CHAPTER 10

virophage is best descirbed as

A satellite virus that inhabits its helper virus (i.e., its host virus)

teratogen

A teratogen is an agent that induces defects during embryonic development Viruses can act as teratogens by crossing the placenta and infecting embryonic cells. The earlier in pregnancy an infection occurs, the more extensive damage is likely to be.

virusoid

Helper virus codes for its capsid

viruses that can induce defects during embryonic or neonatal development (teratogenesis) in humans are:

Herpes simplex virus types I and II Rubella Cytomegalovirus

Viruses that can remain latent (usually in neurons) for many years are most likely

Herpesviruses

viroids

Infectious RNA lacking a capsid

prion

Infectious, incorrectly folded protein

Lysogeny

Lysogeny, a stable, long-term relationship between certain phages and host bacteria, occurs in temperate phages. Temperate phage DNA can exist as a prophage or revert through induction to a lytic cycle

Emerging Viruses

Many emerging diseases are caused by viruses that had been endemic at low levels in localized areas, but have "jumped" species and acquired a new host range and spread; sometimes due to human activities as well—for example, colonizing previously uninhabited jungles.

Replication of Animal Viruses

On the surface of some viruses, proteins are used for attachment to host plasma membranes during adsorption; animal viruses thus gain entry into the cell. Uncoating (loss of the capsid) occurs at the plasma membrane or in the cytoplasm.

envelope

Surrounding lipid bilayer membrane

capsid

Surrounding protein coat

Nucleocapsid

Virion's genome together with capsid

Viroids

Viroids are very different from viruses; each viroid is solely a small RNA molecule. Viroids may cause plant diseases by interfering with mRNA processing.

Virophages

Virophages infect giant viruses, impairing their replication.

naked virus

Virus with a nucleocapsid but no envelope

CLASSIFICATION OF VIRUSES

Viruses are classified by the nucleic acid (DNA or RNA) they contain, other chemical and physical properties, their mode of replication, shape, and host range. Similar viruses are grouped into genera, and genera are grouped into families. Viruses that share the same genome and relationships with organisms generally constitute a viral species.

Prophages such as the lambda (λ) phage

insert into a bacterial chromosome at a specific location

Papovaviridae.

papilloma, polyoma, and vacuolating viruses small, naked, polyhedral dsDNA viruses

RNA Viruses (- Sense)

paramyxoviridae (measles, mumps, respiratory disorders), Rhabdoviridae (rabies); Orthomyxoviridae (influenza) Filoviridae (Marburg and Ebola), Arenaviridae (Lassa fever); Bunyaviridae (hantavirus pulmonary syndrome)

RNA VIRUSES (+ sense)

polio, hepatitis A, rhibovirus, togaviridaem flaviviridae, retroviridae (causes cancer and AIDS)

chemical component that is found in all viruses is:

protein

Herpesviridae

relatively large, enveloped viruses with linear dsDNA 100 types discovered. shingles, chickenpox can lay dormant for years *latency* herpes simplex, vacicella, cytomegalovirus, roseola, Epstein-Barr virus, Kaposi's sarcoma

Hepadnaviridae.

small, enveloped, mostly dsDNA (partially ssDNA) viruses hepatitis B

Parvoviridae

small, naked, linear ssDNA viruses Dependovirus, Parvovirus, and Erythrovirus No known human disease is associated with this genus only known parvovirus to infect humans (predominantly children) is Erythrovirus "fifth disease"

release

the final stage, is facilitated by the enzyme lysozyme. Burst time is the time from adsorption to release of progeny virions; burst size is the number of phage progeny released from one host cell.

retroviruses

(a)Retroviruses cause tumors and leukemia in rodents, birds, and humans. (b)Retroviruses cause acquired immune deficiency syndrome (AIDS) in humans. (c)Retroviruses contain reverse transcriptase to form a complementary strand of DNA, which is then replicated to form double-stranded DNA (dsDNA). (d)dsDNA must migrate to the cell nucleus and integrate into the chromosomes of the host, whereby it becomes a provirus.

Rhabdoviridae

(−) sense RNA virus group, the rhabdoviruses consists of medium-sized, enveloped viruses. Although these viruses have an envelope, the capsid is helical Rhabdoviridae virions contain an RNA-dependent RNA polymerase that uses the (−) sense strand to form a (+) sense strand. Human rabies almost always results from a bite by a rabid animal that is carrying the rabies virus. The Lago virus, which produces disease in bats, and the Mokolo virus, which infects shrews in Africa, are closely related to rabies viruses.

Key characteristic prions:

Ability to form fibrils

DNA Viruses

Adenoviridae (respiratory infections); Herpesviridae (oral and genital herpes, chickenpox, shingles, infectious mononucleosis), the Poxviridae (smallpox), Papovaviridae (warts, some cancers), Hepadnaviridae (hep B)

DNA viruses

Adenovirus Poxvirus Papovavirus Herpesvirus

LATENT VIRAL INFECTION

All herpesviruses have the ability to become latent, remaining dormant. Activation usually involves changes in cell-mediated immunity

Cell strain

All the cells in a cell strain, which are derived from sub-cultured primary cell cultures, are very similar.

Facts about cancer

An estimated 15% of human cancers arise from viral infections. Cancers can be caused by both RNA tumor viruses and DNA tumor viruses. Oncogenes are made up of DNA. Examples of human cancers believed to arise from viral infections include Kaposi's sarcoma, adult T cell leukemia/lymphoma, and cervical cancer.

plate and tail fibers: bacteriophage

Attach phage to specific receptor sites on the cell wall of a susceptible host bacterium

What type of viruses contain the enzyme lysozyme to aid in their infection?

Bacteriophages

Temperate phages

Bacteriophages that can enter into stable, long-term relationships with their hosts are called

benefits enveloped viruses

Because envelopes are acquired from and are therefore similar to host cell membranes, viruses may be "hidden" from attack by the host's immune system. Also, envelopes help viruses infect new cells by fusion of the envelope with the host's cell or plasma membrane.

Viruses and Cancer

Cancer is generally an uncontrolled and/or invasive growth of abnormal cells Tumors, or neoplasms, are benign (noncancerous) or malignant (cancerous). Malignant tumors spread by metastasis. Several animal viruses are thought to cause some forms of cancer, including the Epstein-Barr virus, certain human papillomaviruses, the hepatitis B virus, and some retroviruses, such as HTLV-1.

genome: bacteriophage

Carries the genetic information necessary for replication of new phage particles

satellite viruses

Code for their own capsid protein

virion

Complete virus particle, including envelope if it has one

A type of cell culture that can reproduce for an extended number of generations and is used to support viral replication is a:

Continuous cell line

DNA tumor viruses

DNA tumor viruses contain viral genes whose protein products disrupt the activities of normal host cell proteins that control cell division

Diploid fibroblast strains

Diploid fibroblast strains from primary cultures of fetal tissues produce stable cultures that can be maintained for years; they are used to produce vaccines.

The period of viral growth that occurs from the time of host cell penetration to the time when new virions are first assembled within the host cell is called

Eclipse period

The human virus that has been associated with Burkitt's lymphoma (a malignant tumor of the jaw) is:

Epstein-Barr virus

The replication of animal viruses differs from the replication of bacteriophages in what way?

Once in the host cell, animal viruses undergo a process of "uncoating" whereby the viral genome is separated from its protein coat by proteolytic enzymes; the viral genome in a bacteriophage is ready to go once injected into the bacterial host cell. Compared to bacteriophage replication, synthesis in animal virus replication can take much longer. In the penetration stage, bacteriophages produce lysozyme to weaken the bacterial cell wall and inject their DNA through the tail core into the bacterial cell, whereas animal viruses either fuse their envelope with the host's plasma membrane or enter by endocytosis. the maturation stage in animal enveloped viruses is longer than that of bacteriophage replication.

Oncogenes

Oncogenes are viral genes that cause host cells to divide uncontrollably.

Oncogenes in RNA tumor viruses

Oncogenes in RNA tumor viruses produce proteins in excessive amounts or produce proteins at the wrong times. In either case, infected host cells start uncontrolled cell division.

Lytic cycle

Phages carrying out these stages of replication, leading to host cell destruction, represent a lytic cycle of infection.

Bacteriophages are readily counted by the process of:

Plaque assays

Primary cell cultures

Primary cell cultures come directly from animals and are not subcultured.

Creutzfeldt-Jakob disease (CJD), kuru, scrapie, and mad cow disease are caused by:

Prions

Mammalian Prions

Prions are infectious particles made of protein. Research indicates that prions are normal proteins that become folded incorrectly. Prions cause neurological degenerative diseases, including Creutzfeldt-Jakob disease, kuru, scrapie, mad cow disease, and chronic wasting disease

spike

Projection made of glycoprotein that serves to attach virions to specific receptor sites

Proto-oncogenes

Proto-oncogenes are normal genes that, when under the control of a virus, act as oncogenes, causing uncontrolled cell division

Viruses ds RNA

Reoviridae (upper respiratory and gastrointestinal infections)

tail sheath: bacteriophage

Retracts so that the genome can move from the head into the host cell's cytoplasm

Satellites

Satellites are small RNA molecules unable to replicate without an unrelated helper virus. There are two types: satellite viruses that code for their own capsid protein, and satellite nucleic acids (= virusoids) whose helper virus encodes their capsid. Most satellites are associated with plant viruses.

Delta hepatitis virus

Similar to viroids and virusoids, a defective pathogen requiring the presence of hepatitis B virus for its replication

DNA vs RNA viruses

Synthesis and maturation differ in DNA and RNA viruses. In most DNA viruses, DNA is synthesized in an orderly sequence in the nucleus, and proteins are synthesized in the cytoplasm of the host cell. In RNA viruses, RNA can act as a template for protein synthesis, for making mRNA, or for making DNA by reverse transcription. Virions are assembled in the cell; sometimes viral DNA is incorporated as a provirus into the host cell chromosome.

T-even phages

T-even phages have recognition factors that attach to specific receptors on bacterial cell walls during adsorption. Enzymes weaken the bacterial wall so viral nucleic acid can penetrate it.

Culturing Methods

The discovery of antibiotics to prevent bacterial contamination of chicken embryos and cell cultures and the use of trypsin to separate cells in culture systems into monolayers provided an important impetus to the study of virology.

growth curve of a phage

The growth curve of a phage includes an eclipse period (the time following penetration through biosynthesis) and a latent period (the time after penetration up to release

plaques

The number of phages produced in an infection can be determined by counting the number of plaques produced on a plate of virus-infected bacteria (plaque assay). Each plaque represents a plaque-forming unit.

Retroviridae

The retroviruses are enveloped viruses that have two complete copies of (+) sense RNA They also contain the enzyme reverse transcriptase, which uses the viral RNA to form a complementary strand of DNA, which is then replicated to form a dsDNA. This reaction is exactly the reverse of the typical transcription step (DNA → RNA) in protein synthesis. the human retroviruses invade immune defense cells called T lymphocytes and are referred to as human T cell leukemia viruses (HTLV). Both HTLV-1 and HTLV-2 are associated with malignancies (leukemia and other tumors), whereas the human immunodeficiency virus (HIV-1 and HIV-2 strains) causes acquired immune deficiency syndrome (AIDS).

rubella virus

The rubella virus can be responsible for the death of fetuses and severe birth defects in others; cytomegaloviruses and occasionally herpesviruses also act as teratogens.

cytopathic effect (CPE).

The visible effects that viruses produce in infected host cells are collectively called the cytopathic effect (CPE).

Which of the following properties do viruses have in common with the bacterial section containing Rickettsiae and Chlamydiae?

They are both obligate intracellular parasites.

COMPONENTS OF VIRUSES

Viruses consist of a nucleic acid core and a protein capsid. Some viruses also have a membranous envelope. Viral genetic information is contained in either DNA or RNA—not both. Capsids are made of subunits called capsomeres. A viral capsid and genome form a nucleocapsid. Such viruses are called naked viruses; those with a nucleocapsid surrounded by an envelope are enveloped viruses.

Viral Replication

Viruses generally go through five steps in the replication process: adsorption, penetration, synthesis, maturation, and release. These steps are somewhat different in bacteriophages and animal viruses.

ORIGINS OF VIRUSES

Viruses have arisen, and probably continue to arise, by multiple origins. Viruses act as agents of evolution by their participation in lateral gene transfer.

VIRUS SHAPES AND SIZES

Viruses have polyhedral, helical, binal, bullet, or complex shapes and vary in size from 20 to 300 nm in diameter.

HOST RANGE VIRUSES

Viruses vary in host range and viral specificity. Many viruses infect a specific kind of cell in a single host species; others infect several kinds of cells, several hosts, or both.

Yeast Prions

Yeast prions often act beneficially to the fungi they infect, by an epigenetic effect.

Replication steps

absorption penetration synthesis maturation release

flaviviruses

are enveloped, polyhedral, (+) sense RNA viruses that are transmitted by mosquitoes and ticks. The viruses produce a variety of encephalitides or fevers in humans. The yellow fever virus is a flavivirus Hepatitis C infection is also caused by a flavivirus

togaviruses

are small, enveloped, polyhedral, (+) sense RNA viruses that multiply in the cytoplasm of many mammalian and arthropod host cells. Togaviruses known as arthropodborne viruses are transmitted by mosquitoes and cause several kinds of encephalitis in humans and in horses. T he rubella virus, which causes German measles (rubella), is in this family but is not transmitted by arthropods; rather, it is spread person to person.

WHAT ARE VIRUSES

are submicroscopic obligate intracellular parasites—they replicate only inside a living host cell.

complex capsid

combination of helical and icosahedral shapes

RNA tumor viruses

contain viral genes used for neoplastic transformation and viral replication

drawbacks enveloped viruses

easily damaged; naked viruses are more resistant

Hantavirus

emerging virus in USA transmitted from rodent feces and urine to humans

Bunyaviridae.

enveloped, (−) sense RNA viruses whose genome has three segments transmitted by arthropods, but rodents typically are the principal host hantavirus responsible for hantavirus pulmonary syndrome

Arenaviridae

enveloped, (−) sense RNA viruses, but their genome has only two segments Arenaviruses are carried by rodents Argentinean and Bolivian hemorrhagic fevers and Lassa fever are arenavirus infections

Filoviridae

enveloped, filamentous, single (−) sense RNA viruses can be transmitted from person to person by close contact with blood, semen, or other secretions and by contaminated needles. responsible for Marburg and Ebola diseases

Poxviridae

enveloped, linear dsDNA cause skin lesions; smallpox, molluscum contagiosum, cowbox, monkeypox smallpox- eradicated

Bacteriophage replication

has been thoroughly studied in T-even phages, which are virulent phages.; heads, tails, and tail fibers

hepatoviruses

hepatitis A virus is transmitted via the fecaloral route, with disease arising from the ingestion of contaminated food or water. The major organ infected is the liver.

Enteroviruses

include the polioviruses These viruses are resistant to many chemical substances and can replicate in and pass through the digestive tract unharmed. Unless inactivated by host defense mechanisms, the viruses invade the blood and lymph, spreading throughout the body but especially into the nervous system. Poor sanitation increases the numbers of enteroviruses

Rhinovirus

includes more than 100 types of human rhinoviruses, is one of the genera of viruses responsible for the common cold. Human rhinoviruses do not cause digestive tract diseases because they cannot survive the acidic conditions in the stomach. Instead, they enter the body through the mucous membranes of the nasal passages and replicate in the epithelial cells of the upper respiratory tract.

Polyhedral viruses

many-sided; picornaviruses and adenoviruses

Phage therapy

may replace antibiotics

Orthomyxoviridae

medium-sized, enveloped, (−) sense RNA viruses that vary in shape from spherical to helical have an affinity for mucus. Influenza virus

paramyxoviruses

medium-sized, enveloped, (−) sense RNA viruses, with a helical nucleocapsid. Different genera of paramyxoviruses are responsible for mumps, measles, viral pneumonia, and bronchitis in children and mild upper respiratory infections in young adults.

Adenoviridae.

medium-sized, naked viruses with linear dsDNA the virus appears in the blood and a measles-like rash may develop.

Reoviridae

naked, polyhedral capsid medium-sized dsRNA viruses. rotaviruses are the most common cause of severe diarrhea in infants and in young children under age 2. They also are responsible for minor upper respiratory and gastrointestinal infections in adults.

Picornaviridae.

naked, polyhedral, (+) sense RNA viruses. They include more than 150 species that cause disease in humans. After infection these viruses quickly interrupt all functions of DNA and RNA in the host cell. The Picornaviridae are divided into several groups, including the genera Enterovirus, Hepatovirus, and Rhinovirus.

maturation stage

the viral components are assembled into complete virions

Unlike positive (+) strand RNA viruses, what must negative (−) strand RNA viruses package within virions in order to make mRNA upon infection of a new host cells?

transcriptases

biosynthesis

viral DNA directs the making of viral components.