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Viruses
Chapter 10.17
What you need to know!
 The components of a virus.
 The differences between lytic and
lysogenic cycles.
What’s a Virus?
 Not a living cell but an infectious
particle
 Obligate intracellular parasite
Contains:
1. nucleic acids
2. protein coat
Viruses are Tiny
Protein Coats-Capsids
 Capsids are made from proteins called
capsomeres
 Capsids have many different shapes
depending on the virus:
 Rod shape, Helical, Polyhedral, Icosahedral
 Some viruses will have membrane left
from the host cell
Tabacco Mosaic Virus
 Helical capsid with RNA
Adenovirus
 Respiratory
virus in
animal
 Polyhedral
capsid with
glycoprotein
spikes
Influenza virus
 Membrane
envelope from
host studded
with
glycoproteins
Bacteriophages
 Viruses that infect bacteria
 icosahedral shape of a phage resembles
a lunar landing probe
Viral Genome




DNA double helix
Single stranded DNA
Double stranded RNA
Single stranded RNA
Viral Reproduction
 Viruses are obligate intracellular
parasites
 Isolated viruses cannot reproduce
 They lack the ribosomes and enzymes
for making proteins
 Viruses can only infect limited range of
host
Lytic Cycle
 Virus infects host cell
 Cell constructs virus
 Cell dies and releases the virus
Lysogenic Cycle
 Virus infects host cell
 Virus nucleic acid hides inside host DNA
 A stimulus triggers the virus into the Lytic
Cycle
RNA as genetic material
 Either used directly as mRNA
 Or retroviruses deliver an enzyme called
“Reverse Transcriptase” that converts
RNA into DNA
 HIV
 Due to their single strand, RNA viruses
mutate a lot
HIV – enveloped RNA Virus
 Attaches only to T-cells (WBC)
 Translates RNA into DNA, integration
into cellular DNA called provirus
 Provirus DNA is transcribed to make new
HIV viruses that leave cell
 Provirus never leaves cell
Immune Response and
Vaccinations
 Body produces antibodies that fit specific
virus structure
 Vaccination are treated versions of the
virus that don’t infect and allow the
immune system to develop antibodies
 Example: measles , small pox, Hep B, polio