Microbiology Study Guide – Module 4: Viruses and Prions

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Q1. List general characteristics of viruses relative to viral structure and genome, including reverse transcriptase.

Background

Topic: Viral Structure and Genetics

This question tests your understanding of the basic features that define viruses, including their physical structure, types of genetic material, and the role of enzymes like reverse transcriptase.

Key Terms and Concepts:

Virus: A non-cellular infectious agent with genetic material (DNA or RNA) surrounded by a protein coat (capsid).
Capsid: Protein shell that encases the viral genome.
Envelope: Some viruses have a lipid membrane derived from the host cell.
Genome: Can be DNA or RNA, single- or double-stranded, linear or circular.
Reverse Transcriptase: An enzyme used by some RNA viruses (retroviruses) to synthesize DNA from an RNA template.

Step-by-Step Guidance

Start by listing the main structural components of viruses (e.g., capsid, envelope, nucleic acid type).
Describe the possible forms of viral genomes (DNA vs. RNA, single- vs. double-stranded).
Explain the significance of reverse transcriptase and which viruses use it.
Consider how these features distinguish viruses from other microbes.

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Final Answer:

Viruses are acellular infectious agents composed of a nucleic acid genome (either DNA or RNA, which can be single- or double-stranded) surrounded by a protein capsid. Some viruses also have a lipid envelope derived from the host cell membrane. The genome can be linear or circular. Retroviruses, a type of RNA virus, possess the enzyme reverse transcriptase, which allows them to synthesize complementary DNA (cDNA) from their RNA genome, enabling integration into the host genome. These features distinguish viruses from cellular organisms.

Q2. State why RNA viruses evolve faster than DNA viruses.

Background

Topic: Viral Evolution and Mutation Rates

This question examines your understanding of the molecular mechanisms that influence the rate of evolution in viruses, particularly the differences between RNA and DNA viruses.

Key Terms and Concepts:

Mutation Rate: Frequency at which genetic mutations occur.
RNA-dependent RNA polymerase: Enzyme used by RNA viruses to replicate their genomes.
Proofreading: Mechanism to correct errors during genome replication.

Step-by-Step Guidance

Recall the enzymes used by RNA and DNA viruses for genome replication.
Consider the accuracy and proofreading abilities of these enzymes.
Think about how replication fidelity affects mutation rates and evolution.

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Final Answer:

RNA viruses evolve faster than DNA viruses because their RNA-dependent RNA polymerases lack proofreading ability, leading to higher mutation rates during replication. In contrast, DNA polymerases (used by DNA viruses) have proofreading functions that correct errors, resulting in lower mutation rates. The increased genetic variability in RNA viruses allows them to adapt and evolve more rapidly.

Q3. List the 4 criteria used to classify viruses.

Background

Topic: Viral Taxonomy

This question tests your knowledge of the main characteristics used to categorize and classify viruses in microbiology.

Key Terms and Concepts:

Viral Classification: The system used to organize viruses based on shared features.

Step-by-Step Guidance

Recall the main features of viruses that can vary between different groups.
Think about the type of genetic material, structure, and replication strategy.
Consider how these features are used in official classification systems (e.g., ICTV).

Try solving on your own before revealing the answer!

Final Answer:

The four main criteria used to classify viruses are: (1) type of nucleic acid (DNA or RNA), (2) capsid symmetry (helical, icosahedral, or complex), (3) presence or absence of an envelope, and (4) dimensions of the virion and capsid.

Q4. Explain antigenic shift and antigenic drift, and state how they impact influenza virus evolution and outbreaks.

Background

Topic: Viral Genetics and Epidemiology

This question focuses on the genetic mechanisms that cause changes in viral antigens, particularly in influenza viruses, and how these changes affect disease patterns.

Key Terms and Concepts:

Antigenic Drift: Gradual accumulation of mutations in viral genes.
Antigenic Shift: Abrupt, major change in viral antigens due to reassortment.
Influenza Virus: A segmented RNA virus prone to both drift and shift.

Step-by-Step Guidance

Define antigenic drift and describe the molecular process behind it.
Define antigenic shift and explain how it occurs (e.g., reassortment of gene segments).
Discuss the consequences of each process for influenza virus evolution and outbreaks.
Consider the impact on vaccine effectiveness and pandemic potential.

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Final Answer:

Antigenic drift refers to the gradual accumulation of point mutations in the genes encoding viral surface proteins, leading to minor changes in antigens. Antigenic shift is a sudden, major change resulting from the reassortment of gene segments, often when two different influenza viruses infect the same cell. Antigenic drift causes seasonal influenza outbreaks, while antigenic shift can lead to pandemics due to the emergence of novel viral strains to which the population has little immunity.

Q5. Explain the features of bacteriophage lytic and lysogenic replication.

Background

Topic: Bacteriophage Life Cycles

This question tests your understanding of the two main replication strategies used by bacteriophages (viruses that infect bacteria).

Key Terms and Concepts:

Lytic Cycle: Phage replication that results in host cell lysis and release of new phages.
Lysogenic Cycle: Phage genome integrates into host DNA and replicates with it without killing the host immediately.
Prophage: Integrated phage DNA in the bacterial genome.

Step-by-Step Guidance

Describe the steps of the lytic cycle, focusing on infection, replication, assembly, and lysis.
Outline the lysogenic cycle, emphasizing integration and dormancy.
Compare the outcomes for the host cell in each cycle.

Try solving on your own before revealing the answer!

Final Answer:

In the lytic cycle, a bacteriophage infects a bacterium, replicates its genome, assembles new phage particles, and lyses the host cell to release progeny. In the lysogenic cycle, the phage genome integrates into the host's chromosome as a prophage and replicates along with the host DNA without killing the cell. The prophage can later be induced to enter the lytic cycle.

Q6. Define the term phage conversion and discuss why it is medically important.

Background

Topic: Bacteriophage Genetics and Pathogenicity

This question examines your understanding of how bacteriophage infection can alter bacterial properties, including implications for human health.

Key Terms and Concepts:

Phage Conversion (Lysogenic Conversion): Acquisition of new traits by a bacterium due to prophage genes.
Toxins: Some bacterial toxins are encoded by prophage genes.

Step-by-Step Guidance

Define phage (lysogenic) conversion in your own words.
Explain how prophage genes can alter bacterial phenotype.
Give examples of medically important consequences (e.g., toxin production).

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Final Answer:

Phage conversion is the process by which a bacterium acquires new genetic traits due to the presence of prophage genes, often resulting in the production of toxins or other virulence factors. This is medically important because it can turn harmless bacteria into pathogens, as seen with diphtheria and cholera toxins, which are encoded by prophage genes.

Q7. Describe the generalized steps of animal virus replication.

Background

Topic: Animal Virus Life Cycle

This question tests your knowledge of the main stages involved in the replication of viruses that infect animal cells.

Key Terms and Concepts:

Attachment, Penetration, Uncoating, Biosynthesis, Assembly, Release

Step-by-Step Guidance

List the main stages of animal virus replication in order.
Briefly describe what happens at each stage (e.g., how the virus enters the cell, how its genome is replicated).
Note any differences between enveloped and non-enveloped viruses in the release step.

Try solving on your own before revealing the answer!

Final Answer:

The generalized steps of animal virus replication are: (1) attachment to host cell, (2) penetration (entry), (3) uncoating of the viral genome, (4) biosynthesis of viral components, (5) assembly of new virions, and (6) release from the host cell (by budding for enveloped viruses or lysis for non-enveloped viruses).

Q8. List methods used to culture viruses in a laboratory.

Background

Topic: Virology Laboratory Techniques

This question tests your knowledge of the main laboratory methods for growing and studying viruses.

Key Terms and Concepts:

Cell Culture: Growing viruses in living cells in vitro.
Embryonated Eggs: Fertilized chicken eggs used for virus cultivation.
Laboratory Animals: Sometimes used for viruses that do not grow in cell culture.

Step-by-Step Guidance

Recall the main systems used to culture viruses (e.g., cell lines, eggs, animals).
Consider the advantages and limitations of each method.
Think about which viruses are typically grown using each method.

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Final Answer:

Viruses can be cultured using (1) cell cultures (primary, diploid, or continuous cell lines), (2) embryonated chicken eggs, and (3) laboratory animals. Each method is chosen based on the virus's host range and growth requirements.

Q9. Explain what makes a virus oncogenic and name examples of oncogenic viruses and the cancers they may cause.

Background

Topic: Viral Oncogenesis

This question examines your understanding of how certain viruses can cause cancer and asks for specific examples.

Key Terms and Concepts:

Oncogenic Virus: A virus capable of causing cancer.
Oncogenes: Genes that can transform a normal cell into a cancerous cell.

Step-by-Step Guidance

Define what it means for a virus to be oncogenic.
Explain the mechanisms by which viruses can induce cancer (e.g., insertional mutagenesis, expression of viral oncogenes).
List at least two examples of oncogenic viruses and the cancers they are associated with.

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Final Answer:

A virus is oncogenic if it can cause cancer by integrating its genome into host DNA or by expressing viral oncogenes that disrupt normal cell regulation. Examples include human papillomavirus (HPV), which can cause cervical cancer, and Epstein-Barr virus (EBV), which is associated with Burkitt's lymphoma and nasopharyngeal carcinoma.

Q10. Explain the different drug approaches to managing viral infections and name several antiviral drugs and their targets.

Background

Topic: Antiviral Therapy

This question tests your knowledge of the main strategies used to treat viral infections and examples of antiviral drugs and their mechanisms.

Key Terms and Concepts:

Antiviral Drugs: Medications that inhibit viral replication.
Targets: Viral enzymes (e.g., reverse transcriptase, protease), entry/fusion, genome replication.

Step-by-Step Guidance

List the main strategies for antiviral drug action (e.g., inhibition of entry, genome replication, assembly, release).
Provide examples of drugs for each strategy and their specific viral targets.
Consider why selective toxicity is important in antiviral therapy.

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Final Answer:

Antiviral drugs work by targeting specific steps in the viral life cycle, such as entry (e.g., maraviroc for HIV), genome replication (e.g., acyclovir for herpesviruses, which inhibits viral DNA polymerase), reverse transcription (e.g., AZT for HIV), and protease activity (e.g., ritonavir for HIV). Selective toxicity is crucial to minimize harm to host cells.

Q11. Describe what prions are, name the diseases they cause in humans, and explain how they can be transmitted.

Background

Topic: Prions and Prion Diseases

This question tests your understanding of prions, their unique infectious nature, and their impact on human health.

Key Terms and Concepts:

Prion: Infectious protein particle lacking nucleic acid.
Transmissible Spongiform Encephalopathies (TSEs): Group of fatal neurodegenerative diseases caused by prions.

Step-by-Step Guidance

Define prions and describe their structure (or lack thereof).
List at least two human diseases caused by prions.
Explain the main routes of prion transmission.

Try solving on your own before revealing the answer!

Final Answer:

Prions are infectious proteins that lack nucleic acids and cause neurodegenerative diseases known as transmissible spongiform encephalopathies (TSEs). In humans, prion diseases include Creutzfeldt-Jakob disease (CJD) and variant CJD. Prions can be transmitted through contaminated food, medical instruments, or inherited mutations.