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Biological Sciences

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Viruses

Question
5 out of 6
 

Strategies to fight the AIDS virus include drugs that mimic nucleotides, called nucleotide analogs. Reverse transcriptase incorporates these analogs into the newly formed viral DNA strand. The host cell cannot interpret the DNA correctly, so the virus does not propagate. Unfortunately, this therapy only works for a short time in infected individuals, probably because:

A the outer protein coat of the virus mutates so it is no longer recognized by the immune system.
B the patient’s immune system starts to fight off the analog.
C the reverse transcriptase mutates to prevent incorporation of the analog.
D the viral DNA no longer incorporates into the host DNA.
Ans. C

The action of the analogs is at the level of reverse transcription. If the reverse transcriptase mutates to no longer allow incorporation of the analogs, then the virus can continue on its normal path. It is unlikely the body would mount an immune response to these analogs (choice B) as these analogs are so similar to normal nucleotides. Although the outer protein coat can and does mutate (choice A), this has nothing to do with the analogs. And the virus must incorporate into the host chromosome, so choice D is also incorrect.

Viruses Flashcard List

6 flashcards
1)
The activity of bacteriophages is easily assayed. Bacteria are spread on a solid nutrient agar plate. Under appropriate growth conditions, the bacteria will form a cloudy layer completely covering the dish. In microbiology, this is called a lawn, as opposed to isolated spots, or colonies, of bacteria. If bacteriophages are introduced to the lawn of bacteria, the phages will infect single cells. Once the phages have reproduced, they will lyse the cells and the progeny phage will infect the surrounding bacterial cells. In a relatively short period of time, the phages will have lysed all the bacteria within a radius of the initial infection, causing a clear, circular spot to form in the lawn of bacteria. This is called a plaque. A researcher working with bacteriophages set up the following experiment: Condition 1: Bacteria were spread on agar plates and incubated at 370C for 24 hours. A lawn was clearly visible and no plaques formed. Condition 2: The same strain of bacteria was mixed with a preparation of lambda bacteriophage. The mixture was spread on agar plates and incubated for 24 hours at 370C. An average of twenty plaques formed on the bacterial lawn on each plate. Condition 3: The same strain of bacteria was spread on agar plates and then exposed to UV light. After incubation at 370C for 24 hours, lawns grew and approximately fifty plaques formed on each plate. After incubating the plates from condition 2 for an additional 5 days, the plates became totally clear. What is the most likely explanation for this result?A Newly produced phages infected all bacteria on the dish, thus killing everything.B The bacteria died off due to lack of nutrients.C The bacteria protected themselves from the phages by slowing their growth.D The phage forced all the bacteria to go into the lysogenic cycle and stop growing.
2)
The activity of bacteriophages is easily assayed. Bacteria are spread on a solid nutrient agar plate. Under appropriate growth conditions, the bacteria will form a cloudy layer completely covering the dish. In microbiology, this is called a lawn, as opposed to isolated spots, or colonies, of bacteria. If bacteriophages are introduced to the lawn of bacteria, the phages will infect single cells. Once the phages have reproduced, they will lyse the cells and the progeny phage will infect the surrounding bacterial cells. In a relatively short period of time, the phages will have lysed all the bacteria within a radius of the initial infection, causing a clear, circular spot to form in the lawn of bacteria. This is called a plaque. A researcher working with bacteriophages set up the following experiment: Condition 1: Bacteria were spread on agar plates and incubated at 370C for 24 hours. A lawn was clearly visible and no plaques formed. Condition 2: The same strain of bacteria was mixed with a preparation of lambda bacteriophage. The mixture was spread on agar plates and incubated for 24 hours at 370C. An average of twenty plaques formed on the bacterial lawn on each plate. Condition 3: The same strain of bacteria was spread on agar plates and then exposed to UV light. After incubation at 370C for 24 hours, lawns grew and approximately fifty plaques formed on each plate. What happened in condition 3?A Phage contaminated the dishes causing infection and lysis of the bacteria.B The nutrients in the agar were insufficient to support bacterial growth.C The bacteria were probably a lysogenic strain induced by UV light.D The bacteria were not healthy and died off when exposed to UV light.
3)
The activity of bacteriophages is easily assayed. Bacteria are spread on a solid nutrient agar plate. Under appropriate growth conditions, the bacteria will form a cloudy layer completely covering the dish. In microbiology, this is called a lawn, as opposed to isolated spots, or colonies, of bacteria. If bacteriophages are introduced to the lawn of bacteria, the phages will infect single cells. Once the phages have reproduced, they will lyse the cells and the progeny phage will infect the surrounding bacterial cells. In a relatively short period of time, the phages will have lysed all the bacteria within a radius of the initial infection, causing a clear, circular spot to form in the lawn of bacteria. This is called a plaque. A researcher working with bacteriophages set up the following experiment: Condition 1: Bacteria were spread on agar plates and incubated at 370C for 24 hours. A lawn was clearly visible and no plaques formed. Condition 2: The same strain of bacteria was mixed with a preparation of lambda bacteriophage. The mixture was spread on agar plates and incubated for 24 hours at 370C. An average of twenty plaques formed on the bacterial lawn on each plate. Condition 3: The same strain of bacteria was spread on agar plates and then exposed to UV light. After incubation at 370C for 24 hours, lawns grew and approximately fifty plaques formed on each plate. If a fourth experiment was done, where bacteria were mixed with lambda and exposed to UV light, what would you predict?A The results would be similar to condition 2, with approximately 20 plaques per plate.B The results would be similar to condition 3, with approximately 50 plaques per plate.C There would be approximately 35 plaques per plate.D There would be approximately 70 plaques per plate.
4)
The activity of bacteriophages is easily assayed. Bacteria are spread on a solid nutrient agar plate. Under appropriate growth conditions, the bacteria will form a cloudy layer completely covering the dish. In microbiology, this is called a lawn, as opposed to isolated spots, or colonies, of bacteria. If bacteriophages are introduced to the lawn of bacteria, the phages will infect single cells. Once the phages have reproduced, they will lyse the cells and the progeny phage will infect the surrounding bacterial cells. In a relatively short period of time, the phages will have lysed all the bacteria within a radius of the initial infection, causing a clear, circular spot to form in the lawn of bacteria. This is called a plaque. A researcher working with bacteriophages set up the following experiment: Condition 1: Bacteria were spread on agar plates and incubated at 370C for 24 hours. A lawn was clearly visible and no plaques formed. Condition 2: The same strain of bacteria was mixed with a preparation of lambda bacteriophage. The mixture was spread on agar plates and incubated for 24 hours at 370C. An average of twenty plaques formed on the bacterial lawn on each plate. Condition 3: The same strain of bacteria was spread on agar plates and then exposed to UV light. After incubation at 370C for 24 hours, lawns grew and approximately fifty plaques formed on each plate. Before the phage life cycle was understood, researchers called the results from condition 3 autolysis. They thought an enzyme in the bacteria caused its own destruction and the destruction of surrounding cells. Which of the following would lend support to the viral theory and help to disprove autolysis?A Within a week, all the bacterial cells in condition 3 die.B Viral particles can be purified from the plates in condition 3.C Exposure of bacteria to X rays also causes plaques to form.D With or without exposure to UV light, no bacteria grow at 40C.
5)
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