Introduction to virology

Medical microbiology
Virology
Dr. zaytoon Alkhafaji

Introduction to virology
Definitions
Virology: the science which deals with study of viruses as causative agents of very important diseases that occurs in human, animals, plants and other living organisms (insects, bacteria,…)

Viruses are the smallest infectious agents (ranging from about 20 to 300 nm in diameter) and contain only one kind of nucleic acid (RNA or DNA) as their genome.
Viruses consist of :
1.A nucleic acid genome either DNA or RNA.
2.A protein coat (capsid) that enclosed the genome.
3.In some cases a lipid membrane (envelope)
Virion:A complete infectious virus particle.

General Properties of Viruses
1. Virus particles are very small in size; they are between 20-500 nm (nanometer) in diameter.
2. Viruses are obligatory intra cellular microorganisms.
3. Multiply inside the cells by replicating their genomes which either DNA or RNA, but not both.
4. The virus dose not contain any organelles (ribosomes, t RNA, metabolic enzymes, etc), but they depend on infected cells to provide all their needed organelles.
5. Virus does not affect with antibiotics.
6. Most viruses sensitive to interferon.
7. Viruses can not grow on artificial media, but only in living cells (specific host, Lab. Animals, chicken embryonated eggs & tissue culture).
8. Some viruses cause latent infection.
9. Viruses can not be seen by simple microscope, but only by Electron
microscope (EM).
Classification of viruses
Basis of Classification

The following properties have been used as a basis for the classification
of viruses:
1. Virion morphology, including size, shape, type of symmetry, presence or absence of peplomers, and presence or absence of membranes.

2. Virus genome properties, including type of nucleic acid (DNA or RNA), size of the genome, strand (single or double), whether linear or circular, sense (positive, negative), segments (number, size), nucleotide sequence, percent GC content, and presence of special features (repetitive elements, isomerization, 5?-terminal cap, 5?-terminal covalently linked protein, 3?-terminal poly(A) tract).

3. Genome organization and replication, including gene order, number and position of open reading frames, strategy of replication (patterns of transcription, translation), and cellular sites (accumulation of proteins, virion assembly, virion release).
4. Virus protein properties, including number, size, amino acid sequence, modifications (glycosylation, phosphorylation, myristoylation), and functional activities of structural and nonstructural proteins (transcriptase, reverse transcriptase, neuraminidase, fusion activities).

5. Antigenic properties, particularly reactions to various antisera.
6. Physicochemical properties of the virion, including molecular mass, buoyant density, pH stability, thermal stability, and susceptibility to physical and chemical agents, especially solubilizing agents and detergents.
7. Biologic properties, including natural host range, mode of transmission, vector relationships.

Structure of viral nucleic acids:
Genetic information are stored as the following:
1. Double stranded DNA cells (animal, plants, bacteria and some viruses).
2. Single-stranded DNA in other viruses (phage x 174).
3. Single stranded RNA (myxovirus).
4. Double-stranded RNA (reoviruses).
How we can differentiate between DNA and RNA?
• By DNAase or RNAase
• Between double and single stranded NA; by acridine orange stain, which is yellowish green in double stranded and red orange in single stranded.
Viral shape structure
Nucleocapsid is the arrangement between the viral nucleic acid genome with the capsid, this connection controlled by specific NA genetic information leading to different types of symmetry. Accordingly viruses can classified in to four symmetry structures.
1. Helical symmetry.
2. Cubical symmetry.
3. Binal symmetry.
4. complex symmetry.
viruses can be classified into several families according to the following
characters:
A- Characters of viral nucleic acid (N.A):
1. Type of N.A if it is DNA or RNA.
2. Shape of N.A strand if it is ring or straight, single or double strand, segmented or not.
3. Molecular weight (size &diameter of N.A).
4. Replication site of N.A if it is in the cytoplasm or nucleus of infected cells.
5. Presence of transcriptase enzyme.
B- Characters of viral capsid:
1. Shape & size of capsid.
2. Symmetry of capsid with N.A (Helical, Cubical, Binal, Complex).
3. Site of capsid assembly in side the infected cells.
4. Lipid solvent sensitivity.
5. Number of capsomeres which consisting the capsid.
According to all above characters recently viruses classified into two major groups which are RNA group & DNA group as following:
RNA Viruses families:
1. Picornaviridae
2. Orthomyxoviridae
3. Paramyxoviridae
4. Coronaviridae
5. Reoviridae
6. Retroviridae
7. Rhabdoviridae
8. Birnaviridae
9. Bornaviridae
10. Bunyaviridae
11. Togaviridae
12. Astroviridae
13. Arteriviridae
14. Caliciviridae
15. Flaviviridae
DNA Viruses families:
1. Parvoviridae
2. Anelloviridae (No specific disease associations have been proven).
3. Polyomaviridae
4. Papillomaviridae
5. Adenoviridae
6. Hepadenaviride
7. Herpesviridae
8. Poxviridae

Atypical Virus Like Particles
There are four exceptions to the typical virus as described previously:
Defective viruses Composed of viral nucleic acid and proteins, but cannot replicate without a helper virus.
Pseudoviruses Contain host DNA instead of viral DNA ,its infect the host cell but don’t replicate.
Viriods Consist of a single molecule of circular single strand RNA with no protein coat or envelope. They replicate and causes several diseases in plant but not in human
Prions Smallest known infectious particles, composed only protein .This protein has ability to causes fatal disease called spongiform encephalopathy slowly progressive diseases include :
scrapie disease in sheep, Bovine spongiform encephalopathy in cattle and kuru disease in human