Question 1

What is another term for an RNA-dependent-DNA-polymerase?

Accepted Answer

Reverse transcriptase is another term for RNA-dependent-DNA-polymerase. It is an enzyme that catalyzes the formation of DNA from RNA templates, which is the reverse of the usual process of transcription. Reverse transcriptase is commonly used in research and biotechnology applications, such as the synthesis of cDNA from mRNA for use in cloning or gene expression analysis.

Question 2

Which of the following terms describes the protein shell that surrounds the viral genome?

Accepted Answer

The protein shell that surrounds the viral genome is called a capsid.

Question 3

All viruses that use a negative-sense RNA genome must package which of the following proteins in their virion?

Accepted Answer

All negative-sense RNA viruses require the RNA-dependent-RNA-polymerase enzyme for viral replication, and therefore, must package it into their virion. The other proteins listed are not required for viral replication or assembly.

Question 4

If the genome of a positive-strand RNA virus, which has been purified away from all of the virion proteins, was injected into the cytoplasm of an appropriate host cell, what would happen first?

Accepted Answer

Positive-strand RNA viruses have genomes that can directly serve as mRNA. Therefore, upon entering the cytoplasm of a host cell, the viral RNA can immediately be recognized and translated by the host's ribosomes to produce viral proteins. This is the first step in the replication cycle of such viruses when only the RNA genome is introduced into the cell.

Question 5

The study of which group of viruses led to the discovery of cellular genes that can promoter cancer in animal cells?

Accepted Answer

Retroviruses led to the discovery of cellular genes that can promote cancer in animal cells through the identification of oncogenes.

Question 6

Which of the following is an enzyme that most RNA viruses encode in their genome?

Accepted Answer

RNA viruses replicate their genome using RNA-dependent-RNA-polymerase (also known as RNA replicase), which is encoded in their genome. DNA-dependent-RNA-polymerase (A) and DNA-dependent-DNA-polymerase (C) are found in cells for DNA replication and transcription. RNA-dependent-protein-synthetase (D) does not exist as named, as protein synthesis is carried out by ribosomes, not enzymes.

Question 7

To start an experiment, you do an infection by putting 0.5 ml of a virus stock that has a titer of 6 x 10⁷ pfu/ml onto a plate of animal cells that contains 5 x 10⁶ cells.What is the multiplicity of infection for this experiment?

Accepted Answer

The answer of To start an experiment, you do an...

Question 8

What does the word "virus" mean in Latin?

Accepted Answer

The word "virus" in Latin means "poison".

Question 9

If you wanted to show that an infectious agent is a virus, what experiment could you do?

Accepted Answer

Viruses are much smaller than bacteria and other pathogens, so they can pass through filters that trap larger organisms. The ability of an agent to remain infectious after passing through a porcelain filter, which would trap bacteria and larger particles, indicates that the agent is likely a virus. This method was historically used in the discovery of viruses.

Question 10

Which of the following would not be a nucleic acid form found in a viral genome?

Accepted Answer

An RNA:DNA hybrid is not a common nucleic acid form found in viral genomes. Viruses typically have either DNA or RNA as their genetic material, and it is usually either single-stranded or double-stranded. While there are some viruses that have RNA:DNA hybrids as part of their replication process, it is not a common nucleic acid form found in viral genomes.

Question 11

If the genome of a negative-strand RNA virus, which has been purified away from all of the virion proteins, was injected into the cytoplasm of an appropriate host cell, what would happen first?

Accepted Answer

Negative-strand RNA viruses require their own RNA-dependent RNA polymerase for transcription because host cells do not possess an enzyme capable of transcribing negative-strand RNA into positive-strand mRNA. Since the viral genome has been purified away from all of the virion proteins, including the necessary RNA-dependent RNA polymerase, none of the listed processes can occur immediately upon injection into the host cell.

Question 12

Viruses play a major role in the ecology of the ocean by doing which of the following?

Accepted Answer

Viruses play a major role in the ecology of the ocean by lysing unicellular organisms and releasing carbon and oxygen back into the ocean. This process is called viral shunt and it recycles nutrients and helps maintain the balance of the ecosystem. The other statements are not entirely accurate or complete.

Question 13

The Baltimore classification system describes the relationship between a viral genome and the:

Accepted Answer

The Baltimore classification system categorizes viruses based on their genome type and the method of mRNA synthesis, focusing on the relationship between the viral genome and the production of early mRNAs.

Question 14

Which of the following is a description of a plaque?

Accepted Answer

A plaque is a clear area in a monolayer of infected cells where the cells have died due to the infection. The virus has multiplied and spread to neighboring cells, causing cell death and leaving a visible clearing, or plaque. A plaque assay is a common method used to quantify the number of infectious viral particles in a sample. A, C, D, and E are not descriptions of a plaque.

Question 15

You are given a solution of virus and asked to determine the titer.You carry out a plaque assay with 0.5mls of a 10⁵ dilution of the virus solution and obtain an average of 150 plaques.What is the titer of the solution?

Accepted Answer

The titer is calculated by multiplying the number of plaques by the dilution factor and dividing by the volume plated: (150 plaques x 10⁵) / 0.5 ml = 3 x 10⁷ pfu/ml.

Question 16

Plant viruses use which cellular structures to spread between host cells?

Accepted Answer

Plant viruses use plasmodesmata, small channels that connect adjacent plant cells, to spread between host cells.

Question 17

The word "phage" is a shortened version of the name of a virus that can infect which type of organism?

Accepted Answer

"Phage" is short for bacteriophage, which is a virus that infects bacteria. Therefore, the best choice is A) bacterial cell.

Question 18

When doing a plaque assay with human or animal cells, a layer of nutrient media mixed with agar is put on top of the cells after they have been infected with the diluted virus.What is the major purpose of this agar layer?

Accepted Answer

The major purpose of the agar layer is to limit the movement of virus particles, allowing for the formation of distinct plaque formations which can be counted and used to calculate viral titer. The other options (A, B, and C) are not correct as the cells are already growing in nutrient media before the agar layer is added, and the agar layer does not actively promote virus replication or make infected cells easier to visualize.

Question 19

During a single growth cycle experiment, the titer of extracellular and intracellular virus drops in the first hour.What explains this observation?

Accepted Answer

The drop in titer of extracellular and intracellular virus suggests that the virus is successfully entering the cells and is being uncoated, causing a reduction in the number of infectious virus particles in the medium. This is a normal part of the viral replication cycle. Option A is unlikely as antibodies are not present in the first hour of the experiment, unless they were added intentionally. Option B is also unlikely as cellular enzymes typically act to help the virus replicate, not inactivate it. Option D is unlikely as the virus titer should still be detectable in the medium even if the virus is binding to the cells. Option E is also unlikely as the antiviral defenses of the cell would take time to activate, and would not be immediately effective in the first hour of the experiment.

Question 20

Which of the following processes in cells was NOT discovered by studying a virus?

Accepted Answer

The sequence of the genetic code was not discovered by studying a virus. It was elucidated through experiments on bacterial systems and synthetic RNA molecules, notably by Marshall Nirenberg and Heinrich Matthaei's experiments in the early 1960s, which did not involve viruses.

Quiz 1: Introduction to Virology