Normal Flow: DNA, RNA, Protein  Retrovirus: RNA, DNA, RNA, Protein

Virus Function

  • Virus: small nucleic acid genome enclosed in capsid
  • Penetrates cell → takes over metabolic machinery → assembles new virus copies


Virus Structure

  1. Nucleic Acid: either RNA or DNA which contains viruses information to make progeny                                             
  • DNA/RNA may be double-stranded or single-stranded 
  1. Capsid: viral protein coat enclosing genome, determines genomes size/shape and specificity
  2. Viral envelopes: surrounds capsids, comes from host membranes lipids & proteins, help virus infect host
  • The viral envelope mediates entry into the cell, the capsid enters the nuclear membrane, and the genome is all that enters the nucleus
  • Antibiotics don’t work on viruses bcuz don’t have a cell wall that can be targeted

Types of Viruses

  • Are specific for types of cell they parasitize: some attack only one kind of cell within single species and others attack similar cells from range of closely related species
  • Bacteriophages: infect bacteria, most complex
  • Simpler Viruses
    • Viroids: infect plants, just floaty RNA (no capsids)
      • Plant infections spread by plasmodesmata
    • Prion: infectious misfolded proteins that can transmit incorrect shape

Viruses and Evolution

  • Rapid evolution bcuz produce many copies over short time & less proofreading → mutations can be passed on
  • High rate of mutation increase pathogenicity bcuz host populations don’t evolve defense as fast as viruses evolve
    • Viruses mutate and evolve quickly → new strains on regular/seasonal basis
  • Genetic material of different but related viruses can combine when present in host cell → produce new virus

Living or Nonliving?

  • Living characteristics: have DNA or RNA, can reproduce/replicate, metabolic activity, and genetic recombination but only in hosts cell
  • Nonliving: are not cellular

Replication of Viruses

Lytic Cycle:

  • Used by virulent viruses: builds viral DNA & proteins → self assemble into virus → viruses bust out of the cell and lyse it, releasing lots of viruses → KILLS HOST CELL

Lysogenic Cycle

  • Viral DNA incorporated into the cell’s genome by genetic recombination → host cell duplicates viral DNA & proteins
  • Used by temperate viruses DOES NOT KILL HOST CELL
    • Prophage: viral genes inserted into a bacteria
    • Provirus: viral genes inserted into an animal cell
  • Stays dormant until there are optimal conditions, stress, or external environment stimulus (radiation or chemicals) → releases viruses to create an infection in the lytic cycle
    • So cycle technically in between phase


  • Retrovirus: ssRNA animal virus
  1. Glycoprotein plasma membrane of virus fuses with that of the cell (virus enters cell)
  2. The virus contains protein capsids, RNA, & reverse transcriptase which it releases into the cytoplasm of the host cell
  3. Reverse transcriptase makes DNA complement strand from RNA
  4. dsDNA strand transcribed immediately to manufacture mRNA (lytic) or be incorporated into the host genome (lysogenic)
  • HIV: makes an envelope and leaves through exocytosis

               Over time eats up the host’s membrane destroying the immune system