1. The Position of Viruses in the Biological Spectrum
There is no universal agreement on how and when viruses originated
Viruses are considered the most abundant microbes on earth
Viruses play a key role in the evolution and ecology of Bacteria, Archaea, and Eukarya
Viruses are obligate intracellular parasites

2. Medical Importance of Viruses
Viruses are the most common cause of acute infections
Several billion viral infections per year
Some viruses have high mortality rates
Possible connection of viruses to chronic afflictions of unknown cause
Viruses are major participants in the earth’s ecosystem 2 2

4. General Structure of Viruses
Size range –
most <0.2 μm; requires electron microscope

5. Viral Components: Capsids, Nucleic Acids, and Envelopes
Viruses bear no resemblance to cells
Lack protein-synthesizing machinery
Viruses contain only the parts needed to invade and control a host cell

7. General Structure of Viruses
Capsids
All viruses have capsids - protein coats that enclose and protect their nucleic acid
The capsid together with the nucleic acid are nucleocapsid
Some viruses have an external covering called envelope; those lacking an envelope are naked
Each capsid is constructed from identical subunits called capsomers made of protein

8. Structure of Virus

9. General Structure of Viruses
Two structural types:
Helical - continuous helix of capsomers forming a cylindrical nucleocapsid
Icosahedral - 20-sided with 12 corners
Vary in the number of capsomers
Each capsomer may be made of 1 or several proteins
Some are enveloped

10. Helical Nucleocapsids

12. General Structure of Viruses
Viral envelope
Mostly animal viruses
Acquired when the virus leaves the host cell
Exposed proteins on the outside of the envelope, called spikes, essential for attachment of the virus to the host cell

13. Functions of Capsid/Envelope
Protects the nucleic acid when the virus is outside the host cell
Helps the virus to bind to a cell surface and assists the penetration of the viral DNA or RNA into a suitable host cell
Complex viruses: atypical viruses
Poxviruses lack a typical capsid and are covered by a dense layer of lipoproteins
Some bacteriophages have a polyhedral nucleocapsid along with a helical tail and attachment fibers

15. Nucleic Acids Viral genome – either DNA or RNA, but never both
Carries genes necessary to invade host cell and redirect cell’s activity to make new viruses
Number of genes varies for each type of virus – few to hundreds

16. Nucleic Acids DNA viruses RNA viruses
Usually double stranded (ds) but may be single stranded (ss)
Circular or linear
RNA viruses
Usually single stranded, may be double stranded, may be segmented into separate RNA pieces
ssRNA genomes ready for immediate translation are positive-sense RNA
ssRNA genomes that must be converted into proper form are negative-sense RNA

17. General Structure Pre-formed enzymes may be present
Polymerases – DNA or RNA
Replicases – copy RNA
Reverse transcriptase – synthesis of DNA from RNA (AIDS virus)

18. How Viruses Are Classified
Main criteria presently used are structure, chemical composition, and genetic makeup
Currently recognized: 3 orders, 63 families, and 263 genera of viruses
Family name ends in -viridae, i.e.Herpesviridae
Genus name ends in -virus, Simplexvirus
Herpes simplex virus I (HSV-I)

19. Modes of Viral Multiplication
General phases in animal virus multiplication cycle:
Adsorption – binding of virus to specific molecule on host cell
Penetration – genome enters host cell
Uncoating – the viral nucleic acid is released from the capsid
Synthesis – viral components are produced
Assembly – new viral particles are constructed
Release – assembled viruses are released by budding (exocytosis) or cell lysis

21. Adsorption and Host Range
Virus coincidentally collides with a susceptible host cell and adsorbs specifically to receptor sites on the cell membrane
Spectrum of cells a virus can infect – host range
Hepatitis B – human liver cells
Poliovirus – primate intestinal and nerve cells
Rabies – various cells of many mammals

23. Penetration/Uncoating
Flexible cell membrane is penetrated by the whole virus or its nucleic acid by:
Endocytosis – entire virus is engulfed and enclosed in a vacuole or vesicle
Fusion – envelope merges directly with membrane resulting in nucleocapsid’s entry into cytoplasm

25. Replication and Protein Production
Varies depending on whether the virus is a DNA or RNA virus
DNA viruses generally are replicated and assembled in the nucleus
RNA viruses generally are replicated and assembled in the cytoplasm
Positive-sense RNA contain the message for translation
Negative-sense RNA must be converted into positive-sense message

26. Release Assembled viruses leave host cell in one of two ways:
Budding – exocytosis; nucleocapsid binds to membrane which pinches off and sheds the viruses gradually; cell is not immediately destroyed
Lysis – nonenveloped and complex viruses released when cell dies and ruptures
Number of viruses released is variable
3,000-4,000 released by poxvirus
>100,000 released by poliovirus

28. Damage to Host Cell Cytopathic effects - virus-induced damage to cells
Changes in size and shape
Cytoplasmic inclusion bodies
Inclusion bodies
Cells fuse to form multinucleated cells
Cell lysis
Alter DNA
Transform cells into cancerous cells

31. Persistent Infections
Persistent infections - cell harbors the virus and is not immediately lysed
Can last weeks or host’s lifetime; several can periodically reactivate – chronic latent state
Measles virus – may remain hidden in brain cells for many years
Herpes simplex virus – cold sores and genital herpes
Herpes zoster virus – chickenpox and shingles

32. Mammalian viruses capable of initiating tumors are called oncoviruses
Some animal viruses enter host cell and permanently alter its genetic material resulting in cancer – transformation of the cell
Transformed cells have increased rate of growth, alterations in chromosomes, and capacity to divide for indefinite time periods resulting in tumors
Mammalian viruses capable of initiating tumors are called oncoviruses
Papillomavirus – cervical cancer
Epstein-Barr virus – Burkitt’s lymphoma

33. Multiplication Cycle in Bacteriophages
Bacteriophages – bacterial viruses (phages)
Most widely studied are those that infect Escherichia coli – complex structure, DNA
Multiplication goes through similar stages as animal viruses
Only the nucleic acid enters the cytoplasm - uncoating is not necessary
Release is a result of cell lysis induced by viral enzymes and accumulation of viruses - lytic cycle

34. 6 Steps in Phage Replication
Adsorption – binding of virus to specific molecule on host cell
Penetration – genome enters host cell
Replication – viral components produced
Assembly – viral components assembled
Maturation – completion of viral formation
Release – viruses leave cell to infect other cells 34

37. Lysogeny: The Silent Virus Infection
Not all phages complete the lytic cycle
Some DNA phages, called temperate phages, undergo adsorption and penetration but don’t replicate
The viral genome inserts into bacterial genome and becomes an inactive prophage – the cell is not lysed
Prophage is retained and copied during normal cell division resulting in the transfer of temperate phage genome to all host cell progeny – lysogeny
Induction can occur resulting in activation of lysogenic prophage followed by viral replication and cell lysis

39. Lysogeny
Lysogeny results in the spread of the virus without killing the host cell
Phage genes in the bacterial chromosome can cause the production of toxins or enzymes that cause pathology – lysogenic conversion
Corynebacterium diphtheriae
Vibrio cholerae
Clostridium botulinum

41. Prions and Other Infectious Particles
Prions - misfolded proteins, contain no nucleic acid
Cause transmissible spongiform encephalopathies – fatal neurodegenerative diseases
Common in animals:
Scrapie in sheep and goats
Bovine spongiform encephalopathies (BSE), a.k.a. mad cow disease
Wasting disease in elk
Humans – Creutzfeldt-Jakob Syndrome (CJS)
Extremely resistant to usual sterilization techniques

43. Other Noncellular Infectious Agents
Satellite viruses – dependent on other viruses for replication
Adeno-associated virus – replicates only in cells infected with adenovirus
Delta agent – naked strand of RNA expressed only in the presence of hepatitis B virus
Viroids – short pieces of RNA, no protein coat; only been identified in plants