Study Guide: Viruses, Bacteriophages, and Prions (Microbiology Chapter 13)

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Q1. How do viruses differ from bacteria?

Background

Topic: Differences between Viruses and Bacteria

This question tests your understanding of the fundamental differences between viruses and bacteria, which are two major types of microorganisms studied in microbiology.

Key Terms:

Virus: A non-cellular infectious agent that requires a host cell to replicate.
Bacterium: A unicellular, prokaryotic organism capable of independent life.

Step-by-Step Guidance

Consider the structural differences: Think about whether viruses and bacteria have cellular structures such as cell walls, plasma membranes, and organelles.
Compare their genetic material: Reflect on the type of nucleic acids present in each (DNA, RNA, or both).
Think about their replication: Ask yourself how each reproduces—can they replicate independently, or do they need a host?
Consider their size and complexity: Which is generally larger and more complex?
Review their response to antibiotics: Do both respond to antibiotics in the same way?

Try solving on your own before revealing the answer!

Final Answer:

Viruses differ from bacteria in several key ways:

Viruses are acellular (not made of cells), while bacteria are unicellular prokaryotes.
Viruses require a host cell to replicate; bacteria can reproduce independently by binary fission.
Viruses contain either DNA or RNA (never both), while bacteria have both DNA and RNA.
Viruses are much smaller than bacteria (typically 20–300 nm vs. 0.5–5 μm).
Antibiotics are generally effective against bacteria but not viruses.

These differences are summarized in Table 13.1 of your textbook.

Q2. What is the host range and size of viruses?

Background

Topic: Host Range and Size of Viruses

This question focuses on the diversity of organisms that viruses can infect (host range) and the physical dimensions of viruses.

Key Terms:

Host Range: The spectrum of host cells a virus can infect.
Size: The physical dimensions of a virus, usually measured in nanometers (nm).

Step-by-Step Guidance

Define what is meant by 'host range' in the context of viruses.
Think about what determines a virus's host range (e.g., specific receptors on host cells).
Recall the typical size range for viruses and how this compares to other microorganisms.
Consider examples of viruses with narrow vs. broad host ranges.

Try solving on your own before revealing the answer!

Final Answer:

The host range of a virus refers to the specific types of organisms or cells it can infect, which is determined by the presence of compatible receptors on the host cell surface. Some viruses have a narrow host range (infecting only one species), while others have a broad host range.

Viruses are typically much smaller than bacteria, with sizes ranging from about 20 nm to 300 nm in diameter.

Q3. Characterize different parts of a virion: Nucleic acid, Capsid, Envelope, Spikes.

Background

Topic: Structure of Virions

This question asks you to describe the main structural components of a complete virus particle (virion).

Key Terms:

Nucleic Acid: The genetic material (DNA or RNA) of the virus.
Capsid: The protein coat that surrounds and protects the nucleic acid.
Envelope: A lipid membrane that surrounds some viruses, derived from the host cell membrane.
Spikes: Glycoprotein projections on the envelope or capsid that aid in attachment to host cells.

Step-by-Step Guidance

Identify the function of the nucleic acid in a virion.
Describe the role and composition of the capsid.
Explain what the envelope is and how it is acquired.
Discuss the purpose of spikes and where they are found.

Try solving on your own before revealing the answer!

Final Answer:

Nucleic Acid: Contains the viral genome, which can be DNA or RNA, single- or double-stranded.
Capsid: A protein shell that encases and protects the nucleic acid; made of subunits called capsomeres.
Envelope: A lipid bilayer surrounding some viruses, derived from the host cell membrane during viral exit.
Spikes: Surface glycoproteins that protrude from the envelope or capsid, facilitating attachment to host cells.

Q4. What is the general morphology of viruses? (Helical, Polyhedral, Enveloped, Complex)

Background

Topic: Viral Morphology

This question examines your knowledge of the different shapes and structures that viruses can have.

Key Terms:

Helical: Rod-shaped viruses with a helical arrangement of capsomeres.
Polyhedral: Many-sided viruses, often icosahedral in shape.
Enveloped: Viruses surrounded by a lipid envelope.
Complex: Viruses with complicated structures, such as bacteriophages.

Step-by-Step Guidance

Define each morphological type: helical, polyhedral, enveloped, and complex.
Think of examples for each type (e.g., tobacco mosaic virus for helical).
Consider how the structure relates to the virus's function or mode of infection.
Review diagrams or figures in your textbook to visualize each morphology.

Try solving on your own before revealing the answer!

Final Answer:

Helical viruses: Rod-shaped, with capsomeres arranged in a spiral around the nucleic acid (e.g., tobacco mosaic virus).
Polyhedral viruses: Many-sided, often icosahedral (20 faces), such as adenoviruses.
Enveloped viruses: Surrounded by a lipid envelope, which may have spikes (e.g., influenza virus).
Complex viruses: Have additional structures, such as bacteriophages with a head, tail, and tail fibers.

Q5. Describe how viruses are cultured in the lab (Bacteriophages and Animal Viruses).

Background

Topic: Culturing Viruses

This question tests your understanding of laboratory techniques used to grow and study viruses, which cannot be cultured in the same way as bacteria.

Key Terms:

Bacteriophage: A virus that infects bacteria.
Animal Virus: A virus that infects animal cells.
Plaque: A clear area on a bacterial lawn where viruses have lysed the host cells.

Step-by-Step Guidance

Recall how bacteriophages are cultured using bacterial lawns and agar plates.
Describe the appearance and significance of plaques in bacteriophage culture.
Explain the methods used to culture animal viruses (e.g., in living animals, embryonated eggs, or cell cultures).
Consider why viruses require living cells for growth.

Try solving on your own before revealing the answer!

Final Answer:

Bacteriophages are cultured by mixing them with susceptible bacteria and plating on agar; clear plaques indicate areas where phages have lysed bacteria. Animal viruses are cultured in living animals, embryonated eggs, or cell cultures, since they require living host cells to replicate.

Q6. Multiplication of Bacteriophages: Describe the lytic and lysogenic cycles, their differences, and phage conversion.

Background

Topic: Bacteriophage Life Cycles

This question covers the two main types of bacteriophage replication cycles and the concept of phage conversion.

Key Terms:

Lytic Cycle: The process by which a phage replicates and lyses the host cell.
Lysogenic Cycle: The process by which a phage genome integrates into the host genome and replicates along with it.
Phage Conversion: A change in the phenotype of a host bacterium due to the presence of a prophage.

Step-by-Step Guidance

List the main steps of the lytic cycle (attachment, penetration, biosynthesis, maturation, release).
Describe the lysogenic cycle, focusing on integration of the phage DNA into the host genome (prophage formation).
Compare the outcomes of the lytic and lysogenic cycles for the host cell.
Define phage conversion and consider its significance in bacterial pathogenicity.

Try solving on your own before revealing the answer!

Final Answer:

Lytic cycle: The phage attaches to the host, injects its DNA, takes over the host's machinery to make new phages, and lyses the cell to release progeny.
Lysogenic cycle: The phage DNA integrates into the host genome as a prophage and replicates with the host cell; it can later enter the lytic cycle.
Difference: The lytic cycle results in host cell death, while the lysogenic cycle allows the host to survive and replicate with the prophage.
Phage conversion: Occurs when a prophage imparts new properties to the host bacterium, such as toxin production.

Q7. Multiplication of Animal Viruses: Describe Attachment, Entry, Uncoating, Synthesis, Maturation, and Release.

Background

Topic: Animal Virus Replication

This question asks you to outline the main steps in the replication of animal viruses, noting that the details of nucleic acid synthesis can vary.

Key Terms:

Attachment: Binding of the virus to host cell receptors.
Entry: The process by which the virus or its genome enters the host cell.
Uncoating: Removal of the viral capsid to release the genome.
Synthesis: Production of viral nucleic acids and proteins.
Maturation: Assembly of viral components into new virions.
Release: Exit of new virions from the host cell.

Step-by-Step Guidance

Describe how animal viruses attach to specific receptors on the host cell surface.
Explain the mechanisms of entry (e.g., endocytosis or fusion with the cell membrane).
Discuss the process of uncoating and why it is necessary.
Note that synthesis of nucleic acids and proteins depends on the type of viral genome.
Outline the maturation and release steps, considering how enveloped and non-enveloped viruses differ in release.

Try solving on your own before revealing the answer!

Final Answer:

Attachment: Virus binds to specific receptors on the host cell.
Entry: Virus enters the cell via endocytosis or membrane fusion.
Uncoating: Viral capsid is removed, releasing the genome.
Synthesis: Viral nucleic acids and proteins are produced using host machinery (details depend on genome type).
Maturation: Viral components are assembled into new virions.
Release: New virions exit the cell by budding (enveloped viruses) or lysis (non-enveloped viruses).

Q8. What are oncogenic viruses?

Background

Topic: Oncogenic Viruses

This question tests your understanding of viruses that can cause cancer in their hosts.

Key Terms:

Oncogenic Virus: A virus capable of inducing tumors (cancer) in animals or humans.
Oncogene: A gene that has the potential to cause cancer.

Step-by-Step Guidance

Define what is meant by an oncogenic virus.
Consider how these viruses can lead to cancer (e.g., by integrating into host DNA and disrupting normal cell regulation).
Think of examples of oncogenic viruses (e.g., HPV, EBV).

Try solving on your own before revealing the answer!

Final Answer:

Oncogenic viruses are viruses that can cause cancer by transforming normal cells into cancerous ones, often by integrating their genetic material into the host genome and disrupting normal cell cycle regulation. Examples include human papillomavirus (HPV) and Epstein-Barr virus (EBV).

Q9. What are prions? Give examples of some diseases caused by prions. Explain briefly how prions cause diseases.

Background

Topic: Prions and Prion Diseases

This question focuses on prions, which are infectious proteins, and the diseases they cause.

Key Terms:

Prion: An infectious protein that lacks nucleic acids.
Transmissible Spongiform Encephalopathies (TSEs): A group of fatal neurodegenerative diseases caused by prions.

Step-by-Step Guidance

Define what a prion is and how it differs from viruses and bacteria.
List examples of diseases caused by prions (e.g., Creutzfeldt-Jakob disease, mad cow disease).
Explain the mechanism by which prions cause disease (e.g., by inducing misfolding of normal proteins).

Try solving on your own before revealing the answer!

Final Answer:

Prions are infectious proteins that lack nucleic acids. They cause diseases such as Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy (mad cow disease) in cattle. Prions cause disease by inducing normal proteins in the brain to misfold, leading to brain damage and characteristic spongiform changes.