Viruses: Microbio lecture recording on 18 February 2025

Definition: Viruses that exist naturally within hosts without artificial introduction.
- Examples: Coronavirus and papillomaviruses.
Identification: Determined by the attachment proteins being part of the capsid.

Capsid Proteins:
- Bp1: Virion Protein One, designation indicating its production during the viral life cycle.
Membrane Proteins:
- May involve multiple viral proteins and cellular receptors—some viruses require more than one receptor to infect a host.
Example: HIV typically requires two proteins to bind to host cells.
- CD4: First protein involved in the binding to lung cells.
- CCR5: A chemokine receptor that facilitates entry into cells.

Some viruses engage in a 'double handshake' for entry, requiring two distinct receptors.
Immunity Challenges: Viruses exploit immune cell receptors, evading the immune response.
Adenoviruses: Can enter host cells and replicate after gaining access to immune cells.

Penetration:
- Involves the transport of viruses across cellular membranes, often through endocytosis.
- Endocytosis: Process by which cells internalize substances. Can be general (phagocytosis) or receptor-mediated.
- Clathrin-Coated Pits: Specialized regions facilitating receptor-mediated endocytosis.
Upon internalization, viruses may still require uncoating to release their genome.

Naked Viruses: Utilize strategies to break through endosomal membranes.
Example: Adenoviruses bind to the CAR receptor, leading to internalization and eventual degradation of the endosome to release their genome.
If entry fails, viruses are often directed towards lysosomes, leading to potential loss of infectivity.

DNA Viruses: Majority need to enter the nucleus to replicate.
- Exceptions include poxviruses, capable of replicating in the cytoplasm.
RNA Viruses: Generally replicate within the cytoplasm, as they do not depend on the nuclear machinery.

Positive-Strand RNA Viruses: Mimic host messenger RNA and utilize the host's ribosomes to synthesize proteins.
Negative-Strand RNA Viruses: Require the presence of an RNA-dependent RNA polymerase for replication.
Retroviruses: Notable for transcribing their RNA into DNA, integrating into host cell genomes as proviruses.

Retroviruses: Use reverse transcriptase to convert RNA to DNA, integrating it into the host genome, which can potentially lead to latent infections.
Importance of Proviruses: Integrated viral DNA that can remain dormant, potentially reactivating under favorable conditions.

Latent viruses can remain dormant without causing immediate symptoms.
Some viruses can induce persistent infections, which may lead to cancer—linked with specific types of DNA and RNA virus infections.
Hepatitis B and C Viruses: Associated with liver cancer, highlighting the importance of vaccination.

Cancer association: A significant portion of human cancers are linked to viral infections
- Papillomaviruses: Cause abnormal cell growth via viral integration into host DNA, leading to proliferative signals.
Comparison of Proto-oncogenes and Tumor Suppressors: Viruses may either disrupt or activate these cellular genes, impacting cancer progression.

Vaccination can prevent viral infections that lead to cancer, particularly in predisposed populations (e.g., HPV vaccine).

Challenges in studying viruses: Need live host cells or animals for replication.
Techniques include cell culture methods, in vitro studies, and biosafety measures to handle viral pathogens safely.
Importance of understanding cytopathic effects: Viral infections can visibly alter cell morphology and function, indicating presence and type of infection.