Other classes of viruses, Biology tutorial

The Order Mononegavirales:

Derivation of name is as follows:

'Mono' from the Greek word 'Monos' meaning single

 'Nega' from negative strand or negatively sensed RNA (genome)

'Virales' from viral orders that ends with the suffix - 'ales'

This order includes three viral families of eukaryotic viruses having linear, mono-segmented negatively sensed RNA genome. Three viral families are: The Filoviridae, Paramyxoviridae and Rhabdoviridae and their MW ranges from300 - 100 x 106; S20w = 550 - > 1000. Buoyant density in sucrose = 1. 18 - 120g/cm3 their genome MW = 3.5 - 7 × 106 and comprise approx 0.5 -20% of particle weight. Their Lipid content is approx 15-25% by weight and the composition is dependent on host cell. The carbohydrate is 3-6% by weight where known.


These are small infectious agents which cause different plants diseases. Viroids are encapsidated low molecular weight, covalently closed circular, single stranded infectious positively sensed RNAs. Non-denatural viroid molecules adopt extensive internal base pairing to provide rod-like structures that is 50nm long. The molecular weight = 80 - 122 x 10 S20w8 - 10; Tm in 10mM Na+ is 50 C; density in cesium sulphate is 1 .6g/cm3They comprises about 246 to cover 370 nucleotides. They are rich it G+C content except few members, with central conserved regions. They comprise highly base -paired rod like structure with unique properties.


These are nucleic acid molecules which depend for their multiplication on co-infection of the host cell with the helper virus. Satellite nucleic acids have no appreable sequence homology with the helper virus genome and are not a part of genome. Satellites are different and district nucleic acids from other types of dependent nucleic acid like sub-genomic nucleic acids like defective interfering and messenger it molecules; genome parts and transmission - faulty but independently replicating viruses.

Most reported satellites are related with plant viruses and these have been arbitrarily categorized in 4-kinds according to physical and messenger properties of satellite RNA. These are:

1. TYPE A: The RNA is large (>0.7kb) and encodes the capsid protein which found in satellite specific particles.

2. TYPE B: RNA is large (>0.7kb) and encodes thea non structural protein.

3. TYPE C: RNA is small (< 0.7kb), lacks important mRNA properties

4. TYPE D: RNA is small (<0.7kb), lacks mRNA activity and creates circular molecules in replication.

Viral replication:

Unique feature of virus multiplications is that soon after interaction with host cell, infecting virion is disrupted and its quantifiable infectivity lost. Stage of this growth is known as eclipse period. This duration depends on both particular virus and host cell, and ends with formation of first infectious progeny of virus particles. Viruses have evolved the variety of different strategies for achieving multiplication in their host cells. The replication procedure could be broken down in seven stages which are given below:

i) Absorption/Attachment:

This is the first step on virus infection, in which there is interaction of the virion with the specific receptor site on surface of the cell to be infected. Receptor molecules vary for different viruses. e. g. poliovirus is able to attach only to cells in CNS/ and intestinal tract of primates, HIV binds to CD4 receptor on cells of human immune system. Adsorption is best achieved at 37°c but could also occur at 4°c but very slow. Adsorption is also improved by presence of Magnesium (Mg2+) or Calcium (Ca2+) ions.

ii) Penetration/Entry:

Method of penetration of entry of viruses in host cell is complex in nature. It is achieved by receptor- mediated endocytosis with uptake of ingested virus particles inside endosomes. With syncytia producing viruses, it is by fusion of virus envelope with cell membrane. Penetration in some other node is well known.

iii) Uncoating:

Uncoating takes place concomitantly with or shortly after penetration. At this phase, there is physical separation of viral nucleic acid (or in some cases, internal nucleocapsid) from outer structural components of virion. Infectivity in parental virus is lost at this point. Uncoating is done enzymatically (from lysosome).

iv) Transcription:

At his phase, a specific mRNA should be transcribed from time viral nucleic acid for successful expression and duplication of genetic information included in viral genome. mRNA produced acts as replicative intermediate from viral genome. The procedure transcription could be performed by either host cell mechanism or virus-specified enzyme. Patterns of transcription may vary before (early) at and after (late) virus nucleic acid replication.

v) Synthesis of Viral components:

This is the phase where use host cell component to translate viral mRNA. All viruses specified macromolecules are synthesized in the highly organized sequences. Virus mRNA is translated on cell ribosomes to generate two kinds of virus-protein:

a) Structural proteins that make up virus particle.

b) Non-structural protein, mostly enzymes for virus genome replications. This kind of protein is not found in viral particle.

vi) Assemblage/Morphogenesis:

The newly synthesized viral genomes and capsid polypeptides assemble together to create progeny viruses. Icosahedral capsid can condense in absence of nucleic acid, while nucleocapsids of viruses with helical symmetry can't form without viral RNA. Assembly of viral proteins may occur in cytoplasm or (as in most enveloped viruses) at the plasma membrane. After assemblage of viruses viral structural and necessary proteins sub units, viral becomes mature and ready for liberation or release.

vii) Maturity and release:

On maturation, viral particles escape from cell in numerous ways but most significantly done in two ways:

a) Bursting out in cell thus killing the host cells.

b) Budding out of the host cell which doesn't essentially kill cell.

Mode of transmission viruses and diagnosis of viral infections:

Mode of transmission:

Viruses may be transmitted in the given ways:

i) Direct transmission: This is a person to person method of virus transmission. Main means of transmission comprise droplet or earosol infection (influenza measles), feacal-oral route (enteroviruses), sexual contact (HIV, Hepatitis B), hand-mouth, hand-hand, mouth-mouth and exchange of contaminated blood.

ii) Animal-Animal: Humans are generally the accidental host. It spreads by bites (rabies), earosols infections from rodents contaminated quarters.

Diagnosis of viral infection:

Good diagnostic virology depends on rapid communication between physician and laboratory and on the quality of specimens and information supplied to laboratory. Choice of methods for lab-diagnosis and confirmation of viral infection depends on illness.


All specimens should be safely contained for transport to lab and clearly labeled and must be accompanied by applicable information. Isolation of active virus needs proper collection of suitable specimens, their preservation, both en-route to and in laboratory, and inoculation in appropriate cell cultures, susceptible animals or embryonated eggs.

Neutralization test (Nt-test):

Virus neutralizing antibodies is estimated by adding serum having these antibodies to the suspension of virus and then inoculating mixture in specific cell cultures. Presence of neutralizing antibodies is shown if cell culture fails to develop CPE while control cell cultures that have received virus plus serum free of Neutralizing antibody develop CPE.

Control and treatment of viral diseases:

Interferons (IFN):

Interferons are host-coded proteins (non-toxic antiviral agent) which inhibit viral replication, they are generated by intact animals or cultured cells in reply to viral infection or other inducers. They are generated by all vertebrates' species but usually, normal cells don't generate or synthesize inferferoins except they are induced to do so. They are thought to be body's first line of defense against potent insult by viruses to cells. IFN modulates humoral and cellular immunity and have large cell growth regulatory activities. Interferons could also be induced by DS-RNA, bacteria endotoxin and small molecules like Tilorone.

RNA-viruses are stronger inducers of IFN than DNA-viruses. There are three-main kinds of IFN, Alpha, Beta and Gamma interferons. Alpha IFN and Beta-IFN are resistant to low pH. Beta-IFN and Gamma-IFN are glycosalated but sugars are not essential for biologic activity, so cloned IFN produced in bacteria are biologically active. Different classes of IFN are produced by different cell kinds. Alpha-IFN is synthesized mainly by leukocytes, Beta-IFN are produce mostly by fibroblasts whereas gamma IFN are created only by lymphocytes.

Mode of Action of IFN:

IFN are always host species - specific in function. By contrast, IFN-activity is not specific for the given virus the replication of the wide range of viruses can be inhibited. When IFN is added to cell before infection, there is marked inhibition of viral replication but almost normal cell function. IFN are very effective, so that very small amount is needed for function. It has been evaluated that fewer than 50 molecules of IFN per cell are adequate to induce antiviral states.

IFN act by binding to cell surface receptors, with alpha-lFN and beta-IFN sharing a common receptor and gamma recognizing the distinct receptor. This binding triggers synthesis of numerous enzymes thought to be instrumental in development of antiviral state. Mode of IFN action is from two points.

1. Degradation of Viral mRNA: This is essentially done by the enzyme, endonuclease. Enzyme is activated by presence of oligonuleotide synthetase, 2-5A synthetase both of which is required for oligoadenulic acid, 2 , 5-oligoadenulic acid, formation which in turn, degrades viral mRNA.

2. Inhibition of protein synthesis: The protein kinase phosporylates and inactivates cellular initiation factor, elF-2) and therefore prevents formation of initiation complex required for translation of viral proteins.

Uses of IFN:

1. They have been effective against (prevents) Rhinovirus when controlled intranasally.

2. They inhibit vaccinia infection when controlled intradermally.

3. Chronic active hepatitis because of hepatitis B-Virus could be also prevented but not dramatic effect and also herpes-keratitis Zoster and cytomegalovirus infections.

4. IFN has been shown to have important effect on numerous human tumors like , Sarcoma, breast cancer, lymphoma, myeloma.

Anti-viral drugs:

As viruses are obligate intracellular parasites, a good antiviral agent should be capable of selectively inhibiting viral functions without harming host cells. Seven antiviral drugs are presently licensed for use, like Acyclovir, doxuridine, Amantadine, Vidarabine, Trifluriridine, Ribavirin, and Azidothymidine. All these have one or more side effects on host therefore the ideal antiviral agents remain to be developed. Majority of available antiviral agents are nucleoside analogues. Analogues inhibit nucleic acid replication by inhibition of enzymes of the metabolic pathways for purines and pyrimidines or by inhibition of polymerases for nucleic acid replication.


Immunity to viral infection is based on development of the immune response to specific antigens situated on surface of virus-particles or virus infected cells. Vaccines are available for prevention of several important human diseases but certain general principles apply to most virus vaccine as for use in prevention of human disease.

1. Killed-Virus Vaccine: These are composed of purifying viral preparations to (certain extent and then inactivating viral infectivity in the way that performs minimal damage to viral structural proteins. This is done by treating virus with mild formalin. Killed virus vaccines prepared from the virion stimulate development of circulating antibody against viral protein coats; therefore conferring some degree of resistor ace. For some diseases, only this kind of vaccine is available for the prevention.

2. Live-Attenuated Virus Vaccine: Here viruses used are virus mutants which antigenically overlap with wild-type but are limited in some steps in pathogenesis of disease. Development of virus strains appropriate for live virus vaccines previously was done by selecting naturally attenuated strains or by cultivating virus serially in various hosts and cultures in the hope of deriving an attenuated strain fortuitously.

Future prospects in vaccination:

Molecular biology and modern technologies are combing to make new approaches to vaccine development.

(a) Attenuation of viruses by genetic mapping: This where exploitation is on recombinant or mutants which can be used as live virus vaccines. Introduction on deletion mutations which damage virus doesn't entirely inactivate it must yield the vaccine candidate not likely to revert to virulence.

(b) Use of the virulent virus vectors: Concept here is to use recombinant DNA methods to insert gene coding for protein of interest in the genome of the virulent virus which can be controlled as vaccine.

(c) Gene cloning for Protein Production: The ideal is to clone viral gene. Cloned DNA can then be expressed in prokaryotic and eukaryotic cells if suitably engineered constructions are used.

(d) Local Administered Vaccines: Intranasally administered aerosol vaccines are being developed, mainly for respiratory disease viruses and also for measles virus. It is hope that they will stimulate local antibody at portal of entry of viruses.

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