Viruses Sunita Kaushik Department of Botany DAVCG Yamunanagar

Viruses Sunita Kaushik Department of Botany DAVCG, Yamunanagar

Definition of a Virus Sub microscopic entity consisting of a single nucleic acid surrounded by a protein coat and capable of replication only within the living cells of bacteria, animals or plants.

Definition of a Virus Obligate Intracellular Parasite

Virion Structure Lipid Envelope Nucleic Acid Protein Capsid Virion Associated Polymerase Spike Projections

Virion Morphology • Simple Structure • Repetitive Structure • High Level of Redundancy

Virus Morphology Helical Icosahedral

Virus Replication 1 Virus attachment 1 2 3 5 2 4 5 6 4 3 7 8 7 6 8 and entry Uncoating of virion Migration of genome nucleic acid to nucleus Transcription Genome replication Translation of virus m. RNAs Virion assembly Release of new virus particles

Cytopathic Effect (cpe) Adenovirus Herpes virus

Transmission of Viruses • • • Respiratory transmission ü Influenza A virus Faecal-oral transmission ü Enterovirus Blood-borne transmission ü Hepatitis B virus Sexual Transmission ü HIV Animal or insect vectors ü Rabies virus

In vivo Disease Processes • Cell destruction • Virus-induced changes to gene expression • Immunopathogenic disease

Acute Virus Infection Amount of virus Symptoms Virus Time

Acute Virus Infections • Localised to specific site of body • Development of viraemia with widespread infection of tissues

Poliovirus

Poliovirus Properties of the virus • • Enterovirus. Possesses a RNA genome. Transmitted by the faecal oral route. Cause of gastrointestinal illness and poliomyelitis.

Poliovirus Infection Virus Infection Gut Non-neuronal tissues Viraemia Neuronal tissues Virus excretion in the faeces Paralysis

Incidence of Poliomyelitis A B Number of cases (in thousands) 40 Poliovirus vaccines A: Salk – killed inactivated vaccine. B: Sabin – live attenuated vaccine 30 20 10 0 1950 1960 1970 1980

Influenza A virus Properties of the virus • • • Myxovirus Enveloped virus with a segmented RNA genome Infects a wide range of animals other than humans Undergoes extensive antigenic variation Major cause of respiratory infections

Influenza A virus Infection • • • Spread by respiratory route Virus infects cells of the respiratory tract Destruction of respiratory epithelium ü Secondary bacterial infections • Altered cytokine expression leading to fever ü e. g interleukin-1 and interferon

Spread of influenza virus

Respiratory Tract

Generation of Novel Influenza A Viruses Human H 2 N 2 ANTIGENIC SHIFT Genetic Reassortment Avian H 3 N 8 Point mutation of HA and NA genes ANTIGENIC DRIFT Human H 3 N 2

Viruses and Human Tumours • Epstein Barr Virus ü Burkitt’s Lymphoma • Human papillomavirus ü Benign warts ü Cervical Carcinoma • Human T-cell Leukaemia Virus (HTLV-1) ü Leukaemia • Hepatitis C virus ü Liver carcinoma

Virus-induced tumours Virus Infection [ ] Uninfected Cell ? Uncontrolled cell growth and tumour formation

Virus-induced transformation Normal cells Transformed cells

Virus-Induced Tumours • • Virus infects cell. Virus nucleic acid, as DNA, integrates into cellular genome. Virus causes changes in cellular gene expression. Uncontrolled cell multiplication and tumour formation.

Treatment and Prevention of Virus Infections • Antivirals • Vaccines and immunisation

Antiviral Targets • Attachment/Entry • Nucleic acid replication • Virus protein processing • Virus maturation

Problems with Antivirals • Identification of virus-specific target. • Generation of resistant variants.