Micro Chapter 13 almost complete

Overview of mRNA and Viruses

• mRNA Functionality:

  • mRNA must adhere to cellular rules in its synthesis.

  • The synthesis process varies for different viruses depending on their classification as either DNA or RNA viruses.

  • Viruses lack mechanisms to generate ATP (energy) and depend entirely on the host cell for energy needs.

Differences Between Viruses and Bacteria

• Comparison with Bacteria:

  • Examples: Rickettsia and Chlamydia.

  • These bacteria are obligate intracellular parasites; they must replicate within a host cell and cannot divide independently.

  • They are considered minimal bacteria since they have lost the ability to produce ATP themselves, relying on the host to transport ATP.

  • Rickettsia and Chlamydia possess their own ribosomes.

  • Consider size filtering: bacteria generally don't pass through filters with a pore size of about 0.2 micrometers, whereas viruses do. Key size parameters:

  • Viruses: typically smaller than point two microns, pass through filters.

  • Bacteria: often larger than this size and therefore do not pass.

Viruses as Intracellular Parasites

• Intracellular Parasites:

  • Viruses are categorized as inert on the extracellular phase, lacking metabolic activity until inside host cells.

  • They utilize host cellular machinery—e.g., ribosomes and RNA polymerase—for their replication, leading to their insensitivity to antibiotics.

• Host Range:

  • Each virus has a specific host range; e.g., SARS-CoV-2 infects humans and various animals like tigers and mink due to shared ACE2 protein receptors.

  • Viruses can infect diverse cell types, resulting in varied symptoms (e.g., GI, respiratory, and neurological symptoms).

Virus Classification

• Bacteriophages:

  • Specific viruses targeting bacteria. They attach via receptor sites on bacterial cells (cell wall, fimbriae, flagella).

  • Distinct from animal viruses, which target eukaryotic plasma membranes.

Structure of Viruses

• Complete Infectious Particles (Virion):

  • Composed of nucleic acid (DNA or RNA)—either single-stranded or double-stranded, linear or circular.

  • Has a protein coat (capsid) and may possess an envelope derived from host cell membranes.

  • Spike proteins: embedded in the envelope and recognize host cell receptors.

Size and Morphologies

• Virus Size:

  • Ranges from approximately 20 nm to nearly 1 µm (e.g., Ebola virus).

  • Most viruses fall below 0.2 microns, making them invisible under a light microscope (require electron microscopy).

• Morphologies:

  • Icosahedral: common structure consisting of equilateral triangles (e.g., polio, adenovirus).

  • Helical: long, filamentous structure (e.g., rabies, Ebola).

  • Complex: features a combination of icosahedral and helical aspects (e.g., bacteriophages).

Viruses and Genetic Information

• Viral Genomes:

  • Herpesviruses: double-stranded DNA; does not vary.

  • SARS-CoV-2: single-stranded RNA; its specific genome characteristics lead to varied pathogenic effects.

  • Segmented genomes (e.g., influenza) allow for complex variations and adaptability.

  • Size ranges of viral genomes impact protein coding capabilities and reliance on host machinery.

Capsid and Envelope Formation

• Capsid Structure:

  • Protects nucleic acid; typically exhibits icosahedral symmetry.

  • Naked viruses have only capsids; enveloped ones acquire additional structures from the host cell during assembly (most typically from the plasma membrane).

Virus Lifecycle Considerations

• Entry into Cells:

  • Two primary methods for viruses to enter:

  • Receptor-mediated endocytosis (common for naked viruses).

  • Direct fusion with host cell membrane (common for enveloped viruses like HIV).

• Exit Mechanisms:

  • Enveloped viruses bud from the host cell, utilizing viral proteins in assembly and acquiring host membrane.

Baltimore Classification System

• Categories of Viruses:

  • Class I: Double-stranded DNA.

  • Class II: Single-stranded DNA.

  • Class III: Double-stranded RNA.

  • Class IV: Positive-sense single-stranded RNA (e.g., coronaviruses).

  • Class V: Negative-sense single-stranded RNA (e.g., influenza).

  • Class VI: Retroviruses (e.g., HIV) use reverse transcriptase to convert RNA to DNA.

  • Class VII: Pararetroviruses.

Viral Pathogenesis and Cancer

• Cancer Associations:

  • Some viruses are oncogenic, meaning they have the potential to cause cancer due to mutations from viral integration or host cell interactions (e.g., HPV, Epstein-Barr Virus).

  • Mechanisms of oncogenesis commonly involve viral gene interference with host regulatory mechanisms, potentially leading to uncontrolled cellular growth patterns.

• Risks and Vaccination:

  • Vaccines exist for some cancer-associated viruses (HPV, Hepatitis B) significantly reducing risk of cancer incidence.

  • AWDs related to viral infections include subacute sclerosing panencephalitis from measles and other potential long-term risks.