Chapter 19: FB Note Taking and Study Guide

Viral Structure

Viruses are microscopic infectious agents that require a host cell to reproduce. Their structure is unique and varies among different types, but all viruses share certain key components.

• Size: Viruses are generally much smaller than bacteria, typically ranging from 20 to 300 nanometers in diameter. Their small size allows them to infect host cells efficiently.

• Components: The basic structure of a virus includes:

  • Nucleic Acid Genome: Can be either DNA or RNA, single-stranded or double-stranded.

  • Capsid: A protein shell that encases the viral genome. The arrangement of capsomeres (protein subunits) within the capsid varies among viruses.

  • Envelope: Some viruses possess an outer lipid envelope derived from the host cell membrane, which may contain viral glycoproteins. The presence or absence of an envelope is a key classification feature.

• Classification: Viruses are classified based on their genome type (DNA or RNA), capsid symmetry, presence of an envelope, and host range.

• Example: Influenza virus is an enveloped RNA virus, while adenovirus is a non-enveloped DNA virus.

Additional info: The Baltimore classification system is commonly used to categorize viruses based on their replication strategy and genome type.

Viral and Prion Replication

Viruses and prions replicate by exploiting host cell machinery. The replication strategies differ among bacteriophages, animal viruses, and prions.

Phage Replication

• Lytic Cycle: The virus injects its genome into the host bacterium, hijacks the cell's machinery to produce new viral particles, and ultimately lyses (breaks open) the cell to release progeny.

• Lysogenic Cycle: The viral genome integrates into the host DNA and replicates along with it, remaining dormant until triggered to enter the lytic cycle.

• Comparison: The lytic cycle results in immediate destruction of the host cell, while the lysogenic cycle allows the virus to persist without killing the host.

• Example: Lambda phage can undergo both lytic and lysogenic cycles in Escherichia coli.

Bacterial Defense

• Restriction Enzymes: Proteins that recognize and cut foreign DNA, such as viral genomes, providing a defense mechanism for bacteria.

• CRISPR/Cas System: A bacterial adaptive immune system that captures fragments of viral DNA and uses them to recognize and destroy future infections. CRISPR/Cas has become a powerful tool in biotechnology and medicine.

• Example: CRISPR/Cas9 is widely used for genome editing in research and therapy.

Animal Virus Replication

• Entry: Animal viruses enter host cells via endocytosis or membrane fusion.

• Genome Replication: The replication strategy depends on the type of viral genome (DNA, RNA, retrovirus). For example, retroviruses like HIV use reverse transcriptase to convert RNA into DNA.

• Assembly and Release: New viral particles are assembled and released by budding (enveloped viruses) or cell lysis (non-enveloped viruses).

• Example: HIV is a retrovirus that integrates its genome into host DNA and uses reverse transcriptase for replication.

Additional info: Retroviruses differ from other RNA viruses by their use of reverse transcriptase and integration into the host genome.

Prions

• Definition: Prions are infectious proteins that cause neurodegenerative diseases by inducing abnormal folding of normal cellular proteins.

• Characteristics: Prions lack nucleic acids and replicate by converting normal proteins into the prion form.

• Example: Creutzfeldt-Jakob disease in humans and mad cow disease in cattle are caused by prions.

Table: Comparison of Virus and Prion Replication

Key Equations

• Viral Replication Rate:

• Reverse Transcriptase Reaction (Retroviruses):