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Viruses
Big Idea 3: Living systems store,
retrieve, transmit, and respond to
info essential to life processes.
Essential Knowledge
• 3C3: Viral replication results in
genetic variation, and viral infection
can introduce genetic variation into
the hosts.
Structure of Viruses
• Viruses are not cells
 small infectious particles consisting of nucleic
acid in a protein coat or a membranous
envelope
Viral Genomes
Made of either:
– Double- or single-stranded DNA, or
– Double- or single-stranded RNA
RNA Viruses
Need Reverse Transcriptase: enzyme that makes
double stranded DNA to match w/ host cell
Lack replication error-checking mechanisms 
higher rates of mutation (HIV!)
Herpes Virus-DNA
Ebola Virus-RNA
Capsids and Envelopes
• A protein shell that encloses viral genome
• Built from protein subunits called capsomeres
Fig. 19-3
RNA
DNA
Capsomere
Membranous
envelope
RNA
Head
DNA
Capsid
Tail
sheath
Capsomere
of capsid
Glycoproteins
Glycoprotein
18  250 nm
70–90 nm (diameter) 80–200 nm (diameter)
20 nm
50 nm
(a) Tobacco mosaic (b) Adenoviruses
virus
50 nm
Tail
fiber
80  225 nm
50 nm
(c) Influenza viruses (d) Bacteriophage T4
Viral Replication
• Highly efficient  allow for rapid
evolution and acquisition of new
phenotypes!
Bacteriophage
• AKA: phages,
viruses that infect
bacteria
• Have a long capsid
head that encloses
DNA
• A protein tail attaches
phage to host and
injects DNA inside
Fig. 19-1
0.5 µm
Viruses Need Hosts!
• Intracellular
parasites that
have a host
range (a limited
number of host
cells that it can
infect)
Viral Reproductive Cycles
Virus VC
Fig. 19-4
VIRUS
1 Entry and
DNA
uncoating
Capsid
3 Transcription
and manufacture
of capsid proteins
2 Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
4 Self-assembly of
new virus particles
and their exit from
the cell
Reproductive Cycles of Phages
• 1st Rep Cycle: lytic cycle  Death of host
cell
• Virulent phage: virus that only reproduces via
lytic cycle
Fig. 19-5-1
1 Attachment
Fig. 19-5-2
1 Attachment
2 Entry of phage
DNA and
degradation of
host DNA
Fig. 19-5-3
1 Attachment
2 Entry of phage
DNA and
degradation of
host DNA
3 Synthesis of viral
genomes and
proteins
Fig. 19-5-4
1 Attachment
2 Entry of phage
DNA and
degradation of
host DNA
Phage assembly
4 Assembly
3 Synthesis of viral
genomes and
proteins
Head
Tail Tail fibers
Fig. 19-5-5
1 Attachment
2 Entry of phage
5 Release
DNA and
degradation of
host DNA
Phage assembly
4 Assembly
3 Synthesis of viral
genomes and
proteins
Head
Tail Tail fibers
Reproductive Cycles of Phages
• 2nd reprod. Cycle: latent/lysogenic cycle
• No destroying host cell viral DNA is mixed
into host cell’s chromosome  prophage
• Gives host cell properties like increased
pathogenicity in bacteria
Temperate Phages
Lambda Phage
 Attacks E. Coli
• Phages that use both lytic and lysogenic
cycles
• Env. signal can trigger switch
Fig. 19-6
Phage
DNA
Daughter cell
with prophage
The phage injects its DNA.
Cell divisions
produce
population of
bacteria infected
with the prophage.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Occasionally, a prophage
exits the bacterial
chromosome,
initiating a lytic cycle.
Lytic cycle
Lysogenic cycle
The bacterium reproduces,
copying the prophage and
transmitting it to daughter cells.
The cell lyses, releasing phages.
Lytic cycle
is induced
or
New phage DNA and proteins
are synthesized and
assembled into phages.
Lysogenic cycle
is entered
Prophage
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
Evolution of Viruses
• Maybe plasmids?? (circular DNA in
bacteria and yeasts) and transposons,
small mobile DNA segments
• Mimivirus (double-stranded DNA) is 2nd
largest virus discovered
• Virus evolved before or after cells?
Megavirus (2 strand DNA) Oct. 2011, largest
virus
Vaccines
• Harmless derivatives of pathogenic microbes
that stimulate immune system to mount
defenses against actual pathogen
• Viral infections cannot be treated by antibiotics
With your partner, make a list
of vaccines you have
received!
Epidemics vs Pandemics
• Epidemic: New cases of a disease appear in a
local area  aka outbreak
• If epidemic spreads to another locale 
pandemic
Plant Viruses
Most have an RNA genome
– Horizontal transmission, entering through
damaged cell walls
– Vertical transmission, inheriting the virus from a
parent
Viroids and Prions: Simple
• Viroids: circular RNA molecules that infect
plants and disrupt growth
• Prions: slow-acting, virtually indestructible
infectious proteins that cause brain diseases in
mammals
Ex: Prion Mad Cow Disease
Ex: Prion Creutzfeldt Jakob
Syndrome
Understanding Viruses Movie
http://www.schooltube.com/vide
o/360cafa902fb60e55f61/Under
standing-Viruses