Introduction to Virology

What are viruses Acellular organisms whose genomes consist of nucleic acid, and which obligately replicate inside host cells using host metabolic machinery and ribosomes to form a pool of components which assemble into particles called virions, which serve to protect the genome and to transfer it to other cells.

Characterization of virus (1) A cellular forms, most range in size from 5 to 300nm in diameter. Obligate intracellular parasitic microorganisms which are smaller than bacteria and so could pass through bacterial filters. Highly selective and specific to their hosts. Responsible for several diseases of human, animal, plant, etc. Lack of nuclear membrane and external cell wall Two types recognized to belong to this forms : Viruses. Bacteriophages.

Characterization of virus (2) Viruses are infectious agents with both living and nonliving characteristics. Living characteristics of viruses They reproduce at a high rate, but only in living host cells. They can mutate.

Characterization of virus (3) 2- Nonliving characteristics of viruses a. They are acellular, that is, they contain no cytoplasm or cellular organelles. b. No metabolic enzymes but must replicate using the host cell's metabolic machinery. In other words, viruses don't grow and divide. Instead, new viral components are synthesized and assembled within the infected host cell. c. The vast majority of viruses possess either DNA or RNA but not both.

Nature of viruses Viruses cannot be grown on artificial media. Grow in living organisms or tissue cells which are kept alive in suitable medium.

Composition of viruses (1) Viruses have the same general chemical characteristics, being composed of molecules of nucleic acid (either RNA or DNA) and protein. Virus unit or particle is called a virion. Virion is composed of folded strands of nucleic acid inside a protein coat made of regularly arranged protein subunits called capsomere.

Composition of viruses (2) The simple types of viruses are only nucleoproteins, while the most complex types (as cowpox) contain in addition other compounds such as lipids, carbohydrates and sometimes traces of metals and vitamin-like substances. Viruses contain either DNA or RNA , but both never occur together in one virus. Plant viruses contain only RNA while animal viruses may contain either RNA or DNA . Bacterial viruses (bacteriophage) usually contain DNA.

Viruses have three basic forms

Viruses have three basic forms Complex: Poxviruses: No capsid DNA surrounded by core membrane Bacteriophages: Complex capsid head and tail structures Naked or non-enveloped: Capsid which contains DNA or RNA Enveloped: Outer membrane

Viral taxonomy Viral classification is based on: Shape Type and form of nucleic acid Enveloped or naked Mode of replication Organization of the genome and antigenic differences

Genomic organization of viral nucleic acids RNA viruses RNA single stranded RNA double stranded: one piece or segmented DNA viruses Single stranded Double stranded

Virus infection and replication

Steps of replication 1. Adsorption/attachment: The infective virus has to be bound at first to certain receptors on the outer surface of the host cell. 2. Penetration: There are two views concerning this phase : The whole virus consisting of nucleic acid and protein penetrates into the cell , although the protein part has no role on the further steps of infection. Only the nucleic acid part penetrates into the cell, i.e. the virus has to get rid of its protein coat before penetration Uncoting: Enzymes in lysosome digest capsid, envelope (if present) and release viral genome

Copies of viral proteins Steps of replication Replication: Copies of viral genome Copies of viral proteins Assembly Viral capsid reforms Packaging of genome Release: Budding taking pieces of host cell membrane Lytic host cell burst open releasing many virus particles