AP Bio: Viruses

Introduction to Viruses

• Overview of viruses and their structure.

• Consist of a protein outer shell (capsid) and nucleic acid (DNA or RNA).

• Types of nucleic acids: double-stranded DNA, single-stranded DNA, single-stranded RNA, double-stranded RNA.

Discovery of Viruses

• Historical context self-contained discussions since the 1600s.

• The first virus identified: Tobacco Mosaic Virus (TMV).

  • Significant disease affecting tobacco crops in the 1800s.

  • Early scientists struggled to identify causative agents due to size limitations.

  • Wendell Stanley crystallized TMV in 1935, leading to the first identification of a virus.

Characteristics of Viruses

• Viruses are acellular and much smaller than cells.

• Despite being non-living, they exhibit properties similar to living organisms regarding disease transmission.

• Infection pattern similar to bacteria: must enter a host to cause disease.

• Notable terms:

  • Acellular: lacking cellular structure.

  • Host range: the variety of hosts a virus can infect.

Viral Structure and Function

• Basic structure includes nucleic acid and a protective capsid.

• Some viruses possess an additional envelope derived from host membranes for camouflage.

• Docking sites on capsid important for infection:

  • Determines the virus's host range.

  • Examples: influenza can jump between species (humans, birds, pigs).

Viral Infection and Replication

Lytic Cycle

• Process where a virus hijacks the host's cellular machinery:

  1. Attachment to host cell

  2. Injection of viral DNA

  3. Viral DNA integrates with host DNA, disrupting host gene function.

  4. Virus replicates and assembles new viruses.

  5. Lysis of host cell, releasing new viral particles (e.g., flu virus).

• Virulent viruses are characteristic of the lytic cycle, leading to cell death.

Lysogenic Cycle

• Alternative replication method for some viruses (e.g., herpes):

  1. Following injection, viral DNA integrates into host DNA and remains dormant (prophage or provirus).

  2. Virus can later reactivate and enter the lytic cycle, causing outbreaks.

  3. This allows for persistence in hosts over time, contributing to recurring infections like herpes.

Immune Response and Vaccination

• Immunity can be complex due to mutation potential in viruses:

  • Frequent mutations render vaccines less effective against new strains.

  • Example: the flu virus has many strains; vaccines target the most prevalent strains.

• Epidemics vs. Pandemics:

  • Epidemic: localized outbreak.

  • Pandemic: global outbreak, concerns with high transmissibility (e.g., swine flu, Spanish flu).

Retroviruses

• Example: HIV, which incorporates RNA and requires reverse transcription:

  • Converts RNA to DNA post-infection, contributing to high mutation rates.

  • Challenges in vaccine development due to strain variability.

Prions and Viroids

Prions

• Infectious proteins that cause neurodegeneration (e.g., mad cow disease, Creutzfeldt-Jakob disease).

• Spreads through ingestion of contaminated tissue.

• Catalyze the conversion of normal proteins into pathogenic forms.

Viroids

• Simple infectious agents consisting of circular RNA.

• Primarily affect plant growth, significant in agriculture.

Conclusion

• Viruses represent a diverse group of infectious agents with unique replication strategies and structures.

• Awareness of their impact on health and agriculture essential for managing infectious diseases.