Viruses: Reproduction, Transmission, Viroids, and Prions

Reproduction in Viruses

The process of Reproduction in viruses is of two types

1. Phagic             

2. Pinocytic

A.Phagic: In phagic reproduction, the genetic material of the virus enters the host body. It is of two types

1. Lytic 2. Lysogenic

A.Lytic: In the lytic type of reproduction virus kills or breaks bacterial cells, it causes lysis of host cells i.e bacteria. The virus which causes lysis of the host cell is called virulent E,g T2 Phage.

B.Lysogenic: In the lysogenic, the virus DNA gets integrated into bacterial DNA. This condition is known as a prophage, As the virus DNA is integrated into bacterial DNA it does not do any harm to bacteria. This type of viruses is known as Template viruses e,g λ phage (lambda)

2. Pinocytic: In this type, there is the entry of complete virus inside the host cell except envelop, both genetic materials as well as capsid enters host cell but envelop will remain outside e,g HIV

How does a virus spread?

The virus moves from one host to another for its survival and in search of a suitable host. Some of the modes of the transmission of viruses are as

1. Transmission by vegetative propagation: When plants are grown vegetatively after becoming infected with a virus disease, the pathogen is passed down from generation to generation. Tubers, corms, bulbs, grafting, and cutting can all spread viruses.  

2. Transmission by friction and rubbing: The healthy plants get infected when their parts get rubbed into the infected parts of other plants.

3. Transmission through insects: Insects are the most common and potential agents of virus transmission. the insects transmitting virus disease are called vectors. Insects like whitefly, leafhopper aphids, scale insects, etc are the potent vectors that cause viral diseases.

4. Transmission through pollen grains: If flowers of healthy plants are pollinated with the pollens from infected plants they produce seeds that have virus infection such seeds on germination give rise to infected plants.

5. Transmission by infected agricultural tools: it is the most common method of the transmission of viruses. the tools when used for cutting, pruning, and weeding the plants get infected by viruses.

 Viroids and Prions

The sub-viral agents are those viruses that have a size less than viruses. E,g Viroids, and Prions.

1. Viroids: The Viroids were discovered by Diener in 1971. The Viroids are infectious RNA particles. The capsid protective covering is absent. The molecular weight of viroids is in the range of 115,00 – 130,000 daltons. It contains circular RNA particles and is the smallest pathogens known. The viroids cause diseases in plants i.e they are pathogenic to plants only. E,g PSTD – potato spindle tuber disease, Citrus exocortis, Cucumber pale yellow.

2. Prions: The word prion is derived from proteinaceous infection particles. The name prion was given by Stanley Prusiner in  1982. He received the Noble prize in 1987 for his work on prions. Prions are made of only proteins. They have no nucleic acids i.e RNA OR DNA is absent. PrP is the name given to the proteins that make up prions (prion proteins). These proteins are about 1/100 the size of a tiny virus and include about 250 amino acids.   Prions are thought to be normal proteins that have gotten misfolded, maybe as a result of mutation. The prions cause diseases only to animals. E,g Kuru disease in humans is called laughing death, scrapies disease in sheep. Bovine spongiform encephalopathy mad cow disease (BSE causes Neurodegenerative diseases).

Satellite viruses

A satellite Virus is called a parasite of a parasite. It contains nucleic acid. It is most common in plants but also found in mammals, arthropods, and depends on another virus for its replication. It is a single-stranded RNA sometimes it contains Double-stranded DNA e,g TNV (tobacco necrosis virus), Hepatitis D, Tabacco mosaic satellite virus.

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