Viruses and Bacteria Flashcards

Viruses & Bacteria

Viruses

• Viruses:
  • Smaller than bacteria.
  • Non-living: cannot reproduce on their own.
  • Lack their own cells.
  • Require living cells to reproduce, acting as parasites.
  • Some consider them living due to reproduction, adaptation, and evolution.

Virus Structure

• Genome:

  • DNA or RNA.
  • Single-stranded (ss) or double-stranded (ds).

• Capsid:

  • Protein coat that covers and protects the genome.

• Envelope:

  • May have a former host cell membrane surrounding the capsid.

• Examples of viruses and their structures are given with diagrams including:

  • Rabies Virus (enveloped)
  • Flu Virus (enveloped)
  • HPV (non-enveloped)

Virus Function and Host Infection

• Structure and shape play important roles in how viruses function.
• Each virus type can only infect certain hosts.
  • Surface proteins (ligands) on the virus fit receptor molecules on the host cell.
  • Examples:
    • HPV binds to skin cells via Heparan sulfate proteoglycans (HSPG).
    • Flu binds to α2,3 (bird) and α2,6 (human) sialic acids found on lung cells.
    • SARS-CoV2 binds to ACE2 receptors.

Virus Shapes

• Polyhedral:
  • Resemble small crystals, such as the papilloma virus that causes warts.
• Helical:
  • Long, narrow helical shape, like the tobacco mosaic virus.
• Enveloped:
  • Have an envelope studded with projections, such as the influenza virus and the AIDS-causing virus.

Baltimore Classification System

• Groups viruses based on their genetic material and replication methods.

  • Group I: dsDNA
  • Group II: ssDNA
  • Group III: dsRNA
  • Group IV: ssRNA (+)
  • Group V: ssRNA (-)
  • Group VI: ssRNA (RT) - Retroviruses
  • Group VII: dsDNA (RT)

• Shows how each group relates to mRNA and protein production.

Bacteriophage

• Viruses that infect bacteria.
• T4 virus infects E. coli.
• Structure:
  • Polyhedral-shaped head.
  • Cylindrical tail with leg-like fibers.

Virus Life Cycles

Lytic Cycle

• Host cell bursts, releasing new viruses that infect other cells.
• Lysis = burst.
• Steps (A.E.R.A.R.):
  1. Attachment: Virus attaches to the host cell.
  2. Entry: Virus injects nucleic acid into the cell.
  3. Replication: Virus DNA breaks down host DNA and instructs the cell to make virus parts.
  4. Assembly: Virus parts are assembled into new viruses.
  5. Release: Mature viruses exit and destroy the cell, then infect other cells.

Lysogenic Cycle

• Virus combines its DNA into the host cell's DNA.
• Forms a provirus.
• Virus is dormant as the host cell reproduces (mitosis S phase).
• A trigger can activate the provirus, or it can remain a permanent gene.
• Steps:
  1. Attachment: Virus attaches to the host cell.
  2. Entry: Virus injects nucleic acid into the cell.
  3. Integration: Virus DNA becomes part of host cell's DNA (provirus).
  4. Replication: Host cell replicates with viral DNA.
  5. Lytic Cycle: Virus can enter the lytic cycle when triggered.

Lytic vs. Lysogenic

• Lytic:
  • Causes symptoms quickly.
  • New viruses are made and spread to other cells right away.
  • Examples: Flu, measles, SARS-CoV2.
• Lysogenic:
  • Organism may have no symptoms for many years (dormant).
  • Symptoms develop once the provirus is triggered to enter the lytic stage.
  • Examples: HSV-1 (cold sores), VZV (chicken pox to shingles).

Retroviruses

• Lysogenic.
• Contain RNA.
• Contain reverse transcriptase (RT) for transcribing viral RNA into a DNA copy.
• Force the host cell to make viral DNA.
• Viral DNA is added to the host cell's DNA.
• HIV:
  • Infects white blood cells (T-helper cells).
  • AIDS: Virus enters lytic cycle → white blood cells are destroyed → body cannot fight off other infections.

Immune System, Prions, Viroids, COVID-19

Immune System

• Three types of cells:
  • Eaters: Macrophages (M∅) - Eat pathogens.
  • Helpers: T-helper cells (Th) - Tell the system to build a clone army.
  • Killers:
    • T killer cells (Tk) - Kill virus/pathogen.
    • B-cells - Make antibodies (bombers).
• Process:
  1. Macrophage eats the pathogen.
  2. Macrophage presents the antigen to T-helper cells in lymph nodes.
  3. If the T-helper recognizes the antigen, it mobilizes the killers (Tk and B cells).
  4. Tk and B cells build a clone army.
  5. After about 2 weeks, most of the clone army undergoes apoptosis.
  6. A few