GENERAL VIROLOGY Definition of virus Smallest infectious agent

GENERAL VIROLOGY

Definition of virus • Smallest infectious agent with a size ranging from 20 to 300 nm, • genome, a single nucleic acid either DNA or RNA, but never both.

Differences between Bacteria & Viruses PROPERTY VIRUS BACTERIA Cellular organization NO YES Genome Growth on artificial Either DNA or RNA Both DNA & RNA NO YES Binary Fission NO YES Ribosomes Absent Present Muramic acid Absent Present Antibiotic Resistant Sensitive media

Morphology of viruses • Size: 20 – 300 nm. • Filterable – ability to pass through the filters that hold back bacteria. • Ultramicroscopic – too small to be seen under the light microscope. (Except poxviruses which can be seen under the light microscope when suitably stained).

Relative size of viruses

Structure • A single viral particle is called a VIRION • Nucleic acid is surrounded by protein coat called CAPSID which is made up of structural subunits called CAPSOMERES. • The Capsid and the nucleic acid together are called NUCLEOCAPSID

STRUCTURE OF VIRUS

Shape - Capsid symmetry • Icosahedral (Cubic) - polygon with 12 vertices and 20 facets in the shape of equilateral triangle. E. g. ; Papova, picorna and adenoviruses

• HELICAL - Capsomeres and nucleic acid are wound together to form a helical or spiral tube. E. g. : Rabies virus, Influenza virus

• Complex - Symmetry is complex and not fully understood. E. g: Pox viruses

Virus structures Naked icosahedral Naked helical capsomer Enveloped icosahedral spikes (glycoprotein) protomer nucleic acid Enveloped helical envelope (protein, lipid)

• Viral envelope: Viruses may be enveloped or non – enveloped. • Envelope is the outer covering of the viruses derived from the host cell membrane when the progeny virus is released by budding. • Envelope is a lipid bilayer with virus encoded proteins on the surface.

• Protein subunits may be seen as projecting spikes on the surface of the envelope, which are called peplomers (peplums – envelope). • • A virus may have more than one type of peplomers. • Influenza – Haemagglutinin (HA) & Neuraminidase (NA).

• NUCLEIC ACID: Genome of the viruses can be double stranded (ds) or single stranded (ss). • All DNA viruses are double stranded except Parvoviruses. • All RNA viruses are single stranded with exception of Reoviruses which is double stranded • Single stranded RNA viruses can be • Positive stranded – Genome acts directly as messenger RNA (m. RNA) • Negative stranded – Genome is complimentary to m. RNA.

Susceptibility • Temp - most viruses are heat labile. Hepatitis B virus - 60 C for 1 hour • Stable at low temp at -70 C • p. H - 5 to 9. Enteroviruses are resistant to acidic p. H. • Detergents – enveloped viruses are susceptible • Disinfectants – H 2 O 2, Hyphochlorite, BPL; resistant to phenol

REPLICATION OF VIRUSES • Viruses do not have the enzymes - depend on the synthetic machinery of the host cell for replication. • The replicative cycle can be divided into 7 steps 1. Adsorption or Attachment - The viral attachment protein recognizes specific receptors, which may be protein, carbohydrate or lipid, on the outside of the cell. • e. g. ; Influenza virus Haemagglutinin binding to sialic acid on respiratory epithelium • gp 120 of HIV binding to CD 4 on T cells

Virus attachment

2. Penetration - after adsorption, the coat of the enveloped viruses may fuse with the host cell membrane and release the virus nucleocapsid into the host cytoplasm. • Other viruses may enter the cell by a process of endocytosis, which involves invagination of the cell membrane to form vesicles in the cell cytoplasm.

3. Un-coating - Outer layers of the virion including the Capsid are removed and the nucleic acid is released into the host cell. • Process occurs with the help of lysosomal enzymes of the host cell.

Virus entry & uncoating

4. Biosynthesis of viral Nucleic acid & protein • Transcription of m. RNA from the viral nucleic acid. • Translation of the m. RNA into ‘early proteins’ – early or non-structural proteins are enzymes which initiate and maintain synthesis of virus components. • They may also induce shutdown of host protein and nucleic acid synthesis. • Replication of viral nucleic acid. • Synthesis of ‘late’ proteins – late or structural proteins are components of daughter viron capsids.

• Replication of ss. DNA Viruses - ss. DNA ds. DNA m. RNA Proteins • First single stranded DNA is converted into ds. DNA by producing a complimentary stand. This ds. DNA acts as template for replication & synthesis of m. RNA which are translated into viral proteins. • Replication of ds. DNA Viruses – ds. DNA m. RNA proteins • Only a part of DNA is transcribed into m. RNA which encodes for early proteins required for DNA synthesis.

Hepatitis B Life Cycle Virus particle (+) strand Cell entry (-) strand DNA synthesis (+) strand DNA synthesis 3. 5 kb m. RNA Uncoated DNA genome 3. 5 kb 2. 4 kb 2. 1 kb 0. 7 kb Packaging & export of infectious virus m. RNA Translation of m. RNAs Viral proteins

• Replication of RNA Viruses • In positive stranded ss. RNA viruses, viral RNA directly acts as a template for production of complimentary strand which acts a template for synthesize of viral RNA. • Negative stranded ss. RNA viruses carry their own polymerases for m. RNA transcription. The viral RNA produces complimentary strands which act both as m. RNA & template for synthesis of new viral RNA

• In ds. RNA viruses the viral RNA is transcribed to m. RNA by viral polymerases. • Retroviruses exhibit a unique replicative cycle. ss. RNA is converted into DNA by reverse transcriptase which forms RNA DNA hybrid, which later gets integrated into host cell genome and is called provirus.

Positive (+) RNA virus replicative cycle + - AAA Viral proteins off + strand nucleus + AAA Viral particles

Negative-strand virus replication 3’ (-) Strand 5’ (-) 5’ 3’ (+) Viral genome (-) coded off + strand RNA 5’ (+) 3’ (+) Protein synthesis from + strand RNA Infectious virus particles

5. Assembly / Maturation: • After synthesis of viral proteins and replication of viral nucleic acid the virons are assembled to form daughter virons. • RNA viruses are assembled in the cytoplasm, whereas DNA viruses (except Pox virus) are assembled in the nucleus.

6. Release: • Non-enveloped viruses are released by cell lysis. • Enveloped viruses are released by budding (without cell lysis) during which they acquire their lipoprotein envelop from cell membrane

Virus Assembly

7. Eclipse phase of viruses: • From the stage of penetration of virus into the host cell till the appearance of first infectious virus progeny particle, the virus cannot be demonstrated inside the host cell. • • This period is known as eclipse phase.

Click after each step to view process ATTACHMENT HOST FUNCTIONS PENETRATION VIRAL LIFE CYCLE UNCOATING Transcription Translation REPLICATION ASSEMBLY (MATURATION) RELEASE MULTIPLICATION

classification • Hierarchical. Families have suffix viridae. Genus have suffix virus. Species is important definition.

DNA VIRUSES • Parvo • Papova • Adeno • Herpes • Pox • Hepadna RNA VIRUSES • Picarno • Calici • Reo • Arbo • Toga • Flavi • Arena • Corona • Retro • Bunya • Orthomyxo • Paramyxo

• Viroids: Single stranded circular RNA molecules that lack a protein coat. They are plant pathogens • Prions: They are infectious agents without any nucleic acid. • They are highly resistant to heat, UV rays and nucleases. • They cause slow infections with long incubation period. • Example for prion diseases – Scrapie of sheep, spongiform encephalopathy, Kuru and Creutzfeldt-Jakob disease.