6/18/2013 6:30:00 PM
Chapter 21: Viruses, Viroids and Prions: Infectious Biological Particles
21.1 What is a Virus? Characteristics of Viruses
Viruses are not part of the tree of life because they lack properties of life shared by all
organisms so they themselves are not considered living organisms.
They can’t reproduce on their own
They lack a metabolic system to provide energy for their life cycles
They depend on host cells that they infect for assistance in these functions.
Thus viruses are infectious biological particles instead of organisms.
Structure of virus is simple that is the minimum required to transmit its genome from one
host cell to another.
It is simply one or more nucleic acid molecules surrounded by a protein coat or capsid.
Some capsids may be enclosed within a membrane or envelope derived from their host
Because a virus is not a cell, it does not have a cytoplasm enclosed by a plasma
Look at Figure 21.2 on pg 485 for various viruses and their structure.
Nucleic acid genome of a virus may be either DNA or RNA and can be composed of
either a single strand or double of the DNA or RNA.
Viral genomes range from few genes to over a hundred but all viruses have genes that
encode at least their coat proteins as well as the proteins involved in transcription, genes
required for synthesis of envelope proteins (only in enveloped viruses) and genes
required to create enzymes for nucleic acid replication.
Viruses are two basic structural forms
Helical viruses-protein subunits assemble in a rodlike spiral around the genome. (infect
Polyhedral viruses-the coat proteins form triangular units that fit together (infect
animals, plants and bacteria)
In some, there are protein spikes from the corners to provide host cell recognition
Both can be enveloped in a membrane derived from the host’s membrane. In these
enveloped viruses, proteins synthesized from the viral genome in the host cell are
transported to and embedded in the membrane before the virus particle buds through the
host cell. Allows virus to recognize and bind to host cells.
Viruses are classified into orders, families, genera and species using their size, structure
and how their nucleic acid is replicated. 4000 species into 80 families. Family names end
in –virdae Some bacteria named for the disease they cause. Each virus made of strains based by
Every living organism is likely to be permanently infected by one or more kinds of
Usually virus only affects one virus or closely related viruses; some even only affect one
organ or tissue.
Some are able to infect unrelated species (both naturally and after mutation)
Of 80 viral families, 21 cause human disease
Viruses can also affect animals and plants and the effect can be from undetectable to
Not all viruses are pathogens; some benefit the host
Protective viruses interfere with replication or other functions of pathogenic viruses or
they defend the host.
Bacteriophages are viruses that destroy (other viruses) in incredibly huge numbers
Viruses are vital components of ecosystems and may be dominant entity in certain ones.
For example they affect nutrient cycling through their effect on prokaryotic organisms.
Example: Cyanobacteria dominante the marin phytoplankton helping global
photosynthesis but bacteriophages infected the cyanobacteria, killing them and releasing
nutrients from bacterial cells.
When the bacteriophage infects cyanobacteria, the rate of damage exceed rate of repair
but the rate of photosynthesis is maintained (instead of dropping) based on virus’s
genome which allow it to keep up with the damage.
Done so the host lives long enough to keep the virus cycle going but the outcome is
carbon fixed on Earth.
21.2 Viruses Infect Bacterial, Animal and Plant Cells by Similar Pathways
Virus particles move by random molecular motions until they contact the surface of a
For an infection to occur, the virus (the genome) must enter the cell so the viral genes can
be expressed, leading to replication of genome and assembly of progeny viruses.
New viruses are released from host cell, but rupturing and killing it.
21.2a Bacteriophages are Viruses that Infect Bacteria
Virulent bacteriophages kill their hosts cells during each cycle of infection
Temperate bacteriophages enter an inactive phase inside the host cell and can be passed
on to several generations of daughter cells before becoming active and killing their host.
T-even bacteriophages T2, T4 and T6 have been the most valuable in genetic studies Coats of these bacteriophages separated into head and tail
Head is packed with double stranded linear molecule of DNA
Tail is assembled from several different proteins and has recognition proteins on its tip
that can bind to the surface of host cell. (once tail attaches, DNA genome enters cell)
Infection begins when a T-even phage collides with the surface on an E.coli cell and the
tail attaches to the host cell wall. [Step 1]
Enzyme, lysozyme digests a hole in the cell wall through which the tail injects the DNA
of phage (Proteins remain outside) [Step 2]
Phage uses host cell to express genes and one protein produced early in infection is
enzyme that breaks down the bacterial chromosome. Another gene for DNA polymerase
that replicates the phage’s DNA is expressed and over 100 viral DNA molecules are
synthesized. [Step 3]
Later, the host cell machinery transcribes the phage genes for the viral coat proteins.
Replicated viral DNA is packed to head as head and tail proteins assemble. [Step 5]
Once viral assembly is complete, lysozyme, lyses the bacterial cell wall, causing it to
rupture and the particles to infect other E.coli cells. [Step 6]
Whole series of evens is known as the lytic cycle. [Diagram on pg 488 for Visual
A Scientist’s Favourite Temperate E.coli Bacteriophage Lambda
Infective cycle of bacteriophage lambda is typical of temperate phages.
Phage lambda infect E.coli similarly to T-even phages.
Phage injects its double stranded linear DNA chromosome into bacterium [Step 1]
Inside, the linear chromosomes form a circle and follow one of two paths;
One path is lytic cycle, which is like the lytic cycle of virulent phages.
Starts with Step 1 and 2 (infection), then goes straight to steps 7 through 9 [product and
release of progeny virus] and repeat. To see the steps mentioned see Diagram on pg 489
The second and more common path is lysogenic cycle.
Begins when viral chromosome integrates into the host cell’s DNA by recombination
[Step 1-3 in diagram]
DNA of temperate phage inserts at one or a few places on bacterial chromosomes through
the action of a phage-encoded enzyme that recognized certain sequences in the host
Once integrated, the lambda genes are mostly inactive so no structural components made.
Virus is known as prophage when it is inserted in the host cell DNA. When it replicates
so does the integrated viral DNA, which is passed to daughter cells. [Step 4 & 5] The integrated prophage becomes active through certain environmental signals such as
nutrient availability causing it to go through the lytic cycle.
Enzymes excise the lambda chromosome that is circular and it replicates itself and direct
the production on linear viral DNA and coat proteins.
Active stage culminates in the lysis of the host cell and the release of infective viral
Excision of prophage from host is not always precise, thus sometimes one or more host
cell genes with the viral DNA gets included. They are replicated and carried to a new
Specialized transduction is when the adjacent genes to the integration site(s) of a
temperate phage are cut out by vial DNA and can be included in lytic stage and undergo
Infection of Animal Cells:
Virus’s infection animal cells follow a pattern similar to that for bacteria cells except that
both the viral coat of gene and genome enter the host cells.
Removal of the coat to release gene occurs during or after cell energy (depends on virus)
and the e