Chapter 6 An Introduction to Viruses
Introduction All life-forms can be infected by viruses. Some viruses generate serious epidemics, from dengue fever to influenza to AIDS. Others fill essential niches in the environment, particularly in marine ecosystems. In research, viruses have provided both tools and model systems in molecular biology.
This 11-inch-high limestone Egyptian funerary stele is from Saqqara, 10 miles south of Cairo; Amarna Period, 18 th Dynasty (1403-1365 BCE), Glyptotek Museum, Copenhagen. The stele portrays Roma (or Rema), an Egyptian doorkeeper, and his family giving offerings to the Goddess Astarte. Thought to be the earliest depiction of a victim of poliomyelitis, the man adeptly carries a goblet while supporting himself with a staff. His withered right leg and deformed right foot are characteristic of poliomyelitis.
Ramses V, Pharaoh of Egypt He died ~1145 BCE, presumably of smallpox. His mummified head and torso bear the characteristic lesions of the disease. Smallpox victims included many other rulers throughout history, among them Louis XV of France, Mary II of England, and the Holy Roman Emperor Joseph I.
The search for the elusive virus Louis Pasteur postulated that rabies was caused by a virus (1884) Ivanovski and Beijerinck showed a disease in tobacco was caused by a virus (1890s) 1950s virology was a multifaceted discipline Viruses: non-cellular particles with a definite size, shape, and chemical composition
Viral diseases led to the development of some of the first vaccines Poliovirus causes poliomyelitis, which can lead to paralysis President Franklin Roosevelt established the March of Dimes With its support, Jonas Salk developed the first polio vaccine in 1952
What Is a Virus? A virus is a non-cellular particle that must infect a host cell, where it reproduces. It typically subverts the cell s machinery and directs it to produce viral particles. The virus particle, or virion, consists of a single nucleic acid (DNA or RNA) contained within a protective protein capsid.
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 played a role in the evolution of Bacteria, Archaea, and Eukarya Viruses are obligate intracellular parasites
iruses are ubiquitous in all environments iruses are part of our daily lives Viruses infect all forms of life ost frequent infections of college students: ) Respiratory pathogens such as rhinovirus (the common cold) and pstein-barr virus (infectious mononucleosis) ) Sexually transmitted viruses such as herpes simplex virus (HSV) and apillomavirus (genital warts) ifferent viruses infect every group of organisms ach species of virus infects a particular group of host species, or host ange
Viroids Viroids are RNA molecules that infect plants. They have no protein capsid. Are replicated by host RNA polymerase. Some have catalytic ability.
Prions Prions are proteins that infect animals. They have no nucleic acid component. Have an abnormal structure that alters the conformation of other normal proteins Extremely resistant to usual sterilization techniques
Common in animals Prions Diseases Scrapie in sheep and goats Bovine spongiform encephalopathies (BSE), a.k.a. mad cow disease - transmissible and fatal neurodegenerative disease Humans Creutzfeldt-Jakob Syndrome (CJS)
Viral structure Viruses bear no resemblance to cells Lack protein-synthesizing machinery Viruses contain only the parts needed to invade and control a host cell
Virus Structure The viral capsid is the protein shell of a virus. The capsid encloses the viral genome The capsid delivers the viral genome into the host cell. Different viruses make different capsid forms.
Capsids All viruses have capsids (protein coats that enclose and protect their nucleic acid) The capsid together with the nucleic acid is the nucleocapsid Each capsid is made of identical protein subunits called capsomers Some viruses have an external covering called an envelope; those lacking an envelope are naked
Structural types of capsids Helical Rod or thread-like continuous helix of capsomers forming a cylindrical nucleocapsid
Structural types of capsids Icosahedral - Polyhedral with 20 identical triangular faces Have a structure that exhibits rotational symmetry
Dr.Stepehen Fuller Envelope In some icosahedral viruses, the capsid is enclosed in an envelope, formed from the cell membrane. The envelope contains glycoprotein spikes, which are encoded by the virus. Spikes are essential for attachment of the virus to the host cell Between the envelope and capsid, tegument proteins may be found. Viruses lacking an envelope are naked
Functions of Capsid 1. Protect genome from atmosphere (May include damaging UV-light, shearing forces, nucleases either leaked or secreted by cells). 2. Virus-attachment protein- interacts with cellular receptor to initiate infection. 3. Delivery of genome in infectious form. May simply dump genome into cytoplasm (most +ssrna viruses) or serve as the core for replication (retroviruses and rotaviruses).
Viruses with complex structures These have complex multipart structures T4 bacteriophages: Have an icosahedral head and helical neck Poxviruses lack a typical capsid and are covered by a dense layer of lipoproteins
Types of Viruses
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
DNA viruses Nucleic Acids 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 negativesense RNA
Viral enzymes Pre-formed enzymes may be present Polymerases DNA or RNA Replicases copy RNA Reverse transcriptase synthesis of DNA from RNA (AIDS virus)
Exit and transmission Bacteriophage replication All viruses require a host cell for reproduction. Thus, they all face the same needs for host infection: Host recognition and attachment Genome entry Assembly of progeny virions
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
Steps in phage replication 1. Adsorption binding of virus to specific molecules on host cell 2. Penetration genome enters host cell 3. Replication viral components are produced 4. Assembly viral components are assembled 5. Maturation completion of viral formation 6. Lysis & Release viruses leave the cell to infect other cells
Bacteriophages attach to host cells Contact and attachment are mediated by cell-surface receptors. Proteins that are specific to the host species and which bind to a specific viral component. Bacterial cell receptors are normally used for important functions for the host cell. Example: sugar uptake
Phage reproduction within host cells Most bacteriophages (phages) inject only their genome into a cell through the cell envelope. The phage capsid remains outside, attached to the cell surface. It is termed a ghost.
Bacteriophages can undergo two different types of life cycles 1) Lytic cycle Bacteriophage quickly replicates, killing host cell 2) Lysogenic cycle Bacteriophage is quiescent. Integrates into cell chromosome, as a prophage Can reactivate to become lytic The decision between the two cycles is dictated by environmental cues In general, events that threaten host cell survival trigger a lytic burst
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
Multiplication of Bacteriophage
Animal Virus replication cycles The primary factor determining the life cycle of an animal virus is the form of its genome. DNA viruses Can utilize the host replication machinery RNA viruses Use an RNA-dependent RNA-polymerase to transcribe their mrna Retroviruses Use a reverse transcriptase to copy their genomic (RNA) sequence into DNA for insertion in the host chromosome
Modes of animal viral multiplication General phases in animal virus multiplication cycle: 1.Adsorption binding of virus to specific molecules on the host cell 2.Penetration genome enters the host cell 3.Uncoating the viral nucleic acid is released from the capsid 4.Synthesis viral components are produced 5.Assembly new viral particles are constructed 6.Release assembled viruses are released by budding (exocytosis) or cell lysis
Animal viruses show tissue tropism Animal viruses bind specific receptor proteins on their host cell. Receptors determine the viral tropism. Ebola virus exhibits broad tropism, infecting many kinds of host tissues. Papillomavirus shows tropism for only epithelial tissues. Most animal viruses enter host as virions. Internalized virions undergo uncoating, where genome is released from its capsid.
Adsorption and host range Virus coincidentally collides with a susceptible host cell and adsorbs specifically to receptor sites on the 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 Envelope spike Host cell membrane Capsid spike Receptor Host cell membrane Receptor
Penetration/Uncoating Flexible cell membrane is penetrated by the whole virus 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
Variety in Penetration and Uncoating
Replication and protein production Varies depending on whether the virus is a DNA or RNA virus DNA viruses generally replicate and assemble in the nucleus RNA viruses generally replicate and assemble in the cytoplasm Positive-sense RNA contain the message for translation Negative-sense RNA must be converted into positive-sense message
Papillomavirus (DNA) Life Cycle HPV, a double-stranded DNA virus, enters the cytoplasm, where the protein coat disintegrates. The viral DNA enters the nucleus for replication and transcription by host polymerases. Viral mrna returns to the cytoplasm for translation of capsid proteins, which return to the nucleus for assembly of virions.
Picornavirus (RNA) Life Cycle Picornavirus life cycle. A picornavirus inserts its (+) strand RNA into the cell. Reproduction occurs entirely in the cytoplasm. A key step is the early translation of a viral gene to make RNA-dependent RNA polymerase. The polymerase uses the picornavirus RNA template to make ( ) strand RNA, which then serves as a template for other viral mrnas, as well as progeny genomic RNA, which is replicated in virus-induced vesicles from the endoplasmic reticulum.
HIV (Retrovirus) Life Cycle
Release Assembled viruses leave the 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
Damage to host cell Cytopathic effects - virus-induced damage to cells 1. Changes in size and shape 2. Cytoplasmic inclusion bodies 3. Inclusion bodies 4. Cells fuse to form multinucleated cells 5. Cell lysis 6. Alter DNA 7. Transform cells into cancerous cells
Effects of some human viruses
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
Viral damage Some animal viruses enter the host cell and permanently alter its genetic material resulting in cancer transformation of the cell Transformed cells have an increased rate of growth, alterations in chromosomes, and the capacity to divide for indefinite time periods resulting in tumors Mammalian viruses capable of initiating tumors are called oncoviruses or oncogenic viruses Papillomavirus cervical cancer Epstein-Barr virus Burkitt s lymphoma Hepatitis C virus Liver cancer
Oncogenic viruses Oncogenic viruses transform the host cell to become cancerous. Mechanisms of oncogenesis include: 1) Insertion of an oncogene into the host genome 2) Integration of the entire viral genome 3) Expression of viral proteins that interfere with host cell cycle regulation
Techniques in cultivating and identifying animal viruses Methods used: Cell (tissue) cultures cultured cells grow in sheets that support viral replication and permit observation for cytopathic effects Bird embryos incubating egg is an ideal system; virus is injected through the shell Live animal inoculation occasionally used when necessary
Methods for growing viruses
Tissue culture of animal viruses Animal viruses can be cultured within whole animals by serial inoculation Ensures that the virus strain maintains its original virulence, but process is expensive and laborious. They can also be grown in human cell tissue culture.
Plaque assay of bacteriophages
Plaque assay of animal viruses
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
Detection and treatment of animal viral infections More difficult than other agents Consider overall clinical picture Take appropriate sample Infect cell culture look for characteristic cytopathic effects Screen for parts of the virus Screen for immune response to virus (antibodies) Antiviral drugs can cause serious side effects
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