notes

Objectives of the Lecture

• Understanding Viruses and Their Impact

  • Describe the characteristics and structures of viruses.

  • Describe types of nucleic acid in virions.

  • Identify how viruses are classified.

  • Discuss the specificity of viruses to hosts.

  • Define animal reservoirs.

  • Describe the stages of viral replication.

  • Understand how viruses cause disease (Pathogenesis).

  • Begin to list the nature of viral diseases.

Introduction to Viruses

• Significance and Impact

  • Viruses are responsible for many significant diseases, including recent viral diseases such as COVID-19, AIDS, and SARS.

  • New viral diseases have emerged particularly in the last 60 years.

• Etymology

  • The term "virus" derives from Latin, meaning "venom" or "poison."

  • Originally known as "filterable viruses," as they could remain infectious after passing through filters that retained bacteria.

• Host Range

  • Viruses can infect animals, plants, and bacteria, causing various diseases, with examples including Severe Acute Respiratory Syndrome (SARS) and COVID-19.

Characteristics of Viruses

• Size and Structure

  • Viruses are the smallest infectious agents, ranging from 10 to 100 times smaller than bacteria and approximately 500 times smaller than red blood cells.

  • Invisible under light microscopy; can only be seen with an electron microscope.

  • Lack most enzymes necessary for metabolic processes and can only replicate inside living cells, making them obligate intracellular parasites.

  • Viral replication often results in cell lysis or gradual cell death.

Structure of Viruses

• General Structure

  • Composed of genetic material (either DNA or RNA) and a protein coat known as the capsid.

  • Variants include the presence or absence of an envelope—a lipid bilayer derived from the host cell membrane.

• Classification Based on Structure

  • Naked viruses lack an envelope and are more resistant to environmental conditions, while enveloped viruses have glycoprotein spikes for attachment and entry into host cells.

Viral Nucleic Acid

• Types of Nucleic Acids

  • Viruses can contain single-stranded RNA (ssRNA), double-stranded RNA (dsRNA), single-stranded DNA (ssDNA), or double-stranded DNA (dsDNA).

  • The viral genome does not possess the full complement of genes for a complete virus construction.

• Functions

  • Codes mostly for structural proteins and enzymes necessary for the assembly of new viruses.

  • Directs the host cell's machinery to produce new viral components.

Capsid and Envelopes

• Viral Capsid

  • The protein coat (capsid) protects the nucleic acid and may possess antigenic properties that help viruses attach to host cells.

  • Capsid structures can influence viral classification as they exhibit symmetrical arrangements.

• Envelope Functions

  • The envelope assists in viral attachment to host cells and contains spikes made of glycoproteins that determine antigenicity.

  • Naked viruses are more robust against environmental stress compared to enveloped viruses that are susceptible to disinfectants and extreme conditions.

Key Enzymes in Viruses

• Viral Enzymes

  • Some viruses encode enzymes for nucleic acid synthesis, such as reverse transcriptase, crucial in the replication of retroviruses.

  • Neuraminidase, found in influenza spikes, plays a role in the release of new viral particles.

  • These enzymes are not generally found in host cells and serve as potential targets for antiviral therapies.

Virus Classification

• Classification Criteria

  • Not based on the diseases they cause but rather the type of nucleic acid they carry (dsDNA vs. ssRNA), their shape, and method of replication.

• Host Range and Specificity

  • Viruses exhibit high specificity for their host and cell types.

  • Plant viruses infect plants, bacterial viruses infect bacteria, and animal viruses infect animals, often being specific to particular cells or species.

Animal Reservoirs and Pathogen Transmission

• Host-Specific Viruses

  • Most viruses are host-specific, though some (e.g., rabies, hantavirus) can cross species barriers.

• Animal Reservoir Examples

  • Rabies virus: Transmitted through animal bites.

  • Hantavirus: Influenza-like disease linked to deer mice.

  • Hendra virus: Associated with fruit bats, horses, and humans.

Stages of Viral Replication

• Step 1: Adsorption

  • Attachment of the virus to a host cell’s specific receptor site.

• Step 2: Penetration

  • Virus enters host cell through direct penetration, membrane fusion, or phagocytosis.

• Step 3: Uncoating

  • The virus is dismantled, releasing nucleic acid into the cytoplasm.

• Step 4: Synthesis

  • Viral nucleic acid is replicated, directing synthesis of viral proteins.

• Step 5: Assembly

  • Formation of new viral capsids and nucleic acids within the cell.

• Step 6: Release

  • New viruses are released either by cell lysis or budding off from the host cell.