1. PROPERTIES IMPORTANT IN THE SURVIVAL
OF VIRUSES IN NATURE
1. The capacity to replicate rapidly: but if replication is too rapid, it can be
self-defeating.
2. The capacity to replicate to higher titer: like ARBO-VIRUSES which give
very high vertebrate host viraemia.
3. The capacity to replicate in certain key tissues: the capacity to grow in
immunologically sequestered sites (e.g. Brain or Kidney) or in the cell of
the immune system.
4. The capacity to be shed for long period of time: recrudescence and
intermittent shedding is an additional advantage to the virus (e.g. BHV-1
and BVD infection in cattle).
2. PROPERTIES IMPORTANT IN THE SURVIVAL
OF VIRUSES IN NATURE (Cont)
5. The capacity to elude host defenses: viruses, specially those with
large genomes, have genes that encode proteins capable of interfering
with host antiviral activities (e.g.Sheep Pox virus). Immune Tolerance
is another mechanism (e.g. BVD Virus, FIV in Cats).
6. The capacity to survive after being shed into the external
environment: Canine Parvovirus has environmentally stable
capsid.
7. The capacity to be transmitted vertically: e.g. BVD, ALV….etc.
5. Mutation
Definition:
Errors in copying the viral nucleic
acid that occur during replication
are called Mutation. Whether a
particular mutation is lethal to the
virus, it depends on the resulting
phenotypic characteristics. virus
plaque technique could be used
for studying and isolating virus
variants.
7. Types of Mutation
A. Genotypic(Cont.)
2. Insertion mutations: in
which extra base pairs are
inserted into a new place in
the DNA (e.g Influenza
Viruses)
3. Deletion mutations: in
which a section of DNA is
lost, or deleted.
10. Types of Mutation(Cont.)
A. Genotypic(Cont.)
4. Frame-Shift Mutation:
error occurs at the DNA
level, causing the cordons to
be parsed incorrectly.
5. Missense Mutation: in
which substitution of a single
base pairs results in codon
change, thus giving a single
A.A. substitution.
11. 1. CONDITIONAL LETHAL: multiply under some
conditions but not others - wild-type (wt) grows
under both sets of conditions:
• temperature-sensitive (ts) mutants do not grow at
higher temperature (altered protein).
• host-range mutants do not grow in all the cell
types that the wt does.
B. Phenotypic Changes:
Types of Mutation(Cont.)
12. Types of Mutation(Cont.)
B. Phenotypic Changes (Cont.):
2. Plaque Mutants: a mutant may produce a different type of
plaques in a cell monolayer (e.g. MPV naturally weak RVF virus
mutant).
3. Antibody escape mutants: may become resistant to
neutralization by antibody raised against the wild type (e.g.In
horse persistently infected with equine infectious anemia virus
“EIA”).
N.B. A prominent example of mutations affecting viral antigenicity
is the antigenic drift of Influenza Viruses.
13. 4. Hot Mutants: grow better at elevated temperature than wt
and less susceptible to host fever response.
5. Attenuated Mutants:
- milder (or no) symptoms.
- vaccine development.
- Pathogenesis.
6. Defective Interfering Viruses: demonstrated in most
families (specially RNA Viruses) and need the presence of the
wild-type virus (e.g Influenza Viruses)
Types of Mutation(Cont.)
B. Phenotypic Changes (Cont.):
14. 1. Errors in Replication:
Transitions and Transversions
that occur during replication and
can result from deamination of
Cytosine (A&B).
2. Spontaneous Lesions:
- Depurination.
- Deamination.
- Oxidative Damage to DNA.
Mutagenesis
A. Spontaneous Mutation:
15. 1. Base-Analog Mutagens:
5-Bromo-uracil can substitute
for T, and then wind up binding
with G (B) and results in
miscoding.
AZT a nucleotide analog that
can result in Chain Termination.
Mutagenesis
B. Induced Mutation:
16. 2. Chemical Agents:
Such as nitrous oxide or
nitrosoguanidine.
3. UV and Ionizing Irradiation:
Crosslinks between adjacent
thymines (thymine dimer)
Mutagenesis
B. Induced Mutation:
17. Mutagenesis
C. Site-Directed
Mutagenesis:
The nucleotide at any
prescribed position in a DNA
genome or cDNA transcribed
from an RNA genome can be
changed to any other
nucleotide or it can be deleted
or affected by the nearby
insertion of an additional
nucleotides (s). It can be done
in Bacteriophage vector M13
18. • Mutation.
• Recombination: Exchange of information between two genomes.
• Interaction between viral Gene Products: this my involve not only
their nucleic acids but also their proteins; the products of such interactions
may affect the viral Phenotype.
Genetic Changes
19. RECOMBINATION
1. Intramolecular Recombination:
common in DNA viruses
It involves the exchange of nucleotide sequence between different-but usually closely
related-viruses during replication, e.g. as in Picornaviruses, Coronaviruses and
Togaviruses)
20. RECOMBINATION (Cont.)
2. Genetic Reassortment:
* Observed in RNA Viruses with
segmented genomes (e.g
Bunyaviridae “RVFV”,
Orthomyxoviridae “Infl.”, and
Reoviridae “Rotaviruses and Blue
Tongue Virus”,
* Exchange of segment may occur
when cell are infected
simultaneously with two related
viruses.
21. Mechanism Example
Point Mutation Avian Influenza (H5N2).
Intramolecular Recombination Between WEE and a Sindbis-like
alphavirus tha no longer exist.
Genetic Reassortment Pandemic Human Influenza H1N1 (1918)
originated by gaining genes from avian
viruses.
Intramolecular Recombination and
Mutation
Polio vaccine virus, reversion to virulence
following vaccination.
Genetic Changes and Recent Evidences of
Viral Evolution
23. Recombinant DNA Technology
2. Poxes as Vectors:
* The large genomes of Poxviruses
have allowed the insertion of large
amounts of foreign DNA into non-
essential genome region.
•Vaccinia virus Recombinant
Vectors are generated by
incorporating the foreign DNA into
viral genome via homologous
recombination.
• Proteins are expressed properly
(e.g. have undergone
posttranslational modification)
Foreign gene(s)
(BVD)
27. Practical Achievements of Recombinant
DNA Technology
1. Complete Sequencing of the genomes of representative viruses of all
the families of DNA viruses containing animal pathogens.
2. Characterization of the viral or proviral DNA that are integrated into
the chromosomal DNA of transformed cells and animals infected with
retroviruses.
3. Development of Probes for use in rapid diagnostic assays including
hybridization assays (dot-blot).
4. Application of the PCR to amplify specific viral genome sequences for
diagnostic and research purposes.
5. Production of proteins coded by specific viral genes, using bacterial,
yeast, baculovirus, and animal cell expression systems.
