Viruses, Viriods and Prions

What are Viruses

Viruses are obligate absolute intracellular Parasite which enters the host cell and hijack all cellular mechanisms of the cell.

The term Virus is of Latin origin which means Toxin. They are prokaryotes (I.e. their genetic materials are not enclosed by a nuclear). Viruses was discovered in the late 1950 by Sergei Winogradesky and Sergie Beijerinck although, they were said to be in existence since 1890

Characteristics of a Virus 

 Virus possess the following characteristics features;

• Their size ranges from 20 – 400 nano meters
• They can’t be seen with the naked eye except with the aid of an electron microscope
• They require a host for survival
• They encyst outside a host (They can’t die but rather form cyst, inert chemicals or crystals)
• They are made up of Nucleic acid and protein capsid hence, they are called nucleocapsid
• They are prokaryotes (They lack a nuclear membrane)
• They are host specific
• They lack complex membrane bound organelles like mitochondrion, lysosome, Golgi body, endoplasmic reticulum etc.
• They can't generate energy for themselves because they lack mitochondrion, rather they depend on the host for energy
• They contain nucleic acid either as DNA or RNA but never both
• They have a mode of transmission unique to itself
• They can’t be grown in a cell free medium
• They possess only one characteristics of living things which is Replication (An incomplete reproduction)
• They possess only one organelle which is nucleic acid or chromosomes.

Classification of Virus

The criteria for classifying virus include:

1. Morphology as seen under the electron microscope morphology (Size, shape, structure & symmetry)
2. Biochemistry i.e. Electrophoresis (separation of the virus particles based on size and weight) and Serology (Antigen-Antibody reaction)
3. 
   

   Types of host: Either human or animal host
4. Type of nucleic acid: Is it a DNA or RNA virus
5. Symptoms displayed by the host
6. Mode of transmission: Is it mechanical, through insect, man, animal etc

                                         Shape

Morphology {

                                      Symmetry

Shapes of Virus

• Bullet shaped
• Flexuous phage
• Flexuous filamentous
• Rod Shape

Symmetry of Virus

The symmetry of the virus is the relative arrangement of the capsomere to give the protein capsid.

Note:

What encloses a virus is a Protein Capsid.

A lot of capsomere makes up a protein capsid

A lot of protomers makes up a capsomere

The more complex the symmetry of the virus the more the rigid the infection or disease caused

1)    Helical: Example of a virus with helical symmetry is the TMV (Tobacco Mosaic Virus)

2)    Isomeric/Isometric: Example of a virus with isomeric symmetry is the Adenovirus

3)    Icosahedral:  This virus has 20 facet and example of them include the  Ebola virus, HIV, Polio virus SARS  virus

4)    Polyhedral/complex – T4 phage  Rhabdo and Rota Virus(Viruses Responsible for Rabies)

Process of Viral Replication

Virus undergoes replication and exponential division as soon as it’s enters into a host cell. The virus uses the central dogma of host to replicate more viral protein and viral nucleic acid.

Central dogma is

DNA -> RNA -> Polypeptide

Viral replication is a unique process, because it is the only means by which the central dogma can be reversed.

Virus can alternate between DNA to RNA

Stages involved in viral replication

1)    Attachment

2)    Penetration & uncoating

3)    Replication of the viral genome

4)    Assembly & maturation of the viral particles

5)    Release of exocytosis of the new viral progeny

Attachment:

During this process the Virus attaches to the host cell through lock and key mechanism.

Penetration and coating:

The virus enters the host cell with only the nucleic acid and enzymes.

Replication of the viral genome:

The nucleic acid of the virus is replicated using specific enzyme from the virus and both cells.

Assembly and maturation of the viral particle:

The viral nucleic acid and new protein capsomeres are arranged and packaged together to form numerous new viral progenies.

Release of exocytosis of the new viral progeny:

The newly formed viruses exit the cell through the cell membrane or remain within the cell for a long period of time or come out of the cell. When they burst out of cell membrane the virus is called Lytic Cycle/ Lytic Virus or Virulent Virus. Examples include: Ebola, SARS (Severe Acute Respiratory Syndrome), Rabies and so on.

When they virus remain within the cell for long period time, this kind of virus are called Lysogenic / Lysogenic Virus or Temperate Viruses. Example include HIV/Aids.

Economic Importance of Viruses

1. They cause deadly incredible disease.
2. They serve as model organisms in genetic research.
3. They are used in genetic engineering.
4. They are used in vaccines.
5. They are used in the study of protein synthesis and provide more information about the host.

Prions

prions are infectious Protein. They Replicate, but their mode of replication isn’t understood yet. Prions are said to cause diseases like Bovine Spongiform Encephalopathy (BSE commonly known as Mad cow Disease), which can be cured by garri (Cassava flakes) water other disease include Scrapie Disease (affect sheep), Fatal Familial Insomnia (FFI), Kuru and so on.

Viroid

Viroid are naked RNA i.e. they have RNA macro molecules. They consist of 246 and 357 Ribonucleotides and they are the second smallest organisms. They cause disease like coconut Cadang Cadang, Potato tuber X, Citrus exocortis.

Conclusion

Viruses cause deadly incurable disease. Viral disease can only be managed or prevented.

It should be noted that viruses affect both plants and animals. Prions affects animals only while viroid affects plants majorly.

It should also be noted that in terms of size prions are the smallest organism followed by viroids followed by viruses.

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