Viruses, Viroids, & Prions

Differences Between Viruses and Bacteria

Viruses and bacteria are both microorganisms, but they differ fundamentally in structure, replication, and biological properties.

• Viruses: Non-cellular entities; lack metabolic machinery; require a host cell for replication.

• Bacteria: Cellular organisms; possess metabolic pathways; can reproduce independently by binary fission.

• Example: Escherichia coli (bacterium) vs. Influenza virus.

Criteria Used to Classify Viruses

Viruses are classified based on several criteria, including:

• Type of nucleic acid: DNA or RNA, single or double stranded.

• Capsid symmetry: Icosahedral, helical, or complex.

• Presence of envelope: Enveloped or non-enveloped.

• Host range: Animals, plants, bacteria (bacteriophages).

• Replication strategy: Lytic or lysogenic cycles.

Scientific Nomenclature of Viruses

Virus taxonomy follows specific conventions:

• Genus name: Ends in "virus" (e.g., Enterovirus).

• Family name: Ends in "viridae" (e.g., Herpesviridae).

• Order name: Ends in "virales" (e.g., Mononegavirales).

Lytic and Lysogenic Cycles

Viruses can replicate via two main cycles in host cells:

• Lytic cycle: Virus injects genetic material, replicates, assembles new virions, and lyses the host cell.

• Lysogenic cycle: Viral genome integrates into host DNA (prophage), replicates with host, may later enter lytic cycle.

• Example: Bacteriophage lambda can undergo both cycles.

Complex Viruses

Complex viruses have structures beyond simple helical or icosahedral capsids.

• Definition: Viruses with additional structures such as tail fibers, base plates (e.g., bacteriophage T4).

• Example: Bacteriophages are classic complex viruses.

Prions: Definition and Replication

Prions are infectious proteins that cause neurodegenerative diseases.

• Definition: Misfolded protein capable of inducing misfolding in normal proteins.

• Replication: Prions propagate by converting normal cellular prion protein (PrPC) into the abnormal form (PrPSc).

• Example: Creutzfeldt-Jakob disease.

Viral Envelope: Properties and Origin

The viral envelope is a lipid bilayer surrounding some viruses.

• Properties: Contains viral glycoproteins; aids in host cell entry.

• Origin: Derived from host cell membrane during viral budding.

• Example: Influenza virus is enveloped.

Animal Cell Infection by Viruses

Viruses infect animal cells through a series of steps:

• Attachment: Virus binds to specific receptors on host cell.

• Entry: Via endocytosis or membrane fusion.

• Uncoating: Viral genome released into cytoplasm.

• Replication: Viral genome replicated and proteins synthesized.

• Assembly and Release: New virions assembled and released, often by budding.

Latent Viruses

Latent viruses can remain dormant within host cells.

• Properties: Viral genome persists without active replication; can reactivate.

• Example: Herpes simplex virus can cause recurrent infections.

Glycoprotein Spikes: Role

Glycoprotein spikes are protrusions on viral envelopes.

• Function: Mediate attachment to host cell receptors; determine host specificity.

• Example: HIV uses gp120 spikes to bind CD4 receptors.

Capsid Shapes

Viral capsids protect the genome and determine virus shape.

• Icosahedral: 20-sided structure (e.g., adenovirus).

• Helical: Rod-shaped (e.g., tobacco mosaic virus).

• Complex: Additional structures (e.g., bacteriophage).

Acute, Latent, and Chronic Viral Infections

Viral infections can be classified based on their duration and persistence.

Retroviruses and Reverse Transcriptase

Retroviruses are RNA viruses that use reverse transcriptase to replicate.

• Properties: Contain RNA genome; synthesize DNA from RNA using reverse transcriptase.

• Example: HIV is a retrovirus.

• Key Equation:

Bacteriophage Multiplication Steps

Bacteriophages infect bacteria through a series of steps:

• Attachment: Phage binds to bacterial surface.

• Penetration: Injection of viral DNA into host.

• Biosynthesis: Viral components synthesized.

• Assembly: New phage particles assembled.

• Release: Host cell lysed, phages released.

Sense and Antisense RNA Strands in RNA Viruses

RNA viruses may have sense (+) or antisense (-) strands.

• Sense strand (+): Can be directly translated into protein.

• Antisense strand (-): Must be transcribed into complementary (+) strand before translation.

• Example: Poliovirus (+), Influenza virus (-).

Proto-oncogenes vs. Oncogenes

Proto-oncogenes and oncogenes are related to cancer development.

• Proto-oncogene: Normal gene involved in cell growth and division.

• Oncogene: Mutated or overexpressed proto-oncogene that can cause uncontrolled cell proliferation (cancer).

• Example: ras gene (proto-oncogene) can become an oncogene.