Viruses

How Are Viruses Studied?

"Virology" is the study of viruses; scientists who study viruses are called virologists. Virologists use: electron microscopes to visualize viruses; cell cultures to grow viruses (although some viruses will only grow in animals) techniques from molecular biology, genetics, and immunology; proper safety precautions when studying dangerous viruses.

Etymology

"Virus" is the Latin word for poisonous slime or ooze. Today:"infect" and interfere with software functions. Different names are associated : Vector: viral genome that is engineered to serve as a tool to replicate and express genes. Virion: the complete infectious virus particle

General Phases of Viral Multiplication in Animals

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. This is how viruses acquire the envelope around their nucleocapsids.

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

Virus Families

Animal viruses are categorized according to nucleic acid, capsid, and presence or absence of envelope. 7 DNA families, 15 RNA families DNA viruses causing human disease: enveloped DNA viruses nonenveloped DNA viruses nonenveloped ssDNA viruses

Viruses in Human Infections and Diseases

DNA or RNA molecules are surrounded by capsid; obligate parasites that enter a cell, instruct its genetic and molecular machinery to produce and release new viruses. All DNA viruses are double-stranded except for parvoviruses, which have ssDNA. All RNA viruses are single-stranded except for dsRNA reoviruses.

capsid structure

Depending on capsid shape and arrangement results in 2 types: Helical and Icosahedral. Nucelic Acid Core - can be DNA or RNA

envelope

Envelope is more supple than the capsid Therefore, enveloped viruses are pleomorphic and range from spherical to filamentous. Envelope protects nucleic acid from the effects of enzymes and chemicals when the virus is outside the host cell. Responsible for introducing viral DNA or RNA into host cell.

How Were Viruses First Discovered

In 1892, Russian scientist Dimitri Ivanowsky showed that tobacco mosaic disease was caused by an agent smaller than any known bacteria. In 1898, Dutch scientist Martinus Beijerinck realized that the agent was distinct from bacteria and termed it a virus. The first animal virus, the causative agent of foot-and-mouth-disease, was discovered in 1898 by German scientists Friedrich Loeffler and Paul Frosch. Key to the discovery of plant and animal viruses was the demonstration that viruses pass through filters that retain bacteria and require a host cell to reproduce.

What Organisms and Host Cells Do Viruses Infect?

Infection by viruses- viruses infect bacteria, plants, animals and other living organisms in order to reproduce. a given virus usually infects a limited number of species. within a host organism, usually only a limited number of cell types are susceptible to infection by a given virus Host range- array of host cells that a particular virus can infect. determined by a "lock-and-key" fit between the virus and a receptor on the cell membrane of a host cell

History of viruses

Invasions, wars, and immigrations can introduce viral diseases to different populations Some viral infections have had major global effects: Influenza type A is believed to be responsible for global flu outbreaks in 1918, 1957 and 1968 16th century Spanish explorers brought small pox, measles and influenza viruses to the New World

Heat

Most widely used physical methods of control Heat breaks the atomic bonds of organic molecules (proteins, lipids, nucleic acids) Heat destroys all components of the virus

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 phage genome to all host cell progeny - lysogeny. Induction can occur resulting in activation of lysogenic prophage followed by viral replication and cell lysis.

Ultraviolet radiation

Penetrates the capsid and envelope altering the chemical structure of cytosine and thymine preventing viral replication.

Heavy Metals

React with the protein in the capsid to disrupt its structure, destroying the virus Silver nitrate Mercury (II) chloride

Formaldehyde

Reacts with free amine groups on the nucleotide bases of RNA and DNA preventing nucleic acid activity

teratogenic.

Several viruses can cross the placenta causing developmental disturbances and permanent birth defects

virus size

Smallest living entity capable of reproduction (within a host cell)

Structural Biology

The key to the success in the battle against viruses is understanding different viral mechanisms. Scientists study : Viral structure Viral life cycle Mechanisms of cell infection Viral replication Host cellular proteins needed by the virus (used as antiviral targets)

CAPSID

The protective outer shell Made of capsomers Capsomers - identical subunits made of protein molecules

How Do New Viruses Emerge?

Through mutation and evolution By spreading from one species to a new host species By spreading from an isolated location to more widespread locations Through the detection of previously unrecognized viruses

Retroviruses

Transcription is the process of transferring the genetic information within a strand of DNA to a strand of messenger RNA. This is the case for all DNA viruses Some RNA viruses have enzymes that permit the transfer of genetic information from RNA to DNA (in reverse) These viruses are called retroviruses The enzyme utilized for this process is reverse transcriptase. Retroviruses are unique for their ability to remain with the host cell for relatively long periods. A substantial period of time (days, months, or years) called latency, may pass before the synthesis of viral RNA is activated. During the latency period the infected individual experiences no symptoms or disease. One form of human leukemia is related to a retrovirus The best known retrovirus, the human immunodeficiency virus (HIV) causes AIDS

Viruses In Medicine

Vaccines against viruses enhance the immune response of the host Immune cells are exposed to viral proteins or deactivated virus Immune system creates specific memories Immune system is enhanced and "ready" to defend against the virus Scientists are investigating methods that use viruses for treatment of diseases such as cancer and Alzheimer's disease.

alchohol

Viral envelopes are composed of lipids which are soluble in alcohol. Exposure to alcohol destroys the envelope and inactivates the virus

diseases

Viral infections generally behave in two ways: Viruses reproduce and cause disease immediately (active or lytic). Viruses integrate into the host cell and alternate between periods of activity with symptoms and inactivity without symptoms (dormant or latent).

Are There Infectious Agents Simpler Than Viruses?

Viroids and prions are even simpler than viruses. Viroids have genetic material but no protein coat. Prions are made of protein but have no nucleic acid. Like viruses, viroids and prions are not made of cells. Viroids and prions can cause disease.

What Are Viruses Made Of?

Viruses are composed of nucleic acid, proteins, and sometimes, lipids. Nucleic acid, which can be either DNA or RNA, encodes the genetic information to make virus copies. The nucleic acid is surrounded by a protective protein coat, called a capsid, which is made up of protein subunits called capsomers. An outer membranous layer, called an envelope, made of lipid and protein, surrounds the capsid in some viruses.

Medical Considerations in Viral Diseases

Viruses are limited to a particular host or cell type. Most DNA viruses are budded off the nucleus. Most RNA viruses multiply in and are released from the cytoplasm. Viral infections range from asymptomatic to mild to life-threatening. Many viruses are strictly human in origin, others are zoonoses transmitted by vectors. Many viral infections have rapid course; lytic cycle Some viruses establish long-term persistent infections that last many years or a lifetime.

How Do Viruses Differ From Living Organisms?

Viruses are not living organisms because they are incapable of carrying out all life processes. Viruses are not made of cells can not reproduce on their own do not grow or undergo division do not transform energy lack machinery for protein synthesis

Inactivation of Viruses

Viruses are susceptible to many of the physical and chemical agents routinely used by microbiologists These agents operate on three structures of the virus: Capsid, Nucleic acid, andEnvelope (when present) Inactivated viruses cannot replicate In the scientific sense, viruses are inactivated, not killed.

How Do Viruses Reproduce?

Viruses reproduce via three basic steps. 1. Viruses deliver their genomes into a host cell. 2. Viruses commandeer the host cell transcription and translation machineries and utilize host cell building blocks to copy viral genomes and synthesize viral proteins. 3. Viral genomes and proteins are self-assembled and exit host cells as new infectious particles. Details of each of these steps vary among different virus groups

Icosahedral

a 3 dimensional, 20 sided figure with 12 evenly spaced corners Capsomer shape varies Shape of major virus families. Nucelic Acid strand is packed into the center of the icosahedron, forming a nucleocapsid

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.

2 types of persistent infections

chronic infections - virus is detectable in tissue samples, multiplying at a slow rate; symptoms mild or absent latent infections - after a lytic cycle, virus enters a dormant phase; generally not detectable, no symptoms; can reactivate and result in recurrent infections. Some persistent viruses are oncogenic (causes cancer).

Body defenses

combined action of interferon, antibodies and cytotoxic T cells; frequently results in lifelong immunity

helical capsid

have rod-shaped capsomers that bond together resembling a bracelet. Nucleic Acid strand is coiled

Depend on

host to reproduce (parasites)

Course of viral disease

invasion at portal of entry and primary infection; some viruses replicate locally, others enter the circulation and infect other tissues

what are viruses

microscopic disease causing agents. By understanding mechanisms of virus action, it is possible to control or minimize negative effects of viruses. Discovered in the 1890's. Can be seen with an electron microscope

Common manifestations

rashes, fever, muscle aches, respiratory involvement, swollen lymph nodes

Viruses are

very small (10 - 400nm). not cells. a genome surrounded by a protective protein coat. infectious. dependent on a host cell to provide the machinery they need to reproduce. obligate intracellular parasites. formed from the assembly of newly synthesized components made in a host cell. can infect bacteria, protozoa, fungi, algae, plants, and animals

Cytopathic effects

virus-induced damage to cells Changes in size & shape Cytoplasmic inclusion bodies Nuclear inclusion bodies Cells fuse to form multinucleated cells. Cell lysis Alter DNA Transform cells into cancerous cells

Phenol

Combines with protein in the capsid altering its structure and inactivating the virus Rarely used in its pure form

How Are Viruses Classified?

Genetic material-DNA viruses contain DNA as their genetic material and generally multiply in the host cell's nucleus. RNA viruses contain RNA as their genetic material and generally multiply in the host cell's cytoplasm. Capsid structure- Helical (rod-shaped), Polyhedral, Icosohedral, Complex. Presence or absence of a membranous envelope surrounding the capsid

How Can Viral Diseases Be Prevented and Treated?

Good hygiene Avoid contact with contaminated food, water, fecal material or body fluids. Wash hands frequently. Vaccines Stimulate natural defenses with in the body. Contain a component of or a weakened or killed virus particles. Are developed for many once common illnesses such as smallpox, polio, mumps, chicken pox. Not available for all viruses. Anti-viral drugs (but not antibiotics) Available for only a few viruses. Inhibit some virus development and/or relieve symptoms.