Biology Chapter 1.5: Cell Biology Chapter - Viruses

Cell Theory and Viruses

• Viruses are acellular and do not meet the criteria for living organisms as per cell theory.
• Virus sizes range from 20 nm to 300 nm, while prokaryotes are 1–10 μm and eukaryotes are significantly larger.

Viral Structure

• Viruses consist of genetic material, a protein coat (capsid), and occasionally an envelope made of lipids.
• Genetic material can be circular or linear, and either single- or double-stranded DNA or RNA.
• Enveloped viruses are sensitive to heat and detergents, while non-enveloped viruses are more resistant and persist longer on surfaces.
• Viruses require a host cell for reproduction, acting as obligate intracellular parasites.

Bacteriophages

• Bacteriophages specifically infect bacteria by injecting their genetic material without entering the bacterium.
• They feature a tail sheath and tail fibers that facilitate binding and penetration of the bacterial cell.

Viral Genomes

• Viral genomes vary in size and can range from a few genes to several hundred.
• Positive-sense RNA viruses can be directly translated by host ribosomes, while negative-sense RNA viruses need a complementary RNA strand for translation.
• Retroviruses, such as HIV, carry reverse transcriptase to convert their RNA into DNA, integrating into the host genome for replication.

Gene Therapy

• Retroviruses and transduction techniques are researched for potential gene therapy applications to treat diseases caused by genetic anomalies.

Viral Life Cycle

• Viruses must bind to specific cell receptors to initiate infection.
• Entry methods include membrane fusion or endocytosis for enveloped viruses, while bacteriophages inject their genome through tail sheaths.

Translation and Progeny Assembly

• After entry, viral genetic material is translated into proteins using the host’s cellular machinery.
• DNA viruses typically enter the nucleus for transcription, while RNA viruses translate in the cytoplasm.
• Progeny virions are assembled from replicated genomes and structural proteins, with a single virus producing thousands of new virions.

Progeny Release Mechanisms

• Viral progeny can exit the host cell by:
  • Cell death leading to viral spillover.
  • Host cell lysis due to overwhelming virion numbers.
  • Extrusion through plasma membrane fusion, allowing the host cell to survive.

Lytic and Lysogenic Cycles

• Bacteriophages can switch between lytic (virulent) and lysogenic cycles, with lysogeny involving integration into the host genome.
• Integrated viruses replicate alongside the bacteria during cell division, but can revert to lytic cycles under certain conditions, facilitating gene transfer (transduction) between bacteria.

Prions

• Prions are infectious proteins causing diseases through misfolding of normal proteins, resulting in cellular dysfunction.
• Associated diseases include bovine spongiform encephalopathy, Creutzfeldt–Jakob disease, and familial fatal insomnia.

Viroids

• Viroids consist of small circular single-stranded RNA and typically infect plants by silencing genes, disrupting protein synthesis.
• Some human-associated viroids, like hepatitis D virus (HDV), require co-infection with hepatitis B virus (HBV) to exert pathogenic effects.