Download 1. Discuss the contributions of Mayer.

Chapter 18 Reading Quiz
1. Which viral reproductive cycle destroys the
host cell? lytic
vaccine
2. A(n) ______
is a harmless variant or
derivative of a pathogen that stimulates the
immune system to mount a defense.
prion
3. An infectious protein is called a(n) ______.
4. How many different species a virus can
Host range
infect is called its _______.
5. Viruses that show up suddenly are referred
emerging viruses.
to as ______
Discovery of viruses….
• Mayer demonstrated that the stunting
disease of tobacco plants was contagious 
thought it was caused by an unusually small
bacteria
- later findings demonstrated that the
disease could not be bacteria-caused, but
must be a particle much smaller (and unlike) a
bacterium
- the infectious particle was finally
crystallized and observed and is now known as
the tobacco mosaic virus (TMV) 
structural components of viruses and why
viruses are obligate parasites.
•
•
•
•
•
It is a genome enclosed in
a protective coat
It is organized as single
nucleic acid molecules
May have 4 to several
hundred genes
Simple composition 
1. Capsid – protein coat that
encloses the viral genome
2. Envelope – membrane that
cloaks some viral capsids
(head, sheath, DNA, tail
fibers)
Viruses express their
genes and reproduce only
within a living cell 
how viruses recognize host cells.
• They recognize their host cell by a
complementary fit between external
viral proteins and specific cell surface
receptor sites 
what happens when a virus infects a host cell.
• A viral
infection
begins
when the
genome of
a virus
makes its
way into a
host cell 
the lytic and lysogenic reproductive cycles
using phage T4 and phage  as examples.
Lytic Cycle
• Viral replication cycle that
results in the death (or lysis)
of the host cell
• T4  phage attaches to cell
surface, phage contracts
sheath and injects DNA,
hydrolytic enzymes destroy
the host cell’s DNA, phage
genome directs the host cell
to make phage components
and cell lyses and releases
phage particles
Lysogenic Cycle
• A viral replication cycle that
involves the incorporation of
the viral genome into the
host cell genome
• λ phage binds to the surface
of ecoli and injects the DNA
and inserts it by genetic
recombination 
defenses bacteria have against phage infection.
• Bacterial mutations can change receptor
sites to avoid recognition
- this in turn prevents infection
• Restriction nucleases in bacteria
recognize and cut up foreign DNA
- self-destruction is avoided because
bacterial DNA is chemically altered
(methylated) 
variations in replication cycles of animal viruses.
Enveloped Viruses are
characterized by:
1.
2.
3.
4.
Attachment
Entry
Uncoating
Viral RNA & protein
synthesis
5. Assembly and release
RNA as viral genetic
material:
- RNA viruses can be
complicated (like
retroviruses)
- mRNA or the strand that
corresponds to mRNA is the
+ strand and it has the
nucleotide sequence that
codes for proteins
- The – strand is a template
for synthesis of the + strand

the role of reverse transcriptase in
retroviruses.
• It is the enzyme that transcribes DNA
from an RNA template
• The newly integrated DNA is now a
“provirus” and never leaves the host
genome
Viral genomic RNA
(reverse transcriptase)
Viral DNA
Evolution of viruses
• Probably evolved several times AFTER
the first cells
• Probably came from little bits of nucleic
acids that moved from cell to cell –
maybe plasmids and/or transposons
(mobile genetic elements)
evidence that viruses probably evolved
from fragments of cellular nucleic acid.
• Genetic material of different viral families is
more similar to host genomes than to that of
other viral families
• Some viral genes are identical to cellular
genes
• Viruses of eukaryotes are more similar in
genomic structure to their cellular hosts than
to bacterial viruses
• Viral genomes are similar to cellular genetic
elements like plasmids and transposons 
what vaccines are and how they work.
• Vaccine  a “harmless” variant or derivative
of pathogenic microbes that stimulate the
immune system to mount defenses against the
actual pathogen
• Once you have a viral infection though, it’s
pretty much up to your immune system
• There are some anti-viral drugs
– AZT interferes with DNA synthesis by reverse
transcriptase
– Acyclovir stops viral polymerase in herpesvirus
how viruses may cause disease symptoms
• Viruses damage or kill cells (viral infection 
lysosome releases hydrolytic enzymes)
• They can be toxic or cause infected cells to
produce toxins
• Cause varying degrees of cell damage
• Immune system reacts, causing fever, aches,
inflammation
Emerging viruses
•
•
•
•
HIV in San Francisco in the 1980s
Ebola in 1976 in Africa
West Nile virus in 1999 in North America
H1N1 (swine flu) in 2009; H5N1 (bird flu) in 1997
(mortality rate of 50%!)
– Named for the surface proteins
• 16 types of hemaglutinin; 9 types of neuraminidase
• How they show up so suddenly…
– Mutation of existing viruses
– Dissemination from an isolated human population
– Spread from other animals (about ¾ of new human
diseases)
horizontal and vertical routes of viral
transmission in plants.
• Horizontal  route of viral infection in
which an organism receives the virus
from an external source
• Vertical  route in which an organism
inherits a viral infection from its parent

Viroids & prions
• Viroids – tinier circular RNA molecules
that cause errors in plant growth
regulation
• Prions – infectious proteins - misfolded
–
–
–
–
Act very slowly (ten-year incubation period)
Pretty much indestructible
Cause mad-cow, Creutzfeldt-Jakob disease
Not much is known about these yet…