Characterizing and Classifying Viruses, Viroids, and Prions: Study Notes

Characterizing and Classifying Viruses, Viroids, and Prions

Characteristics of Viruses

Viruses are minuscule, acellular infectious agents that possess either DNA or RNA as their genetic material. They are responsible for a wide range of diseases affecting humans, animals, plants, and bacteria. Unlike cellular organisms, viruses cannot carry out metabolic processes, grow, or respond to their environment independently. Instead, they rely on host cells to reproduce and increase their numbers.

• Acellular: Lacking cytoplasmic membrane, cytosol, and organelles.

• Extracellular State (Virion): Nucleic acid surrounded by a protein coat (capsid), sometimes with a phospholipid envelope.

• Intracellular State: Envelope and capsid removed; virus exists as nucleic acid.

• Host Specificity: Most viruses infect only particular host cells due to affinity between viral surface proteins and host cell receptors.

• Generalists: Some viruses infect many kinds of cells or hosts.

Figure: Relative sizes of cells, bacteria, and viruses, illustrating the minuscule size of virions compared to cellular organisms.

Are Viruses Alive?

There is ongoing debate among scientists regarding the status of viruses as living entities. Viruses lack many characteristics of life, such as growth, self-reproduction, responsiveness, and metabolism. However, they can invade cells, take control of host cellular machinery, possess their own genomes, and evolve over time.

Genetic Material of Viruses

Viruses contain either DNA or RNA, which may be single-stranded (ss) or double-stranded (ds), and can be linear or circular. The type of nucleic acid is a primary criterion for virus classification. Viral genomes are much smaller than those of cells and contain very few genes.

Capsid Morphology and Shape

The protein coat, or capsid, is composed of subunits called capsomeres. Capsid shape is used in virus classification and can be helical, polyhedral, or icosahedral (20 sides). The capsid protects viral nucleic acid and facilitates attachment to host cells.

Viral Envelope

Some viruses acquire an envelope from the host cell during replication or release. The envelope consists of a phospholipid bilayer and proteins, including virally coded glycoproteins (spikes) that play a role in host recognition. Enveloped viruses are more fragile than naked (nonenveloped) viruses, but the envelope provides some protection from the immune system.

Classification of Viruses

Viruses are classified based on:

• Type of nucleic acid

• Presence of an envelope

• Shape

• Size

Viral genera are organized into families, but relationships among viruses are not fully understood by taxonomists.

Table: The Novel Properties of Viruses

Viruses	Cells
Inert extracellularly; active only inside host cell	Metabolize on their own
Do not divide or grow	Divide and grow
Acellular	Cellular
Obligate intracellular parasites	Can be free-living
Genome can be dsDNA, ssDNA, dsRNA, ssRNA	Contain both DNA and RNA
Highly chromosome size varies: 10 nm to over 500 nm	200 nm to 12 cm in diameter
Have a proteinaceous capsid around genome, some have envelope	Surrounded by a phospholipid membrane and cell wall
Replicate in an assembly-line manner using host enzymes and organelles	Self-replicating by asexual and/or sexual means

Families of Human Viruses

Human viruses are classified into families based on their nucleic acid type and other properties. DNA viruses and RNA viruses are further subdivided by strand type and representative genera.

Table: Families of Human Viruses (DNA Viruses)

Family	Strand Type	Representative Genera (Diseases)
Parvoviridae	Single	Erythrovirus (parvovirus infection)
Herpesviridae	Double	Herpes simplex virus, Epstein-Barr virus, Cytomegalovirus, Varicella-zoster virus
Poxviridae	Double	Orthopoxvirus (smallpox)
Polyomaviridae	Double	Polyomavirus (progressive multifocal leukoencephalopathy)
Adenoviridae	Double	Mastadenovirus (respiratory infections)
Hepadnaviridae	Double and single	Orthohepadnavirus (hepatitis B)

Table: Families of Human Viruses (RNA Viruses)

Family	Strand Type	Representative Genera (Diseases)
Picornaviridae	Single, +	Enterovirus (polio), Hepatovirus (hepatitis A)
Caliciviridae	Single, +	Norovirus (gastroenteritis)
Flaviviridae	Single, +	Flavivirus (yellow fever, hepatitis C)
Coronaviridae	Single, +	Coronavirus (respiratory infections)
Orthomyxoviridae	Single, -, segmented	Influenzavirus (influenza)
Paramyxoviridae	Single, -	Paramyxovirus (measles, mumps)
Rhabdoviridae	Single, -	Lyssavirus (rabies)
Reoviridae	Double, segmented	Rotavirus (gastroenteritis)

Viral Replication Cycles

Viruses replicate by recruiting host cell machinery. The replication cycle varies between bacteriophages and animal viruses.

Bacteriophage Replication: Lytic Cycle

The lytic cycle results in the destruction of the host cell and the release of new virions. The five stages are:

1. Attachment

2. Entry

3. Synthesis

4. Assembly

5. Release

Figure: The lytic cycle of bacteriophage replication, showing sequential stages from attachment to release.

Lysogenic Replication Cycle (Lysogeny)

Some bacteriophages undergo lysogeny, where the viral genome integrates into the host chromosome as a prophage. The host cell continues to grow and divide, passing the viral genome to daughter cells. Induction triggers the lytic cycle.

Figure: The lysogenic cycle, showing integration of the viral genome and subsequent cell division.

Replication of Animal Viruses

Animal viruses follow similar basic replication pathways but differ due to the presence of envelopes and the eukaryotic nature of host cells. Entry mechanisms include direct penetration, membrane fusion, and endocytosis. Uncoating is required to release the viral genome.

Figure: Three mechanisms of entry of animal viruses: direct penetration, membrane fusion, and endocytosis.

Synthesis of DNA and RNA Viruses

DNA viruses often replicate in the nucleus, while RNA viruses replicate in the cytoplasm. Strategies differ based on the type of nucleic acid:

• dsDNA viruses: Replicate genome in nucleus; proteins made in cytoplasm.

• ssDNA viruses: ssDNA folds to form dsDNA, which is replicated.

• +ssRNA viruses: RNA acts as mRNA.

• –ssRNA viruses: RNA must be transcribed to +ssRNA before translation.

• Retroviruses: Use reverse transcriptase to synthesize DNA from RNA.

• dsRNA viruses: Each strand serves as a template for replication.

Assembly and Release

Most DNA viruses assemble in the nucleus; most RNA viruses assemble in the cytoplasm. Enveloped viruses cause persistent infections, while naked viruses are released by exocytosis or lysis.

Latency of Animal Viruses

Some animal viruses remain dormant in host cells as latent viruses or proviruses. Latency can last for years, and incorporation into host DNA is permanent. Examples include HIV, chickenpox, hepatitis B, and herpes virus.

Role of Viruses in Cancer

Viruses can contribute to cancer by activating oncogenes, promoting cell division, or interfering with tumor repression. About 20–25% of human cancers are caused by viruses. Examples include Burkitt's lymphoma, Hodgkin's disease, Kaposi's sarcoma, and cervical cancer.

Culturing Viruses in the Laboratory

Viruses cannot grow in standard media and must be cultured inside host cells. Methods include mature organisms, embryonated eggs, and cell cultures.

Viroids

Characteristics of Viroids

Viroids are extremely small, circular pieces of single-stranded RNA that are infectious and pathogenic in plants. Unlike viruses, viroids lack a capsid and do not code for proteins. Viroid RNA adheres to complementary plant RNA, leading to degradation and disease.

Figure: Comparison of healthy and viroid-infected potatoes, illustrating the impact of viroid infection on plant health.

Prions

Characteristics of Prions

Prions are proteinaceous infectious agents. Normal cellular prion protein (PrP) has α-helices, while disease-causing prion PrP has β-sheets. Prion PrP induces refolding of cellular PrP into the disease-causing form.

Prion Diseases

Prion diseases, or spongiform encephalopathies, are characterized by large vacuoles in brain tissue, giving a spongy appearance. Examples include BSE (mad cow disease), scrapie, kuru, CWD, and vCJD. Prion diseases are transmitted by ingestion, transplantation, or contact with infected tissues. There is no standard treatment.

Figure: Comparison of normal and prion-infected brain tissue, showing characteristic spongy appearance.

Prion Sterilization

Normal sterilization procedures do not deactivate prions. Prions are destroyed by incineration or autoclaving in concentrated sodium hydroxide. The European Union has approved the use of enzymes to remove prions from medical equipment.

Additional info: The notes have been expanded with academic context to ensure completeness and clarity for exam preparation.