1. Introduction to
Virology

2. 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.

3. 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.

4. 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.

5. 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.

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

7. 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.

8. 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.

9. Viruses have three basic forms

10. 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

11. 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

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

13. Virus infection and replication

14. 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

15. 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