VIRUSES

Ch. 10: Viruses, Viroids, and Prions

Introduction to Viruses

  • Virus Characteristics:

    • Viruses are acellular entities, meaning they are not composed of cells.

    • They require a living cell to reproduce.

    • Viruses must utilize the host cell's replication machinery, such as ribosomes and enzymes.

    • Conceptually described as a "Box of Directions."

Components of Viruses

  • Two Main Components:

    • Capsid:

    • The outer portion comprised of proteins.

    • Often described as the "BOX" of the virus.

    • It encloses the nucleic acid core and may contain additional viral enzymes.

    • Can be surrounded by a lipid envelope.

    • May have protein spikes that facilitate attachment to a host cell.

    • Viruses exhibit a variety of shapes including:

      • Helix

      • Sphere

      • Polyhedron

      • More complex forms.

    • Core:

    • Consists of nucleic acid, which can be either DNA or RNA. This is the "DIRECTIONS" part of the virus.

Virus Specificity and Types

  • Host Cell Specificity:

    • Viruses exhibit specificity towards their host cells.

    • Bacteriophages:

    • These viruses infect bacteria and reproduce within bacterial cells.

    • They reproduce through two cycles:

      • Lytic Cycle:

      • Virus is replicated, transcribed, and translated, leading to the cell bursting to release new viruses.

      • Lysogenic Cycle:

      • Phage DNA integrates into the host chromosome, referred to as a prophage.

      • The prophage is passed on to daughter cells and may later switch to the lytic cycle.

    • Animal Cell Viruses:

    • Include viruses such as influenza, chicken pox, SARS-CoV-2, and HIV.

    • Plant Cell Viruses:

    • An example includes the tobacco mosaic virus.

Bacteriophage Life Cycle Illustrations

  • Lytic Cycle Phases:

    1. Phage attaches to the bacterial cell.

    2. Phage injects its DNA.

    3. The phage DNA circularizes.

    4. New phages are assembled.

    5. New phage DNA and proteins are synthesized.

    6. The cell lyses, releasing phages that may infect other cells.

  • Lysogenic Cycle Phases:

    • Upon environmental stress or multiple cell divisions, the prophage within the bacterial chromosome replicates normally, copying it at each division.

Health Implications of Viruses

  • Many viruses are known to cause diseases in animals and plants. Examples include:

    • Chicken Pox:

    • A DNA virus that belongs to the herpes family.

    • Flu Viruses:

    • Typically utilize RNA as their genetic material.

    • HIV:

    • Classified as a retrovirus because it transcribes RNA to DNA.

  • Viral Replication Methods:

    • Some animal viruses acquire a bit of the host cell membrane during replication by a process called budding rather than lysis.

Emerging Viruses and Public Health

  • Emerging Viruses:

    • Defined as viruses that appear suddenly, catching the medical community off guard. Examples include:

    • SARS-CoV-2

    • HIV (Human Immunodeficiency Virus), which leads to AIDS (Acquired Immunodeficiency Syndrome).

  • Checkpoint Question:

    • Why does the current year's flu shot not provide immunity for next year's flu?

Historical Context of Emerging Viruses

  • Notable emerging viruses by year:

    • 1981: AIDS

    • 1918: H1N1 flu

    • 1996: West Nile virus

    • 2002: Severe acute respiratory syndrome

    • 2009: H1N1 flu

    • 2015: Zika fever

    • 1997: Avian flu

    • 1976: Ebola

Specific Virus Examples

  • SARS-CoV-2:

    • Structure includes:

    • Nucleocapsid

    • Envelope

    • RNA virus with genomic +ssRNA

    • Contains spike glycoprotein (S), membrane protein (M), and small envelope protein (E).

Chicken Pox Specifics

  • Varicella-zoster Virus:

    • Characterized by:

    • Glycoprotein spikes

    • Polyhedral capsid

    • Lipid envelope

    • Double-stranded DNA genome.

HIV and Reverse Transcription

  • HIV:

    • A retrovirus that utilizes RNA as a template to synthesize DNA.

    • The synthesized DNA integrates into the host chromosome.

    • Contains an enzyme, reverse transcriptase, responsible for reverse transcription—the process of synthesizing DNA from an RNA template.

  • Checkpoint Questions for HIV:

    1. Why is reverse transcriptase a good target for anti-AIDS drug therapy?

    2. What challenges exist in developing a reliable vaccine for HIV?

HIV Structural Characteristics

  • HIV Structure includes:

    • Envelope with glycoprotein spikes

    • RNA organized into two identical strands

    • Reverse transcriptase enzyme.

Summary of Virus Replication Processes

  • Animal Cell Protein Synthesis:

    • Transcription and translation processes converting DNA to mRNA and then to proteins.

  • Influenza & SARS Protein Synthesis:

    • RNA virus synthesis processes resulting in viral proteins from RNA templates.

  • HIV Protein Synthesis:

    • Involves reverse transcription from RNA to DNA, leading to the generation of mRNA and proteins.

Conclusion

  • Understanding viruses, their structure, replication, and the associated health implications is crucial for developing medical responses and public health strategies.