Describing Viruses, Prions & Viroids... Are they alive?!

Virus: Tiny infectious a cellular agent having either DNA or RNA (Does not have a cell, but has a genome).

• They are much smaller and less complex than cells
• Virons do not grow
• They have no nucleus, cytoplasm, or membranes
• They do not carry out cellular functions
• Virons do not generate metabolic energy
• They are obligate intracellular parasites, meaning that they require a host cell to reproduce

Viruses can be harmful!

• Virulent- disease causing.
• Temperate- not immediately disease causing.
• Viruses play a role in causing some cancers

Viruses may be helpful

• A temperate virus introduces nucleic acid from its former host into a new host, thus changing the genetic code of the host. By this process of transduction, viruses cause genetic variation within a host population.
• Certain varieties of flowers have been developed using viruses to alter the genetic code.

Example: HIV, Rinovirus (common flu), Influenza (the flu)

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Prion: Proteinaceous infectious particle that lacks nucleic acids and replicates by converting similar normal proteins into new prisons. (Mutation cause protein to have different folding properties)

Example: Classic CJD (Creutzfeldt-Jakob Disease) is a human prion disease. It is a neurodegenerative disorder with characteristic clinical and diagnostic features.

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Viroids: An infectious entity affecting plants, smaller than a virus and consisting only of nucleic acid without a protein coat.

Structure

• Viroids are infectious agents composed exclusively of a single piece of circular single stranded RNA which has some double-stranded regions

• Because of their simplified structures both prions and viroids are sometimes called subviral particles. Viroids mainly cause plant disease but have recently been reported to cause a human disease.

Example: Citrus exocortis, Hop stunt viroid and Coconut cadang-cadang viroid.

Are they alive?!

Viruses, Prions and Viroids may behave like living things, acellular particles. Viruses, viroids and prions are not consider to be living organisms because they are incapable of carrying out all life process.