virus classifications and typing (2).pdf

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1. VIRUS CLASSIFICATION
CHAPTER 2
HASSAN Mohamed
Rift valley university

2. CLASSIFICATION OF VIRUSES
The following criteria are used to classify viruses:
1. Morphology –structure of capsid
ex.presence or absence of envelope
2. Size of the virion
3. Tissue tropism of virus
4. Genome composition –DNA / RNA
ex. ds/ss DNA and ds/ss RNA

3. TAXANOMY
Taxonomic groups –family, subfamily, genus and
species
2. The names of virus families (family) are italicized end
in Latin suffix –viridae
3. The genera (genus) end in the suffix –virus
4. The species –English common name

4. MORPHOLOGY
Viruses are divided into three groups, based on the morphology of
the nucleocapsid and the arrangement of capsomeres.
Symmetry of viruses
this arrangement of capsomers in the virus.
shapes of virus are rod ,spherical and complex .
Rod shaped virus-helical symmetry
Spherical virus-icosahedron
Rod + spherical-complex

5. HELICAL SYMMETRY
Helical capsids are made up of a single type of protein subunit stacked around a
central axis to form a helical structure.
The helix may have a hollow center, which makes it look like a hollow tube. This
arrangement results in rod-shaped or filamentous virions.
These virions can be anything from short and very rigid, to long and very flexible.
The well-studied tobacco mosaic virus (TMV) is an example of a helical virus.

6. ICOSAHEDRAL
Icosahedral capsidsymmetry gives viruses a spherical appearance at low
magnification, but the protein subunits are actually arranged in a regular
geometrical pattern, similar to a soccer ball; they are not truly spherical.
An icosahedral shape is the most efficient way of creating a hardy structure.
This shape is used because it can be built from a single basic unit protein
which is used over and over again. This saves space in the viral genome.
Adenovirus, an icosahedral virus.

7. COMPLEX VIRUSES
Complex viruses possess a capsid which is neither purely helical, nor purely
icosahedral, and which may have extra structures such as protein tails or a complex
outer wall.
Viral protein subunits will self-assemble into a capsid, but the complex viruses DNA
also codes for proteins which help in building the viral capsid.
Many phage viruses are complex-shaped; they have an icosahedral head bound to
a helical tail. The tail may have a base plate with protein tail fibers. Some complex
viruses do not have tail fibers.

8. EXAMPLES

9. CLASSIFICATION ACCORDING TO SIZE
Ranges of sizes is used to classify viruses:
20 nm to 500 nm (spherical)
12 nm to 300-2000 nm (rod like)
Easily observed with electron microscope
Ex.1 Mimivirusis 500 nm Infects algae
Ex.2 Parvovirus is 20 nm in diameter Infects algae

10. TYPE OF HOST
Virus are named according to the host cells
a. Bacteriophages: infect bacterial cells
b.Plantviruses infect plant cells
c. Animal viruses are subgroupedby the tissues they
attack:
a. Dermotrophic: if they infect the skin
b. Neurotrophic: if they infect nerve tissue

11. GENOME
BALTIMORE CLASSIFICATION OF VIRUSES
The division of the viruses into classes based on genome
type and mode of replication and transcription suggested by
David Baltimore –Seven Baltimore classes.
Major groups of viruses are distinguished first by their
nucleic acid content as either DNA or RNA
RNA and DNA viruses can be single-stranded (ssRNA,
ssDNA) or double-stranded (dsRNA, ssDNA)

12. CONT-
Class I
Double-stranded (ds) DNA viruses are in class 1
dsDNA Viruses: dsDNA genomes possess two anti-parallel
strands in a double-helix structure
The production of mRNA and genome replication in such viruses
occurs as it would from the host genome.

13. CONT-
Class II
Single-stranded(ss) DNA viruses.
ssDNA Viruses: ssDNA genomes possess only a single strand of
DNA.
These viruses form a double stranded DNA intermediate
during replication and this intermediate used for
transcriptionwhich process of creating mRNA.

14. CONT-
Class III
Class III (double-stranded RNA viruses)
dsRNA Viruses: dsRNA genomes possess two anti-parallel strands
much like DNA in a double-helix-like structure

15. POSITIVE & NEGATIVE SENSE RNA
Positive sense RNA can be translated directlyinto protein upon
uncoating of the virion in the cell
Negative sense RNA must be transcribed by a virus coded, virion
packaged RNA dependent RNA polymeraseimmediately following
uncoating.

16. HOW DO THESE VIRUSES
REPLICATE?
Cellular RNA polymerases do not catalyze formation of RNA from an RNA template but from
DNA template.
RNA viruses whether plus, negative or double stranded require a specific RNA-dependant
RNA polymerase.

17. CONT-
Class IV
Positive-strand of RNA viruses
Positive ssRNAViruses: Orientation of genes in the RNA genome is such that they can be directly
translated to proteins much like a host cell's mRNA
Viral genome is of the plus configuration and hence serve directly as mRNA.
The viruses required other protein, therefore mRNA encodes a virus specific and RNA dependent RNA
polymerase.
Once synthesized, this polymerase makes complementary minus strands of RNA and then use as
template to make more plus strand.

18. CONT-
Class V (negative strand RNA virus)
ssRNAViruses: ssRNAgenomes possess only a single strands which can be organized in different orientations
mRNA must be first synthesized, however cells does not have RNA polymerase.
To solve this issue ,these viruses contain enzyme in the virion, enters cell along with the genomic RNA. Therefore, in this
case complementary plus strand is synthesized by RNA dependantRNA polymerase and used as mRNA.
Plus strand used as template to make more negative strandgenome.

19. CONT-
Class VI
Single-stranded RNA genome that replicates with DNA intermediate.
This RNA virus require reverse transcriptase to copy the information found in RNA to
DNA.
Class VII
Double-stranded DNA genome that replicates with RNA intermediate.
Required reverse transcriptase
Mechanism producing mRNA is similar in virus Class I

20. EXAMPLE OF REVERSE TRANSCRIPTASE

21. EX

22. BALTIMORE CLASSIFICATION OF VIRUSES