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Viruses
1883- Adolf Mayer- Studied Tobacco Mosaic Disease
- It stunts the growth of tobacco plants & gives the
leaves a mottled, or mosaic, coloration
- Disease was contagious; he concluded it was tiny
bacteria, unable to be seen w/ a light mic.
- But, could only reproduce w/in host cell.
- Alcohol did not inactivate the disease, which is
generally lethal to bacteria
1935- Wendell Stanley crystallized & observed the particle, Tobacco
Mosaic Virus (TMV)
Basic Size & Structure
* Tiniest Virus is 20 nm, (there are 1000 nm in 1 μm;
bacteria are generally between 1-10 μm)
*They consist of a nucleic acid covered w/ a protein coat,
sometimes those are covered by a membranous envelope
Basic Viral Genomic Info.
* Many different kinds
* Double or single-stranded DNA
* Double or single-stranded RNA
* Anywhere from 4-100 genes
Capsids & Envelopes
* Capsid- Protein shell
* Rod-shaped (helical), Polyhedral, or more complex
* Built from large # of protein subunits, called
capsomeres.
*Ex- TMV- Rod-shaped capsid made from over 1000
molecules of the same protein.
*Ex-Adenoviruses- 252 identical protein molecules
arranged in a capsid w/ 20 triangular faces;
an icosahedron
* Some have viral envelopes that cloak their capsids.
* Viruses are obligate intracellular parasites; can reproduce only w/in
a host cell
* They lack enzymes for metabolism or ribosomes for
protein production
*Bacteriophage- Virus that attacks a bacterial cell (also called a
phage)
Host specificity- Viruses ID host by a “lock & key” fit between
proteins on outside of virus & specific receptor molecules on surface
of host cell
- Some viruses have host range large enough to
include many species
- Some have host range of only one species
- Viruses of many eukaryotes are tissue specific
- Human cold virus- infect lining of upper
respiratory tract
- AIDS- infects white blood cells
Viral Infection: An Overview
1) Genome of a virus enters a cell
2) Viral genome reprograms cell equipment to make
copies of itself (ie. DNA polymerase & nucleotides)
3) Also has cell machinery manufacture viral proteins
4) Copies of viral genome & proteins come together to
form new viruses
5) Viruses leave cell to infect other cells
Steps #1-5 occur in 2 ways
I. Lytic Cycle (Figure 18.4)
* Phage reproductive cycle that kills the host cell.
* After virus has used the cell’s machinery to replicate,
the bacterium lyses (breaks open) & releases new
viruses to infect other cells
* These phages are called virulent phages
Study diagram of Lytic Cycle
II. Lysogenic Cycle (Figure 18.5)
* Phage reproductive cycle that does not kill the host
* Phages capable of both lytic & lysogenic cycles are
called temperate phages
Ex- Phage λ (lambda)
1) Phage binds to surface of E. coli & injects DNA
2) DNA forms a circle.
3) Thru genetic recombination, viral DNA is
incorporated into cell’s DNA. Phage is then known
as a prophage
4) Every time E. coli divides, it passes viral DNA on
to daughter cells.
5) Thru various environmental triggers, DNA leaves
E. coli DNA & initiates a lytic cycle that kills the
cell.
Classification of Viruses depends on;
a) type of nucleic acid it carries (DNA or RNA; ds or ss)
b) presence or absence of a membranous envelope
Viral Envelopes- Typically a lipid bilayer, w/ glycoprotein spikes to
bind to specific receptor sites on host
* Viral envelope fuses w/ plasma membrane
* capsid & genome then enter host cell.
Study Figure 18-6
Retroviruses- ssRNA viral genome
* Contain reverse transcriptase, which transcribes DNA
from the viral RNA template
* Newly made DNA then integrates as a provirus into
the cell’s chromosome
* RNA polymerase transcribes viral DNA into mRNA
that is used to make viral proteins & as genome for
new viruses
* Ex- HIV
Damage a virus inflicts on an animal
* Depends on tissue affected
* If it can regenerate quickly, not much damage.
* Ex- Common cold- epithelium of respiratory tract
* If it can’t regenerate, lots of damage
* Ex- Poliovirus- affects nerve cells
Vaccines
* Harmless variants or derivatives of pathogenic
microbes
* Stimulate immune system to remember the pathogen
for when the real one shows up.