human body is damaged 10,000 times or more. Given the possible DNA repair mechanisms, which order listed below would be most effective at repairing these as quickly as possible in order to prevent mutations from being carried forward in DNA replication?

Proofreading by DNA polymerase, glycosylase enzyme activities, excision repair, SOS repair

If you were carrying out a penicillin enrichment culture and you forgot to add penicillinase before plating the sample onto nutrient agar, what would happen? A) You would get the same results whether you add this enzyme of not because penicillin naturally rapidly degrades in agar.

Prototrophs and auxotrophs would both be killed by the penicillin; only PenR mutants would grow and you would not enrich for auxotrophs.

DNA repair mechanisms occur

both eukaryotes and prokaryotes

In conjugation, transformation, or transduction, the recipient bacteria is most likely to accept donor DNA

from the same species of bacteria.

Bacteria that have properties of both the donor and recipient cells are the result of

genetic recombination

To increase the chance of detecting carcinogens in the Ames test, the test substance is treated with

ground up rat liver

Cells that are His-, StrR could be isolated from a mixed sample by using a medium containing

histidine AND streptomycin

Please select the INCORRECT statement regarding mutation.

missense mutation is also called a synonymous mutation, meaning no change in the amino acid encoded.

The source of variation among microorganisms that were once identical is

mutation

To increase the proportion of auxotrophic mutants in a population of bacteria, one may use

penicillin enrichment

H. influenzae has genes that it needs to make pili for attachment to host cells. These pili genes are not always expressed; sometimes the bacterium turns them off and does not produce pili. This is an example of

phase variation

The characteristics displayed by an organism in any given environment is its

phenotype

The properties of a cell that are determined by its DNA composition are its

phenotype

Colonies of the bacterium Serratia marcescens are red when incubated at 22°C but white when incubated at 37°C. This is an example of

phenotypic change.

Thymine dimers are removed by

photoreactivation repair AND excision repair.

Antibiotics

provide an environment in which preexisting mutants survive.

A clever technique that streamlines the identification of auxotrophic mutants is

replica plating

The F plasmid carries the information for

sex pilus

Further tests indicate that your patient has a methicillin-resistant Staphylococcus aureus (MRSA) infection. You explain to the patient that these bacteria are not impacted by penicillin or the related drug methicillin. This resistance is thought to have arisen originally by a random change in the cell's chromosome through a process called

spontaneous mutation

The designation his- refers to

the genotype of a bacterium that lacks a functional gene for histidine synthesis AND bacteria that are auxotrophic for histidine.

The diploid character of eukaryotic cells may mask the appearance of a mutation since

the matching chromosome may carry the correct version of the gene.

Direct selection involves inoculating cells onto growth media on/in which

the mutant but not the parental will grow

The formation of a covalent bond between two adjacent thymines is caused by

UV radiation

Chemical mutagens that mimic the naturally occurring bases are called

base analogs

X-rays

Cause breaks in DNA molecules

On which of the following DNA strands would UV radiation have the most effect?

AATTAGTTC

Which change in a gene's DNA sequence would have the least effect on the eventual amino acid sequence produced from it?

Addition/deletion of three consecutive nucleotides

A quick microbiological test for potential carcinogens was developed by

Ames

The material responsible for transformation was shown to be DNA by

Avery, MacLeod, and McCarty.

DNA transfer by conjugation is more efficient in a liquid medium setting, subjected to very mild agitation (stirring), rather than on an agar plate format. Why?

Direct cell-to-cell contact is required for this process, and this is more likely to be achieved in the fluid liquid format than on an agar plate (especially for relatively non-motile types of bacteria).

Double-stranded DNA enters the recipient cell during transformation.

False

Organisms termed his- are considered prototrophic for histidine.

False

The Ames test determines antibiotic sensitivity of a bacterium.

False

Which of the following statements about spontaneous mutation is TRUE?

If the mutation rate to antibiotic A is 10-9 per cell division, and to antibiotic B is 10-6 per cell division, the probability of the cell being resistant to both medications is 10-15.

Is it as effective to take two antibiotics sequentially for an infection as it is to take them simultaneously, as long as the total length of time of the treatment is the same?

It depends. Provided that the majority of the infectious agent is killed off by the first drug, the likelihood that the few that are left would not also be killed by the second drug is low. However, simultaneous treatment should be more effective at eliminating all the microbes in the shortest time possible, and with the least probability of selection for multiple drug resistance mutations.

To maximize the number of thymine dimer mutations following UV exposure, should you keep human cells in tissue culture in the dark, in the light, or does it matter at all?

It doesn't matter-human cells don't possess the enzymes needed for photorepair of thymine dimers.

On further reading, you learn that strains of H. influenzae vary genotypically. Many of them have silent mutations. Select the TRUE statement regarding this type of mutation.

It is a point mutation—a single base pair is altered, causing the change of one amino acid in the protein that does not affect the function of that protein.

Which is NOT true about mismatch repair?

It removes both strands in the mismatch area.

Which is TRUE about a crown gall tumor?

It results from the incorporation of bacterial plasmid DNA into the plant chromosome.

Two bacterial genes are transduced simultaneously. What does this suggest about their proximity to each other within the original host genome?

It's highly likely that the two genes are located next to each other in the original host cell chromosome. Since transduction relies on either mispackaging of bits of host cell DNA into non-functional virus units, or improper excision of lysogenic phage DNA from a host cell chromosome (carrying parts of the host cell DNA with it), the genes must lie close to each other to be transduced into a new cell simultaneously.

insertion sequence

are the simplest type of transposon, code for a transposase enzyme, AND are characterized by an inverted repeat.

The lab results indicate that the pus from your patient's "bite" contains clusters of spherical cells that stained purple in the Gram stain. You tell your patient that his wound contains bacteria from the genus ______ and is classified as a ________ organism.

Staphylococcus; Gram-positive

Your patient asks why the penicillin he has been taking hasn't helped his infection. Which of the following is a possible explanation for your patient?

The bacteria causing the infection are resistant to penicillin; a different antibiotic is needed.

You mix two bacterial stains in a tube of glucose-salts agar. One strain is His−, Val−, while the other strain is Trp−, Leu−. You previously showed that neither strain grows on glucose-salts agar. After incubating the tube, you plate a sample onto a new glucose-salts agar plate. Several colonies grow. What do you know is TRUE about these colonies?

The bacteria in the colonies are His+, Val+, Trp+, Leu+.

A bacterial strain is resistant to infection by a bacteriophage. Which statement is FALSE?

The bacterial host DNA is protected from restriction enzyme degradation by phosphorylation.

You make two agar plates: one is a nutrient agar plate (plate A) that contains histidine and penicillin. The other is a glucose salts agar (plate B) that also contains penicillin. You inoculate a sample onto both plates using replica plating technique, incubate the plates, and compare the growth after 48 hours. There are 12 colonies on the nutrient agar plate and 11 colonies on the glucose salts medium. Which of the following statements is INCORRECT?

The prototrophs are resistant to penicillin but the auxotrophs are sensitive to this antibiotic

Some bacteria have a higher incidence rate of thymine dimer mutations following exposure to UV light than others. What might be going on here to lead to this outcome?

They may simply have a higher proportion of T nucleotides next to each other in their DNA than other bacteria, leading to more possible dimers being formed AND they may have a weaker expression of photoreactivation enzymes, leading to formation of more thymine dimers.

Which of the following about transposons is NOT true?

They were first recognized in fungi.

What concerns you most about the strain of H. influenzae that you may be exposed to while you are away is antibiotic resistance. The genes for resistance can be acquired by this organism as naked DNA from the environment, an example of

Transformation

Which of the following statements is CORRECT regarding transformation?

Transformation is the uptake of "naked" DNA from the environment.

Crown gall is caused by a prokaryote plasmid that can be expressed in plant cells.

True

DNA polymerase is able to proofread the DNA sequence.

True

Each gene mutates at a characteristic frequency.

True

F plasmids and oftentimes R plasmids are both able to code for production of a pilus.

True

Mutations are likely to persist after SOS repair, but not after light-induced or excision repair.

True

Plasmids often carry the information for antibiotic resistance.

True

The restriction-modification system always has two genes involved, the cutting enzyme and the methylating enzyme.

True

Transposons may leave a cell by incorporating themselves into a plasmid.

True

A mutation in E. coli results in the loss of both restriction endonucleases and modification enzymes. Would you expect any difference in the frequency of gene transfer via transduction FROM Salmonella INTO this E. coli strain?

Yes—the loss of the restriction endonucleases would leave the recipient E. coli unable to break down "invading" viral DNA from the transducing phage, AND the loss of the modification enzymes would leave the recipient E. coli unable to tag its own DNA as "self," leaving the viral DNA untagged and recognizable as "foreign," and targeted for destruction. Together, these would lead to higher rates of successful transduction.

The study of the crown gall tumor found

a bacterial plasmid promoter that was similar to plant promoters.

Intercalating agents

act during DNA synthesis AND often result in frameshift mutations.

The largest group of chemical mutagens consists of

alkylating agents

In conjugation the donor cell is recognized by the presence of

an F plasmid

Competent cells

are able to take up naked DNA, occur naturally, AND can be created in the laboratory.

The Ames test is useful as a rapid screening test to identify those compounds that

are mutagens

Not all bacteria can take up DNA from the environment. Those that can are referred to as

competent

Gene transfer that requires cell-to-cell contact is

conjugation

The transfer of vancomycin resistance from Enterococcus faecalis to Staphylococcus aureus is thought to have involved

conjugation AND transposons

Chemical mutagens often act by altering the

hydrogen bonding properties of the nucleobase.

Your patient asks whether there is any way for him to get rid of this infection. You tell him that he will be given a different antibiotic such as doxycycline. This antibiotic works by binding to a cell structure called the 30S ribosomal subunit, which stops the bacterial cells from growing by

inhibiting protein synthesis

The clustered, regularly interspaced short palindromic repeats (CRISPR) system in bacterial cells has been called the "immune" system of bacteria. CRISPR protect bacteria from a repeat infection from the same phage because bacterial cells

integrate fragments from the phage DNA in their own chromosomes and target for destruction any DNA that contains the same fragments in the future.

Planar molecules used as chemical mutagens are called

intercalating agents

Among the easiest of the mutations to isolate are those which

involve haploid chromosomes AND involve antibiotic resistance.

Indirect selection

is necessary to isolate auxotrophic mutants.

Replica plating

is useful for identifying auxotrophs AND uses media on which the mutant will not grow but the parental cell type will.

DNA is protected from restriction enzymes by being

methylated

Irradiation of cells with ultraviolet light may cause

thymine dimers

The mechanism by which genes are transferred into bacteria via viruses is called

transduction

You explain to your patient that there has been an increase in the number of strains of S. aureus that are now resistant to methicillin. The gene conferring resistance has moved from one strain to another via mobile gene elements, an example of which is a(n) ________.

transposon

Segments of DNA capable of moving from one area in the DNA to another are called

transposons

In order for insertional inactivation to occur, the transposon must be placed

within the gene in question