chapter 6 viruses
interferon
Natural human chemical that inhibits viral replication; used therapeutically to combat viral infections and cancer.
satellite viruses
Other fascinating viruslike agents in human disease are defective forms called satellite viruses that are actually dependent on other viruses for replication. One remarkable example is the adeno-associated virus (AAV), so named because it was originally thought that it could only replicate in cells infected with adenovirus;
viruses called oncogenic and effect all sell is transformation.
Some animal viruses enter their host cell and permanently alter its genetic material, leading to cancer. Experts estimate that up to 20% of human cancers are caused by viruses. These viruses are termed oncogenic, and their effect on the cell is called transformation.
naked viruses
Viruses that consist of only a nucleocapsid are considered naked viruses (
provirus
When viral DNA is incorporated into the DNA of the host, it is called a provirus.
viruses contain either DNA or RNA but not both.
The sum total of the genetic information carried by any organism is known as its genome. So far, one biological constant is that the genetic information of living cells is carried by nucleic acids (DNA, RNA). Viruses, although not technically alive and definitely not cells, are no exception to this rule, but there is a significant difference. Unlike cells, which contain both DNA and RNA, viruses contain either DNA or RNA but not both.
when the RNA or DNA of a virus invade the cell they take control over the host cells synthetic and metobolic machinery.
The synthetic and replicative phases of animal viruses are highly regulated and extremely complex at the molecular level. The viral nucleic acid takes control over the host's synthetic and metabolic machinery.
To complete the cycle, assembled viruses leave their host in one of two ways
-Nonenveloped and complex viruses that reach maturation in the cell nucleus or cytoplasm are released when the cell lyses or ruptures. -Enveloped viruses are liberated by budding or exocytosis2 from the membranes of the cytoplasm, nucleus, endoplasmic reticulum, or vesicles. During this process, the nucleocapsid binds to the membrane, which curves completely around it and forms a small pouch. Pinching off the pouch releases the virus with its envelope
prions
A common feature of these conditions is the deposition of distinct protein fibrils in the brain tissue. Researchers have hypothesized that these fibrils are the agents of the disease and have named them prions
virion
A fully formed virus that is able to establish an infection in a host cell is often called a virion.
nucleocapsid
All viruses have a protein capsid, or shell, that surrounds the nucleic acid in the central core. Together, the capsid and the nucleic acid are referred to as the nucleocapsid
RNA or DNA
Although most viruses follow this same pattern, a few exhibit distinctive and exceptional forms. Notable examples are the parvoviruses, which contain single-stranded DNA, and reoviruses (a cause of respiratory and intestinal tract infections), which contain double-stranded RNA. In fact, viruses exhibit wide variety in how their RNA or DNA is configured. DNA viruses can have single-stranded (ss) or double-stranded (ds) DNA; the dsDNA can be arranged linearly or in ds circles. RNA viruses can be double-stranded but are more often single-stranded
lyses
An average-size E. coli cell can contain up to 200 new phage units at the end of this period. Eventually, the host cell becomes so packed with viruses that it lyses—splits open—thereby releasing the mature virions
direct fusion of the viral envelope with the host cell membrane
Another means of entry involves direct fusion of the viral envelope with the host cell membrane (as in influenza and mumps viruses) (figure 6.13b). In this form of penetration, the envelope merges directly with the cell membrane, thereby liberating the nucleocapsid into the cell's interior.
predominant one among scientists
Another viewpoint proposes that even though viruses do not exhibit most of the life processes of cells, they can direct them and thus are certainly more than inert and lifeless molecules. This view is the predominant one among scientists today.
cells need specific virus receptors to be invaded by virus.
Cells that lack compatible virus receptors are resistant to adsorption and invasion by that virus. This explains why, for example, human liver cells are not infected by the canine hepatitis virus and dog liver cells cannot host the human hepatitis A virus. It also explains why viruses usually have tissue specificities called tropisms (troh′ -pizmz) for certain cells in the body. The hepatitis B virus targets the liver, and the mumps virus targets salivary glands.
Cultures of animal cells usually exist in the primary or continuous form.
Continuous cell lines tend to have altered chromosome numbers, grow rapidly, and show changes in morphology; they can be continuously subcultured, provided they are routinely transferred to fresh nutrient medium.
Creutzfeldt-Jakob disease (CJD)
Creutzfeldt-Jakob disease (CJD) afflicts the central nervous system of humans and causes gradual degeneration and death. It is transmissible—but by an unknown mechanism. Several animals (sheep, mink, elk) are victims of similar transmissible diseases. Bovine spongiform encephalopathy (BSE), or "mad cow disease,
Cultures of animal cells usually exist in the primary or continuous form.
Cultures of animal cells usually exist in the primary or continuous form. Primary cell cultures are prepared by placing freshly isolated animal tissue in a growth medium. The cells undergo a series of mitotic divisions to produce a monolayer on the surface of the dish.
helical and icosahedral.
Depending on how the capsomers are shaped and arranged, this assembly results in two different types for animal viruses: helical and icosahedral.
capsomers ( make up the capsid structure that forms the virus wall.)
In general, each capsid is constructed from identical subunits called capsomers that are constructed from protein molecules. The capsomers spontaneously self-assemble into the finished capsid. Depending on how the capsomers are shaped and arranged, this assembly results in two different types for animal viruses: helical and icosahedral.
where the virus DNA and RNA are replicated
In general, the DNA viruses (except poxviruses) enter the host cell's nucleus and are replicated and assembled there. With few exceptions (such as retroviruses), RNA viruses are replicated and assembled in the cytoplasm.
penetration by endocytosis
In penetration by endocytosis (figure 6.13a), the entire virus is engulfed by the cell and enclosed in a vacuole or vesicle.
Some of these scientists argue that these organisms are closely enough related to each other, and distant enough from other viruses, that they constitute a fourth domain of life, or at the very least, a new order.
In the last 10 years, scientists have discovered a group of viruses that they call nucleocytoplasmic large DNA viruses (NCLDVs)
In vivo In vitro
In vivo means that incubations are carried out in lab animals or embryonic bird tissues (whole organisms). In vitro refers to the use of cell (or tissue) culture methods
adsorption or attachment of virus tocell wall.
Invasion begins when the virus encounters a susceptible host cell and adsorbs specifically to receptor sites on the cell membrane. (Note the difference between the words absorb, as in what paper towels do, and adsorb, which means "attach.") The membrane receptors that viruses attach to are usually glycoproteins the cell requires for its normal function. For example, the rabies virus binds to the acetylcholine receptor of nerve cells, and the human immunodeficiency virus (HIV) attaches to the CD4 protein on certain white blood cells
viruses bear no real resemblance to cells
It is important to realize that viruses bear no real resemblance to cells and that they lack any of the protein-synthesizing machinery found in even the simplest cells. Their molecular structure is composed of regular, repeating subunits that give rise to their crystalline appearance
envelope (not all viruses)
Members of many families of animal viruses possess an additional covering external to the capsid called an envelope, which is usually a modified piece of the host's cell membrane
virus
Microscopic, acellular agent composed of nucleic acid surrounded by a protein coat.
spongiform encephalopathies
Not all noncellular infectious agents have typical viral morphology. One group of unusual forms, even smaller and simpler than viruses, is implicated in chronic, persistent diseases in humans and animals. These diseases are called spongiform encephalopathies because the brain tissue removed from affected animals resembles a sponge
naming conventions for viruses
Note the naming conventions: Virus families are written with -viridae on the end of the name, and genera end with -virus.
induction
On occasion, in a process called induction, the prophage in a lysogenic cell will be activated and progress directly into viral replication and the lytic cycle. Lysogeny is a less deadly form of parasitism than the full lytic cycle and is thought to be an advancement that allows the virus to spread without killing the host.
viroids
Plants are also parasitized by viruslike agents called viroids that differ from ordinary viruses by being very small (about one-tenth the size of an average virus) and being composed of only naked strands of RNA, lacking a capsid or any other type of coating.
lysogeny
Special DNA phages, called temperate phages, while they can participate in a lytic phase, also have the ability to undergo adsorption and penetration into the bacterial host and not undergo replication or release immediately. Instead, the viral DNA enters an inactive prophage state, during which it is inserted into the bacterial chromosome. This viral DNA will be retained by the bacterial cell and copied during its normal cell division so that the cell's progeny will also have the temperate phage DNA
plaques
The areas where virus-infected cells have been destroyed show up as clear, well-defined patches in the cell sheet called plaques (see figure 6.23). Plaques are essentially the macroscopic manifestation of cytopathic effects
tobacco virus
The first substantial revelations about the unique characteristics of viruses occurred in the 1890s. First, D. Ivanovski and M. Beijerinck showed that a disease in tobacco was caused by a virus (tobacco mosaic virus).
The general phases in the life cycle of animal viruses are
The general phases in the life cycle of animal viruses are adsorption, penetration, uncoating, synthesis, assembly, and release from the host cell.
virus organization
The general plan of virus organization is the utmost in simplicity and compactness. Viruses contain only those parts needed to invade and control a host cell: an external coating and a core containing one or more nucleic acid strands of either DNA or RNA and, sometimes, one or two enzymes.
The length of the entire multiplication
The length of the entire multiplication cycle varies from 8 hours in polioviruses to 36 hours in some herpesviruses.
how to group viruses
The main criteria presently used to group viruses are structure, chemical composition, and similarities in genetic makeup.
The primary purposes of viral cultivation are
The primary purposes of viral cultivation are 1. to isolate and identify viruses in clinical specimens; 2. to prepare viruses for vaccines; and 3. to do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells.
helical capsids
The simpler helical capsids have rod-shaped capsomers that bond together to form a series of hollow discs resembling a bracelet. During the formation of the nucleocapsid, these discs link with other discs to form a continuous helix into which the nucleic acid strand is coiled
filterable virus
These early researchers found that when infectious fluids from host organisms were passed through porcelain filters designed to trap bacteria, the filtrate remained infectious. This result proved that an infection could be caused by a cell-free fluid containing agents smaller than bacteria and thus first introduced the concept of a filterable virus.
polymerases
They may come with preformed enzymes that are required for viral replication. Examples include polymerases (pol-im′ -ur-ace-uz) that synthesize DNA and RNA, and replicases that copy RNA.
transformed cells
Transformed cells have an increased rate of growth; alterations in chromosomes; changes in the cell's surface molecules; and the capacity to divide for an indefinite period, unlike normal animal cells. Mammalian viruses capable of initiating tumors are called oncoviruses. Some of these are DNA viruses such as papillomavirus
Viruses
Viruses are a unique group of biological entities known to infect every type of cell, including bacteria, algae, fungi, protozoa, plants, and animals, and are extremely abundant on our planet
cytopathic effects
Virus-induced damage to the cell that alters its microscopic appearance is termed cytopathic effects (CPEs).
Viruses are different
Viruses are different from their host cells in size, structure, behavior, and physiology. They are a type of obligate intracellular parasite that cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release quantities of new viruses
Complex capsids
While most viruses have capsids that are either icosahedral or helical, there is another category of capsid that is simply called complex. Complex capsids, found in the viruses that infect bacteria, may have multiple types of proteins and take shapes that are not symmetrical.
icosahedron
icosahedron (eye-koh-suh-hee′-drun)—a three-dimensional, 20-sided figure with 12 evenly spaced corners. The arrangements of the capsomers vary from one virus to another. Some viruses construct the capsid from a single type of capsomer, while others may contain several types of capsomers
the synthesis of new genome
the synthesis of new genomes and mRNAs for translation differs among the various types of RNA viruses. Note that the retroviruses turn their RNA genomes into DNA. This step is accomplished by a viral enzyme called reverse transcriptase and has important implications in infections with these viruses, one of which is HIV.
chronic latent stage
viruses remain in a chronic latent state,3 periodically becoming reactivated. Examples of this are herpes simplex viruses (cold sores and genital herpes) and herpes zoster virus (chickenpox and shingles). Both viruses can go into latency in nerve cells and later emerge under the influence of various stimuli to cause recurrent symptoms.