VIROLOGY Viruses: Structure, Genetics, Culture and Diseases

VIROLOGY

Viruses: Structure, Genetics, Culture and Diseases.

An illustration From T. MADHAVAN, M.Sc., M.L.I.S., M.Ed., M.Phil., P.G.D.C.A., Lecturer in Zoology..

a power . presentation from T. MADHAVAN, & K. CHANDRASEKARAN., Lecturers in Zoology.

this session optimized for XI std An Illustration for the students who aspire for medical studies.

Viruses.smaller than the bacteria [ranges from 20-300 nm].

incapable of independent growth in artificial media.

can grow only in plant or animal cells or in microorganisms, referred as obligate intracellular parasites reproduce in these cells by replication.

Depend on host cells to carry out these vital functions.

Bacterial viruses are called bacteriophages.

Bacteriophages infect the bacteria and multiply within the bacterial

body and cause the lysis of bacteria [lytic cycle] or integrate themselves with the bacterial genome [lysogeny]




Viruses: Structure.

A virus contains only one type of nucleic acid which is either DNA or RNA. It can be double- stranded DNA, single- stranded DNA, double-stranded RNA or

single- stranded RNA.The nucleic acid can be circular or linear.The central core of nucleic acids surrounded by a protein covering called capsid




The central core of nucleic acids surrounded by a protein covering called capsid which is made up of units called capsomeres.

Proteins making up the capsomere is determined by the viral nucleic acid.

A virus may also be covered by an envelope composed of lipids, proteins and carbohydrates.

The envelope may also have spikes projecting from the surface which is again determined by the nucleic acid.

capsid




Symmetry in viruses

Viruses exhibit a characteristic symmetry.

Spherical viruses are isohedral. {Adeno viruses SV15, polio viruses and blue tongued viruses, made up of lippoproteins }

Rod shaped are helical. {TMV and Animal viruses that cause measles, mumps, rabies and influenza, packed within fringed lipoprotein, glycoprotein envelopes}

Certain groups of viruses are complex in symmetry. {Seen in Pox viruses, T–bacteriophages, these have different proteins and lipoproteins }







The general life cycle of a virus can be described by: Recognition of Host Genetic Material Enters Host Replication Using Host Nucleotides Protein Synthesis Using Host Enzymes, Ribosomes, tRNA, ATP Self-assembly of Capsids And Packaging of Genome Release From Host There are many variations of this cycle depending on

the type of virus and the host.

General Lifecycle

Viral Genetics.







Viral genome contains all the genetic information either in RNA or DNA, but never both.

Virions contain only a single copy of the nucleic acid.

Retro viruses are diploid having two identical single stranded RNA genomes.

Structure of the nucleic acid in the virion may be either linear or circular.




Culture of Animal Viruses.

Viruses can grow only in living cells.

Chicken Embryo technique is commonly employed for culture.

The yolk sac is a general ideal medium for the growth of viruses.

Viral cultures are 3 types:

1. Primary cell culture,

2. Diploid cell strains,

3. Continuous cell lines.




Primary cell culture are derived from normal tissue of an animal such as mouse, hamster, chicken and monkey or human being.Cells from these tissues are processed and cultured the first monolayer is referred to as Primary cell culture.A monolayer is confluent layer of cells covering the surface of a culture vessel.

Diploid cell strains are derived by primary cell cultures from the specific tissues like kidney or lung which is of embryonic origin.These diploid cells are the most employed host of choice for the production of human vaccines.

Continuous cell lines are capable of an infinite number of doublings. Such cell lines may arise with the mutation of a cell strain orFrom the established cell cultures from malignant tissue.Many viruses, which are difficult or impossible to grow have been cultured in continuous cell lines.




Specimen Virus1

Blood CMV, enteroviruses2,3, HSV3, VZV3

CSF and CNS tissues Enteroviruses, mumps virus, HSV, CMV

Dermal lesions HSV, VZV, adenovirus, enteroviruses

Eye HSV, VZV, adenovirus, enteroviruses, CMV

Genital HSV, CMV

Mucosal HSV, VZV

Oral HSV, VZV

Rectal HSV, VZV, enterovirus

Respiratory tract

upper Adenovirus, rhinovirus, influenza, parainfluenza, enteroviruses, RSV, reovirus, HSV

lower Adenovirus, influenza, parainfluenza, RSV, CMV4

Stool Enteroviruses, adenoviruses

Tissues CMV, HSV, enteroviruses

Urine CMV, adenovirus, enteroviruses, mumps

Abbreviations:

HSV - herpes simplex virus

CMV - cytomegalovirus

VZV - varicella-zoster virus

RSV - respiratory syncytial virus

2Enteroviruses: cox sackie virus, poliovirus, echovirus, and enterovirus.

3Rarely isolated.

4Usually in immunocompromised hosts.

Viruses Typically Isolated From Clinical SpecimensViruses Typically Isolated From Clinical Specimens




Rabies virus belongs to the order Mononegavirales, viruses

with a nonsegmented, negative-stranded RNA genomes.

Within this group, viruses with a distinct "bullet" shape are

classified in the Rhabdoviridae family, which includes at least

three genera of animal viruses, Lyssavirus, Ephemerovirus,

and Vesiculovirus.

Rabies: Virus and Disease

Rabies virus is parasite of domestic and wild animals.

This virus belongs to the Rhabdo virus family.

Rabies virus is transmitted to human through a bite of an infected animal { Dogs, cats, bats are the mammalian sources for this virus}




The basic structure and composition of rabies virus

Rhabdoviruses are approximately 180 nm long and 75 nm wide. The rabies genome encodes five proteins:

nucleoprotein (N), phosphoprotein (P), matrix protein (M), glycoprotein (G) and polymerase (L).

All rhabdoviruses are have two major structural components: a helical ribonucleoprotein core (RNP) and a surrounding envelope.




Symptoms of Rabies disease

headache,

high fever,

alternating excitement and depression,

muscular spasms in throat and chest,

Hydrophobia etc.,Incubation period is about 3 to 8 weeks (It may vary).

Mortality rate from rabies is 100%

New rabies vaccine is both safe and immunogenic.




Cancer & Viruses

Viruses have been identified as one of the casuative agents for cancer or tumour.

Such oncogenic viruses are

Adeno viruses,

Polyoma viruses,

Simia virus 40 (SV40),

Epsteon-Bar virus (EBV) [herpes virus],

RNA sarcoma virus are oncogenic RNA viruses Rous sarcoma}.




Hepatitis -B.

HBV is a mostly double-stranded DNA virus in the Hepadnaviridae family.

HBV causes hepatitis in human and related virus in this family cause hepatitis in ducks, ground squirrels and woodchucks.

The HBV genome has four genes: pol, env, pre-core and X that respectively encode the viral DNA-polymerase, envelope protein, pre-core protein (which is processed to viral capsid) and protein X.

The function of protein X is not clear but it may be involved in the activation of host cell genes and the development of cancer.




Risk Factors for HBV Infection

Hepatitis B is endemic in parts of Asia.

HBV is transmitted horizontally by blood and blood

products and sexual transmission.

It is also transmitted vertically from mother to infant in

the pre-natal period.




POXA range of pox viruses cause febrile illnesses in man

and animals with a prominent vesicular rash.

The most prominent of them was smallpox virus (variola) which caused a severe disease in man but

which has now been eliminated by intensive vaccination.

Poxviruses are very large, brick-shaped viruses about 300 x 200 nm (the size of small bacteria).

They have a complex internal structure - a large double-stranded DNA genome

Smallpox belongs to the Orthopox virus genus.




SMALLPOX (the only disease that has been globally eradicated)

Source : only from close contact with infected persons. No animal reservoir or vector. No latency in man.

Spread : by close contact, eg. within households, via droplet infection of pharyngeal secretions (ulcers in pharynx).

Incubation : 10 - 12 days.




Major illness : Rather similar to severe chicken pox (varicella).

Abrupt onset of fever and prostration with a macular rash on the third day (head, limbs, hands and feet rather than trunk, including palms and soles).

Progressed to vesicles which become pustular, ulcerated, scabbed, healed with scarring ("pock marked face"). Also inside mouth. 16 to 30% mortality. Milder subtypes were also seen.




ChickenpoxChickenpox




Chickenpox is a common disease caused by the varicella-zoster virus (VZV) which is a part of the herpes virus

family spreads in the air through coughs or sneezes or through contact with fluid from inside the chickenpox blisters.

Chickenpox, which occurs most often in the late winter and early

spring, is very contagious.

A person usually has only one episode of chickenpox in his or

her lifetime.

But the virus that causes chickenpox can lie dormant within the body

and can cause a different type of skin eruption later in life called

shingles, also referred to as herpes-zoster.




http://plato.acadiau.ca/courses/biol/Microbiology/virus.htm

http://nsm1.utdallas.edu/bio/gonzalez/lecture/micro/virology.htm

Acknowledgement //

References…

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VIROLOGY Viruses: Structure, Genetics, Culture and Diseases