Reproduction And Transmission In Viruses

Viruses do not have their own metabolic activity and, hence, use cellular ATP, ribosomes, tRNA and certain biosynthetic products of the host cell. Thus, viruses multiply only inside the host cell. During multiplication, nucleic acid of the virus is replicated and the proteins required for the viral coat are also formed. The process of multiplication can be explained by bacteriophages with the following major steps:

• The phage attaches itself to the wall surface of the bacterium with the help of tail fibres and end plate.
• The tail sheath contracts and the phage DNA is injected into the bacterial cell. The empty protein coat, called ghost, remains attached to the bacterium for sometime.
• Phage DNA now takes over the protein synthesis machinery of bacterial host and inactivates the DNA of the bacterium.
• Phage DNA replicates and also synthesizes new proteins for its capsid. The protein coats are assembled spontaneously around phage DNA. This process continues till large numbers of phage particles are formed.
• Ultimately the bacterial cell bursts by the process called lysis and new phage particles (200 to 1000) are liberated. These particles are capable for infecting other bacteria of the same type.
• The entire process of multiplication takes about 30 minutes. It is called latent period.

The viruses are transmitted through various agencies, such as seeds, grafting, vegetative propagation, insects, mechanical means, nematodes, etc.

Plant diseases

A variety of symptoms are caused by viral infections. These include local lesions, clearing of veins, mosaic formation, ring spotting, chlorosis, distortion, necrosis, breaking of blossoms, stunting and premature defoliation.

Table : Viral diseases of plants

Disease Causal agent

Tobacco mosaic Tobacco Mosaic virus

Abutilon mosaic Abutilon mosaic virus

Papaya mosaic Papaya mosaic virus

Cucumber mosaic Cucumber mosaic virus

Potato mild mosaic Potato Virus X

Potato rugose mosaic Potato virus Y

Potato leaf curl Potato leaf curl virus

Potato leaf roll Potato leaf roll virus

Little leaf of brinjal Brinjal little leaf virus

Rosette of groundnut Groundnut mosaic virus

Bunchy top of Banana Banana bunchy top virus

Sugarcane mosaic Sugarcane (or Saccharum) Virus I

Animal disease

Diseases caused in mammals include small pox, chicken pox, poliomyelitis, influenza, measles, yellow fever, encephalitis, infectious hepatitis, viral bronchitis, viral enteritis, common cold, mumps, AIDS, etc.

Table : Viral diseases of man

Name of the Disease Causal Virus

Common cold Rhino viruses

Influenza Influenza virus

German measles (Rubella) Rubella virus

Measles Measles virus

Poliomyelitis (Polio) Polio virus

Small pox Variola virus

Yellow fever Arbovirus

Viruses are ultramicroscopic, non-cellular, infectious, non-cytoplasmic most primitive, filtrable, obligate parasites.

Smaller than bacteria and can pass through bacteria proof filters.

Devoid of most of the biosynthetic enzymes.

No growth outside host, no division.

Uses metabolic energy of host.

Contain only one type of nuclei acid either DNA or RNA.

Viruses are host specific because infect only a single species and definite cells of the host.

Transmissible from diseased to healthy organisms.

Effective in very small doses. Most of them are highly resistant to germicides and extreme physical conditions.

Exception to the cell theory.

Genetic element remain enclosed inside a protein coat known as capsid.

Simplest known form of life.

Connecting link between living and non-living.

General structure of viruses

Shape and Size

The shape of different types of viruses varies considerably. They may be spherical or golf ball like (poliovirus, herpes virus), rod shaped (TMV), tadpole like (bacteriophages), helical (influenza virus) polyhedral (adenovirus).

The size ranges from 10 nm to 300 nm.

Plant viruses, in general, are smaller than bacterial or animal viruses.

Chemical structure and composition

Viruses are made up of nucleic acid core enclosed inside a protein coat.

Nucleic acid

Nucleic acid can be either DNA or RNA. The nucleic acid may occur as single or double stranded.

Virus Type of nucleic acid Virus Type of nucleic acid

Herpes Double stranded DNA Measles Single stranded RNA

Chicken pox Double stranded DNA Mumps Single stranded RNA

Hepatitis B Double stranded DNA Polio Single stranded RNA

Cyanophages Double stranded DNA TMV Single stranded RNA

Influenza virusSingle stranded RNA Mycophages Double stranded RNA

Rabies Single stranded RNA Reovirus Double stranded RNA

HIV Single stranded RNA Wound tumour virus Double stranded RNA              

Capsid or Protein coat

The protein coat is called capsid. It is made up of many identical sub units called capsomeres. The capsomeres are composed of either one or several types of proteins.

Host specificity of viruses is due to the proteins of the capsid.

In a virus particle, the capsomeres are arranged in a very symmetrical manner and give a specific shape to a particular virus.

Some large virus particles (i.e., virions) have an additional covering of lipids or lipoproteins outside the capsid. Such virions are called enveloped (e.g., influenza virus, mumps virus) and those without this additional covering are refered to as naked (e.g., TMV).

CRYPTOGRAM of viruses : Gibbs (1966) Gibbs & Harrison (1968) proposed a system of classification called cryptogram.

It is a code that has 4 pairs of symbols which give definite information in coded form on some properties of viruses.

Pair I : Denotes type of nucleic acid in virus [ R = RNA, D = DNA) / strands of nucleic acid [1 = single, 2 = double]

Pair II : Molecular weight of nucleic acid in millions/percentage of nucleic acid in virus [1 = 1 million and 1 percent, 2 = 2 million & 2 percent & so-on]

Pair III : Shape of the virus/shape of the capsid [S = spherical, E = Elongated, X = complex]

Pair IV : Kind of host/kind of vector [B = Bacterium, S = Seeded plants, I = Invertebrates, V = Vertebrates, F = fungus, O = No vector, X or = vector not known, Ap = aphid, Fu = fungus, Ne, nematode, TH = thrips etc.

Cryptogram of TMV :

R/1 : 2/5 : E / E : S / O

cryptogram of coliphage T4

D/2 : 130 / 40 : X / X : B / O

Viroids

Smallest known infectious agents (a subviral agents)

Infective nucleic acid entity.

They are naked RNA molecules of low molecular weight.

Cause disease only in cultivated higher plants like PSTD. (Spindle Tuber disease of potato).

• Chrysanthemum chlorotic mottle stunt.
• Pale fruit of tomato
• Bunchy top
• Citrus exocortis etc.

Viroid diseases are persistent infections i.e. there is no recovery of infected plants and  viroids can be isolated from the diseased planties long as plant survives.

Transmitted by mechanical means.

Discovered by Diener and Raymer. PSTD was first viroid.

RNA is short single stranded circular highly folded.

RNA of viroid has inverted base sequences at their ends and thus resemble retrovirus.

Prions

Proteinaceous infections particle sub viral entity devoid of their own genetic material.

Discovered by Alper et al (1966)

Stanley B. Prusiner : Studied isolated and identified the causal agent of Scrapie, Kuru and mad cow disease in 1982 and war awarded nobel prize for prions in 1997.

Prusiner called this infectitious agent as prions.

Produced by mutation in the normal prp gene of host DNA.

It produce abnormal glycoprotein called Prion protein (Prp)

Replicate very slowly (1-4 years) in the spleen & lymphatic system where they pass to brain & damage the cells.

Mad cow disease : [BSE or Bovine spongiform encephalopathy]

CJD [Creutz feldt Jakob disease]

KURU [laughing death]

Prions are resistant to nuclease, proteases radiations, disinfectants (phenol)

Virusoids

These are viroids (small circular RNA) linked with larger RNA of a true virus.

They replicate with in their host and do not cause infection.

Discovered by Randles.

Virion

An inert intact complete virus particle outside the host in cell free environment is called virion.

Pseudo virion

A pseudo virion contains host cell DNA instead of the viral genome. Such virions are formed during viral replication and they donot replicate.

Pasteur developed a vaccine against rabies. Joseph Miester was the first victim, who received antirabies vaccine.

Edward Jenner developed first successful vaccine against small pox by using cowpox virus.

Virucides : Most of the agents used as antiviral agents are pyrimidine analogue used against DNA virus infections.