viruses

Viruses

Nature of Viruses

  • Basic Structure:

    • All viruses consist of a nucleic acid core encased in a protein coat.

    • Viruses do not have cytoplasm and are not classified as cells.

  • Nucleic Acid:

    • Can be either DNA or RNA.

    • Forms can be circular or linear, single-stranded (SS) or double-stranded (DS).

Virus Structure

  • Protein Sheath (Capsid):

    • Most viruses form a protective capsid around their nucleic acid core.

    • This capsid is made up of multiple copies of 1 to a few types of proteins.

  • Specialized Enzymes:

    • Some viruses carry specific enzymes within their nucleic acid core.

    • Example: Retroviruses use reverse transcriptase.

  • Viral Envelope:

    • Many animal viruses acquire an envelope derived from the host cell membrane containing viral proteins.

Viral Shapes

  • Capsid Shapes:

    • Common shapes include:

    • Icosahedral: Spherical shape, e.g., adenovirus.

    • Helical: Rodlike or threadlike, e.g., Tobacco Mosaic Virus (TMV).

  • Complex Viruses:

    • Some, like T-even bacteriophages, have a binal symmetry, integrating both helical and icosahedral shapes.

Viral Hosts

  • Obligate Intracellular Parasites:

    • Viruses require host cells for survival and reproduction.

    • Their life cycle inherently involves residing inside a host cell.

  • Host Range:

    • Viruses have a limited range of organisms they can infect, defined by specific tissues they target (tissue tropism).

    • Example: Rhabdovirus (rabies) targets neurons.

Viral Genomes

  • Types of Nucleic Acids:

    • Viral genomes can be either DNA or RNA.

    • They can exist as double-stranded (DS) or single-stranded (SS), with configurations being circular or linear.

DNA Viruses

  • Most are double-stranded.

  • Replicate within the nucleus of a eukaryotic host cell.

  • Notable examples include smallpox and herpes simplex viruses.

RNA Viruses

  • Predominantly single-stranded.

  • Typical examples include influenza, measles, and the common cold.

  • Replicate in the host cell's cytoplasm, with a high mutation rate due to the error-prone nature of RNA replication, making treatment challenging.

Retroviruses

  • Structure includes a single-stranded RNA genome that undergoes reverse transcription to become double-stranded DNA.

  • Reverse transcriptase is employed to copy viral RNA into DNA.

  • Once integrated, this viral DNA can be maintained forever within the host's DNA; HIV is a well-known retrovirus.

Viral Replication

  • Unlike other organisms, viruses lack ribosomes and enzymes needed for protein/nucleic acid synthesis.

  • They hijack the host cell's cellular machinery to express:

    • Early genes

    • Intermediate genes

    • Late genes

  • Final outcomes result in the assembly and release of new viruses.

General Viral Life Cycle

  • The virus acts as a set of genetic instructions, tricking the host cell to produce new viruses.

  • Viruses only reproduce inside cells; they are metabolically inactive (virions) outside of cells.

Are Viruses Alive?

  • Discussion points around their classification as living or non-living beings.

  • Considerations on their evolutionary position.

Virus Classification

  • Taxonomy:

    • The International Committee on Taxonomy of Viruses (ICTV) categorizes viruses based on:

    • Order

    • Family

    • Subfamily

    • Genus

  • Classification Factors:

    • The diseases they cause and the hosts they infect.

    • Differences in genome expression.

    • The Baltimore classification method details viral classification by nucleic acid type and replication methods.

Classification by Disease or Host

  • This method has limitations:

    • Not all viruses are pathogenic.

    • Some viruses exhibit different disease expressions under varying conditions.

    • Example: The common cold can stem from several different viruses.

Bacteriophage

  • Definition: Viruses that specifically infect bacteria, commonly referred to as phage.

  • They are incredibly diverse and primarily categorized by their specific bacterial hosts.

  • Most extensively studied phages include E. coli-infecting viruses (T1, T2, etc.).

  • They exhibit two reproductive life cycles:

    • Lytic Cycle

    • Lysogenic Cycle

Lytic Vs. Lysogenic Life Cycle

  • Lytic Cycle:

    • Phage attaches to the bacterial outer surface, injects viral genome, takes over cell machinery to produce viral components, assembles new virions, and finally lyses the host cell to release mature virus particles.

  • Lysogenic Cycle:

    • Virus remains dormant (latent) without immediately destroying the host.

    • Viral genome integrates into the bacterial chromosome as a prophage, replicating along with host cell DNA.

    • Can be triggered to enter the lytic cycle under certain conditions.

Induction and Phage Conversion

  • During the lysogenic cycle, expressed viral genes may alter the host's characteristics (phenotype).

  • Example: Certain lysogenic phages can introduce pathogenic traits, such as cholera toxin coding genes into Vibrio cholerae, transforming non-pathogenic strains into pathogenic variants.

Viral Infections: Persistent vs. Acute

  • Persistent Infections:

    • Characterized by latent or chronic manifestations over time.

    • Examples include some infections that can remain dormant.

  • Acute Infections:

    • Involve rapid virus replication and sudden onset of symptoms, which can escalate to outbreaks, epidemics, or pandemics.

Review Questions

  1. Explore foundational concepts about viruses and how they differ from bacterial cells.

  2. Define zoonotic diseases and provide viral examples.

  3. Identify virus shapes based on diagrams.

  4. Contrast the lytic and lysogenic cycles of bacteriophages.

  5. Discuss details regarding HIV structure, reverse transcriptase function, and implications on health.