Although they appear to behave as living things, viruses, vioids and prions are nonliving particles. Read on and discover more about these 'smart' parasites.

Acellular Particles

Viruses, viroids and prions are not considered to be living organisms because they are incapable of carrying out all life processes. The factors that distinguish them as nonliving are as follows.

Acellular Particles…

• are not made of cells
• cannot reproduce on their own
• do not grow or undergo division
• do not transform energy
• lack machinery for protein synthesis
• are so small that they can only be seen with an electron microscope

Viruses

What Are Viruses?

Viruses are composed of nucleic acid, proteins and, in some cases, lipids as well. Nucleic acid, which can be either DNA or RNA, encodes the genetic information that is necessary to make copies of the virus.

The nucleic acid is surrounded by a protective protein coat, called a capsid. Some viruses are also surrounded by an outer membranous layer, called an envelope, made of lipid and protein.

Intracellular and Extracellular Viral Form

Viruses exist in one of two states; Extracellular and Intracellular. Before it invades a host cell, a virus is in the ‘extracellular state’. An extracellular virus is called a virion (vie-ree-on). In this form, the virus consists of a protein coat (capsid) surrounding nucleic acid. In addition, some viruses have phospholipid envelope surrounding the capsid. This outermost layer provides protection and recognition sites for host cells.

Once the virus invades a host cell it is in an ‘intracellular state.’ In this state, the capsid is removed and the virus exists as only as nucleic acid (genetic material).

How Do Viruses Reproduce?

Viruses reproduce via four basic steps. Viral reproduction includes:

1. Delivery of the viral genomes into a host cell
2. Commandeering of the host cell transcription and translation machinery
3. Use of the host cell’s building blocks to copy viral genomes and synthesize viral proteins
4. Viral genomes and proteins then self-assemble and exit host cells as new infectious particles.

Viroids and Prions

A virus is a very simple, yet successful infectious agent. Yet there are acellular paticle that are even more basic than viruses: viroids and prions.

Viroids have genetic material but no protein coat. Conversely prions are made of protein but have nonucleic acid. Like viruses, viroids and prions are not made of cells but can cause disease.

What Is a Viroid?

Viroids are smallest known agents of infectious disease. Whereas conventional viruses are made up of nucleic acid encapsulated in protein (capsid), viroids are uniquely characterized by the absence of a capsid.

In spite of their small size, viroid ribonucleic acids (RNAs) can replicate and produce characteristic disease syndromes when introduced into cells.

Thus far this type of acellular particle has only been identified in association with plants and is not at present a threat to humans.

What Is a Prion?

A prion is a disease-causing agent, discovered by Stanley Prusiner, which is responsible for various fatal neurodegenerative diseases called transmissible spongiform encephalopathies.

Prions are an abnormal form of a normally harmless protein found in mammals and birds. These abnormal proteins can enter the brain through infection, usually after being ingested, or it can arise from a mutation in the gene that encodes the protein. Unlike all other types of infectious agents in that they appear to lack nucleic acid (DNA or RNA).

Once present in the brain, prions cause normal proteins to refold into abnormal shapes. As these abnormal proteins multiply, they destroy neurons and eventually cause brain tissue to become riddled with holes. Prions can only be destroyed through incineration.

Diseases caused by prions include:

• Creutzfeldt-Jakob (kroits-felt yock’-ub) disease
• Mad Cow Disease
• Scrapie (neurological disease of sheep and goats)

More Information on Microbes

You can also find other informative microbe-related articles at Suite101, including Prokaryotic and Eukaryotic Cells, Viral Pneumonia, What is a Virus and the Bacterial Cell Wall or see the excellent websites of Science Prof Online or Cells Alive.

Sources

Bauman, R. (2005) Microbiology.

Park Talaro, K. (2008) Foundations in Microbiology.