Study Guide: Viruses (Microbiology Week 6, Chapter 13)

Viruses

Introduction to Viruses

Viruses are acellular infectious agents that require host cells for replication. They exhibit unique mechanisms for infecting specific hosts and have diverse structural and functional features.

• Definition: Viruses are microscopic entities composed of genetic material (DNA or RNA) enclosed in a protein coat, sometimes with a lipid envelope.

• Host Specificity: Viruses are specific to their host cells due to interactions between viral surface proteins and host cell receptors.

• Example: HIV infects human T-helper cells by binding to the CD4 receptor.

Structure and Function of Viral Components

Viruses possess distinct structural components that determine their infectivity and replication strategies.

• Capsid: The protein shell that encases the viral genome, providing protection and aiding in host cell attachment.

• Envelope: A lipid membrane derived from the host cell, present in some viruses, containing viral glycoproteins essential for entry into host cells.

• Viral Envelope Function: Facilitates fusion with host cell membranes and evasion of immune responses.

• Classification: Viruses are classified based on their nucleic acid type (DNA or RNA), capsid symmetry, presence or absence of an envelope, and replication strategy.

• Example: Influenza virus is an enveloped RNA virus.

Bacteriophage Replication Cycles

Bacteriophages, viruses that infect bacteria, exhibit two main replication cycles: lytic and lysogenic.

• Lytic Cycle: The phage injects its DNA, replicates, assembles new virions, and lyses the host cell to release progeny.

• Lysogenic Cycle: The phage DNA integrates into the host genome (prophage), replicates with the host, and can later enter the lytic cycle.

• Five Stages of Lytic Replication:

  1. Attachment

  2. Penetration

  3. Biosynthesis

  4. Maturation

  5. Release

• Comparison Table:

Feature	Lytic Cycle	Lysogenic Cycle
Phage DNA fate	Replicates independently	Integrates into host genome
Host cell outcome	Lysis and death	Cell survives, divides normally
Progeny production	Immediate	Delayed (upon induction)

Viral Replication in Animal Cells

Animal viruses replicate through various mechanisms, often involving fusion or endocytosis.

• Entry: Animal viruses enter cells via receptor-mediated endocytosis or membrane fusion.

• Release: New virions exit cells by lysis or budding, acquiring an envelope in the latter process.

• Latent and Chronic Infections: Some animal viruses establish latency (e.g., Herpes simplex virus), while others cause chronic infections (e.g., Hepatitis B virus).

• Oncogenic Viruses: Certain viruses can induce cell transformation, leading to cancer (e.g., Human papillomavirus).

Retroviruses

Retroviruses are RNA viruses that reverse transcribe their genome into DNA, integrating into the host genome.

• Reverse Transcriptase: Enzyme that synthesizes DNA from RNA template.

• Example: HIV is a retrovirus causing AIDS.

• Replication Equation:

$\text{RNA} \xrightarrow{\text{reverse transcriptase}} \text{DNA} \xrightarrow{\text{integration}} \text{Host genome}$

Prions

Prions are infectious proteins that cause neurodegenerative diseases by inducing abnormal folding of normal proteins.

• Definition: Prions are misfolded proteins lacking nucleic acids, responsible for transmissible spongiform encephalopathies.

• Diseases Caused by Prions:

  • Creutzfeldt-Jakob disease

  • Kuru

  • Bovine spongiform encephalopathy ("mad cow disease")

  • Scrapie (in sheep)

• Comparison with Viruses: Prions lack genetic material and do not elicit immune responses, unlike viruses.

Challenges in Culturing Viruses

Culturing viruses requires living cells, presenting unique difficulties compared to bacteria.

• Obligate Intracellular Nature: Viruses cannot replicate outside host cells.

• Methods: Use of cell cultures, embryonated eggs, or live animals.

• Ethical and Practical Issues: Maintaining sterile conditions, cost, and host specificity.

Summary Table: Viruses vs. Prions

Feature	Viruses	Prions
Genetic Material	DNA or RNA	None
Structure	Protein coat, sometimes envelope	Protein only
Replication	Requires host cell machinery	Induces misfolding of host proteins
Diseases	Wide variety (e.g., influenza, HIV)	Neurodegenerative (e.g., CJD, BSE)

Key Terms

• Bacteriophage: Virus that infects bacteria.

• Capsid: Protein shell of a virus.

• Envelope: Lipid membrane surrounding some viruses.

• Prophage: Phage DNA integrated into bacterial genome.

• Prion: Infectious protein causing neurodegenerative disease.

Additional info: Academic context and definitions have been expanded for clarity and completeness.