Lecture 15, Part 1: Viruses

Administrative Details

• Date: Monday, October 21st
• Lecture: Part 1 on Viruses
• Attendance Code: 414
• Word of the Day: Phage
• Homework:
  • Assignment 7 was due last Friday
  • Homework on Chapter 13 (Viruses) this week
  • Upcoming: Assignment 9 on Chapter 15 (Microbial Pathogenesis)

Course Structure

• Chapters in units 3 and 4 are dense and complex
• Focus on class discussions and provided study guides
• Unit 3 Schedule:
  • Current: Viruses
  • Next: Human microbiome, Disease and Epidemiology, Mechanisms of Pathogenicity
  • Review sessions planned due to complexity

Exam Information

• Exam 2: Fewer students have taken it, harder than the first exam

Virology Overview

• Types of Viruses:
  • Bacteriophages: Prokaryotic viruses
  • Animal Viruses: Infect eukaryotic cells
  • Envelope vs Non-envelope viruses
• Virus Multiplication:
  • DNA/RNA
  • Enveloped or non-enveloped
  • Positive or single-stranded
• Virus Structure:
  • Essential components: DNA/RNA and protein coat
  • Some have envelopes or spikes (glycoproteins)

Virus Characteristics

• Obligatory Intracellular Parasites: Need host to replicate
• Components:
  • No ribosomes or ATP generating mechanism
  • Certain RNA viruses have enzymes within a capsid

Host Specificity and Virus Size

• Most viruses infect specific cell types
• Size comparison: Nanometer scale, smaller than bacteria and red blood cells

Virion Structure

• Nucleic Acid: DNA or RNA, single/double-stranded
• Capsid: Protein coat made of capsomeres
• Envelope and Spikes: Optional, glycoproteins

Virus Morphology

• Shapes: Polyhedral (e.g., Mastodonovirus), Helical (e.g., Ebola)
• Complex Viruses: Bacteriophages with structures like capsid head, sheath, tail fibers

Virus Culture and Identification

• Culture Methods:
  • Must grow in living cells
  • Bacteriophages form plaques on bacteria
  • Animal viruses in living cells, eggs, or cultures
• Identification Methods:
  • Cytopathic effects
  • Serological tests and antibodies
  • Nucleic acid sequencing via PCR

Bacteriophage Cycles

• Virulent vs Temperate Phages
  • Lytic Cycle (Virulent): Causes cell lysis
  • Lysogenic Cycle (Temperate): Phage DNA incorporates into host DNA

Animal Virus Multiplication

• Steps:
  • Attachment
  • Penetration by endocytosis/fusion
  • Uncoating
  • Biosynthesis and maturation
  • Release by budding/rupture
• Replication Differences:
  • Endocytosis or fusion dependent on envelope presence

RNA Virus Replication

• Challenges to Central Dogma: Differing replication mechanisms
• Positive and Negative-Stranded RNA:
  • Positive-stranded RNA can serve directly as mRNA
  • Negative requires transcription to positive mRNA first
• Double-stranded RNA: Involves both strands

Retroviridae

• Reverse Transcriptase: Enables RNA to DNA synthesis and host integration
• Provirus Formation: Integration into host DNA
• Release: Budding with host cell membrane

Next Steps

• Continue Part 2 on Wednesday
• Office hours available for further questions.