Recording-2025-03-14T20:07:23.851Z Lecture 13.1

Overview of Viruses

• Introduction to the topic of viruses.

• Discussion of the COVID virus as a widely recognized example.

• Purpose of the lecture:

  • Define what viruses are.

  • Explore different kinds of viruses and parts of the viral cycle.

  • Understand how viruses reproduce inside cells and the damage they can cause.

Definition of a Virus

• A virus is classified as an acellular obligate intracellular parasite:

  • Acellular: Lacks cells, making it technically not alive.

  • Obligate: Must enter a host cell to reproduce; it is not optional.

  • Intracellular: Reproduces inside host cells.

  • Parasite: Feeds on host cell, often harming it in the process.

• Clarified that viruses cannot reproduce independently; therefore, they rely on host cells.

• Example of another obligate intracellular parasite: Chlamydia (classified as a bacterium, not a virus).

Basic Viral Reproduction

• Overview of the viral reproduction process:

  • The virus must first infect a host cell.

  • After infection, the virus will use the host's resources to reproduce, stealing nutrients and enzymatic tools.

  • Consequently, host cells can become weakened or killed due to resource depletion.

• Comparison of viral reproduction vs. bacterial growth:

  • Viruses do not reproduce in a doubling manner (like bacteria). Instead, virus replication is marked by a step-like growth pattern.

  • An initial period of apparent inactivity followed by a sudden burst of new viral particles (virions).

Viral Growth Curves

• Logarithmic Scale: Representation of viral counts shows a slow initial phase followed by rapid increase.

• Synthesis Time: Viruses may take about 25 minutes for synthesis before bursting out of the host cell.

• Destruction of Host Cells: New virions are released simultaneously, which may lead to the death of host cells.

Components of a Virus

• Three main components generally found in viruses:

  1. Core (Nucleic Acid):

     • Contains either DNA or RNA.

     • Most viruses consist of either DNA or RNA, very rarely both.

  2. Capsid (Protein Coat):

     • Protective layer that holds the nucleic acid core.

     • Comes in various shapes, including icosahedral (20-sided).

  3. Envelope:

     • Not found in all viruses.

     • Many human-infecting viruses, like COVID-19, possess an envelope.

     • Envelope often includes glycoproteins (spikes) that aid in attaching the virus to host cells.

Types of Viruses: Naked vs. Enveloped

• Naked Virus:

  • Comprises a core and capsid only, showcasing a simpler structure.

  • Examples include various icosahedral viruses visible through electron microscopy.

• Enveloped Virus:

  • Consists of a core, capsid, and envelope with glycoproteins.

  • Envelopes can vary in shape, such as helical wrapping structures around nucleic acids.