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