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VIRUSES
 Viral replication results in genetic variation, and viral
infection can introduce genetic variation into the hosts.
How does viral replication differs from other
reproductive strategies??
How does viral replication generate genetic
variation?
 The genome of viruses can be ssDNA, dsDNA, ssRNA, or dsRNA
 Viruses are called DNA viruses or RNA viruses depending on
their nucleic acid.
 Made of a protein shell (capsid) enclosing the nucleic acid. Capsids are
built from a large number of protein subunits and exist in a variety of
shapes.
 Some viruses, especially those which infect animals, have a membrane
(called an envelope) surrounding the capsid. The envelope is derived
from the host cell membrane and contains both viral and host proteins
and glycoproteins.
 Viruses lack organelles.
 They are intracellular parasite - able to reproduce only inside a host cell.
 Infection begins when the viral genome enters the host cell and begins to
direct the host cell machinery to make new capsid proteins and copies of
the viral genome.
 To produce new virus particles, the virus gains all the necessary enzymes,
nucleotides, amino acids, energy, etc. from the host.
 After the capsid proteins and nucleic acid are synthesized, they usually
spontaneously assemble into new virus particles.
 Hundreds or thousands of new viruses
exit the cell, often destroying the cell in
the process.
 The new virus particles are then free to
infect more cells.
This life cycle ends with the death of the host cell.
Viruses with this type of life cycle are called
virulent viruses.
1. The virus attached to the cell and injects it’s nucleic
acid.
2. The viral genes take over the cell, directing it to
make viral proteins and nucleic acid which then selfassemble into new viruses.
3. One viral gene codes for an enzyme which digests
the cell wall.
4. Without a cell wall, the bacterial cell lyses (hence,
lytic cycle) as a result of the osmotic uptake of water.
5. The lysed cell then releases up to 200 phages to
infect nearby cells.

In this cycle, the viral genome is copied without killing the
cell.

Viruses with this type of life cycle are called temperate
viruses.

The bacteriophage lambda provides a good example. This
virus can follow either a lytic or lysogenic cycle.
1.
2.
3.
4.
5.
6.
The phage attaches to the surface of the host cell (E.
coli) and injects it’s DNA. During a lytic cycle the
phage follows the pattern outlined earlier.
During a lysogenic cycle, the viral genome is
incorporated into the host cell genome and is called
a prophage.
One gene codes for a protein that suppresses most
of the other prophage genes.
Each time the bacterium divides, the phage DNA is
replicated and passed on to the daughter cells.
This way, the virus increases in numbers without
destroying the host cells. (diabolical)
A chemical or environmental trigger of some kind
causes the prophage to exit the lysogenic phase
and begin a lytic infection.

Viruses with envelopes
1.
Once the virus binds with the host cell, the envelop fuses with
the host cell plasma membrane, transporting the capsid and
nucleic acid into the cell.
2.
After cellular enzymes remove the capsid, the viral genome
takes over the cell.

Viruses with envelopes
3. The ER makes proteins for the new envelopes and clusters
them in patches on the plasma membrane.
4. New viruses bud (like exocytosis) from the areas of these
clusters. Thus, the envelop is derived from the host cell
membrane although some of the proteins contained in it are
of viral origin.
Note that this cycle does not necessarily kill the host cell.
 RNA viruses known as retroviruses contain an enzyme cal led
reverse transcriptase which can transcribe DNA from an RNA
template.
 The host RNA polymerase then transcribes the DNA into RNA which
function as both mRNA for the synthesis of viral proteins as well as
for new viruses.
EQ: How does
viral replication
generate genetic
variation?
1.
Virus replication allows for mutations to occur through usual
host pathways.
2.
RNA viruses lack replication error-checking mechanisms,
and thus have higher rates of mutation.
3.
Related viruses can combine/recombine information if they
infect the same host cell.
4.
Documented evidence: HIV is a well-studied system where
the rapid evolution of a virus within the host contributes to
the pathogenicity of viral infection.
 1. Viruses transmit DNA or RNA when they infect a host cell.
 • Transduction in bacteria
 • Transposons present in incoming DNA
 2. Some viruses are able to integrate into the host DNA and
establish a latent (lysogenic) infection. These latent viral
genomes can result in new properties for the host such as
increased pathogenicity in bacteria.
 When viruses attack a cell, they often pick up some of the host’s
genome and thereby alter their own.
 Are often called, “Jumping Genes” – which are DNA sequences
that move from one location on the genome to another.
1. How does viral replication differs from
other reproductive strategies??
2. How does viral replication generate
genetic variation?