1. Introduction to Virology
The Nature of Viruses
Why Study Viruses
A Brief History of Virology
Detection and Measurement of Viruses
Virus Replication Cycle

2. The Nature of Viruses
Viruses consist of a nucleic acid genome packaged in a protein coat
Fig. 1.1 Schematic diagram of virus particles.

3. The Nature of Viruses
Viruses are dependent on living cells for their replication
Virus particles break down and release their genomes inside the cell
Virus genomes are either RNA or DNA, but not both

4. Why Study Viruses
Viruses are important disease-causing agents

5. Why Study Viruses Viruses can infect all forms of life
Viruses are the most abundant form of life on Earth

6. Why Study Viruses
The study of viruses has led to numerous discoveries in molecular and cell biology
Study of gene expression in small DNA viruses led to the identification of promoters for eukaryotic RNA polymerases
Research on the replication of bacteriophage and animal virus DNAs laid the foundations for understanding the enzymes involved in cellular DNA replication
RNA splicing in eukaryotic cells was first discovered by studying messenger RNAs of DNA viruses
Study of cancer-producing viruses led to the isolation of numerous cellular oncogenes and the understanding that cancer is caused by their mutation or unregulated expression

7. A Brief History of Virology
The scientific study of viruses is very recent

8. A Brief History of Virology
Viruses were first distinguished from other microorganisms by filtration
The crystallization of tobacco mosaic virus challenged conventional notions about genes and the nature of living organisms

9. A Brief History of Virology
The “phage group” stimulated studies of bacteriophages and helped establish the field of molecular biology
Study of tumor viruses led to discoveries in molecular biology and understanding of the nature of cancer

10. A Brief History of Virology

11. A Brief History of Virology

12. Detection and Measurement of Viruses
Most viruses were first detected and studied by infection of intact organisms
The plaque assay arose from work with bacteriophages
Fig. 1.3 Plaque assay.

13. Detection and Measurement of Viruses
Eukaryotic cells cultured in vitro have been adapted for plaque assays
Hemagglutination is a convenient and rapid assay for many viruses
Fig. 1.4 Hemagglutination assay.

14. Detection and Measurement of Viruses
Virus particles can be seen and counted by electron microscopy
The ratio of physical virus particles to infectious particles can be much greater than one

15. Virus Replication Cycle
An example of a virus replication cycle: mouse polyomavirus
Fig Replication cycle of mouse polyomavirus.

16. Virus Replication Cycle
Analysis of viral macromolecules reveals the detailed pathways of virus replication
Fig Steps in virus replication cycle.

17. Steps in the Virus Replication Cycle
Virions bind to receptors on the cell surface
2. The virion (or the viral genome) enters the cell
3. Early viral genes are expressed: the Baltimore classification of viruses

18. Baltimore classification of viruses
Fig Baltimore classification of viruses.

19. Steps in the Virus Replication Cycle
4. Early viral proteins direct replication of viral genomes
5. Late messenger RNAs are made from newly replicated genomes
6. Late viral proteins package viral genomes and assemble virions
7. Progeny virions are released from the host cell

20. Key Terms Plaque assay Budding Plaque-forming units (PFU) Capsid
Plasmodesmata
Positive (or plus) strand
Retrovirus
Reverse transcriptase
RNA-dependent DNA polymerase
RNA-dependent RNA polymerase
Scaffolding proteins
Sialic acid
Tropism
Uncoating
Virion
Budding
Capsid
Confocal microscope
DNA-dependent RNA polymerase
Envelope
Filterable virus
Hemagglutination assay
Hepadnavirus
Lysis
Multiplicity of infection (m.o.i.)
Negative (or minus) strand
Obligatory intracellular parasite
Oncogene