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

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

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

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

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

Structure of Virus

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

Helical Nucleocapsids

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

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

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

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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