Ch. 13 Viruses, Viroids, and Prions: Structure, Classification, and Replication

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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 numerous diseases affecting humans, animals, plants, and bacteria. Viruses cannot carry out metabolic pathways, grow, or respond to their environment independently; instead, they recruit host cell machinery to replicate. - Acellular: Lacking cytoplasmic membrane, cytosol, and organelles. - Extracellular State (Virion): Nucleic acid surrounded by a protein coat (capsid), sometimes enveloped by a phospholipid membrane. - Intracellular State: Envelope and capsid removed; virus exists as nucleic acid. Comparison of plant, bacterial, and viral structures

Hosts of Viruses

Most viruses infect specific host cells due to the affinity between viral surface proteins and host cell receptors. Some viruses are highly specific, while others (generalists) infect multiple cell types or hosts. All organisms are susceptible to some form of virus. Relative sizes of cells, bacteria, ribosomes, and viruses

Are Viruses Alive?

There is scientific debate regarding the status of viruses as living entities. While they can invade cells, control host functions, possess genomes, and evolve, they lack independent growth, self-reproduction, responsiveness, and metabolism.

Comparison: Viruses vs. Cells

Viruses	Cells
Inert outside host cell; become active inside	Metabolize independently
Do not divide or grow	Divide and grow
Acellular	Cellular
Obligate intracellular parasites	Can be free-living or parasitic
Genome: dsDNA, ssDNA, dsRNA, ssRNA	Genome: DNA and RNA
Size: 10–500 nm	Size: 200 nm–12 cm
Protein coat (capsid), sometimes envelope	Surrounded by phospholipid membrane and cell wall
Replication: host enzymes and organelles	Self-replicating by binary fission/sexual means

Table comparing properties of viruses and cells

Genetic Material of Viruses

Viral genomes may be DNA or RNA, single- or double-stranded, linear or circular. This diversity is a primary basis for virus classification. Viral genomes are much smaller than those of cells and contain few genes.

Capsid Morphology and Shape

The protein coat (capsid) is composed of capsomeres and determines the virus's shape, which is used in classification. Common shapes include helical, polyhedral, and icosahedral (20-sided). The capsid protects viral nucleic acid and facilitates attachment to host cells.

Viral Envelope

Some viruses acquire an envelope from the host cell membrane during replication or release. The envelope consists of a phospholipid bilayer and proteins, including virally coded glycoproteins (spikes) that aid in host recognition. - Enveloped Virion: Has a membrane; more fragile, provides immune protection. - Naked Virion: Lacks membrane; released by lysis or exocytosis.

Classification of Viruses

Viruses are classified based on nucleic acid type, presence of envelope, shape, and size. Viral genera are organized into families, but relationships among viruses are not fully understood.

Families of Human Viruses

Family	Strand Type	Representative Genera (Disease)
Parvoviridae	Double	Orthopoxvirus (smallpox)
Herpesviridae	Double	Herpes simplex, Epstein-Barr, Cytomegalovirus
Papillomaviridae	Double	Human papillomavirus (warts, cervical cancer)
Adenoviridae	Double	Mastadenovirus (respiratory infections)
Hepadnaviridae	Double and single	Orthohepadnavirus (hepatitis B)
Polyomaviridae	Double	Polyomavirus (BK virus, JC virus)
Parvoviridae	Single	Erythrovirus (parvovirus B19)

Table of DNA virus families

Family	Strand Type	Representative Genera (Disease)
Picornaviridae	Single, +	Enterovirus (polio), Hepatovirus (hepatitis A)
Caliciviridae	Single, +	Norovirus (gastroenteritis)
Flaviviridae	Single, +	Flavivirus (yellow fever, hepatitis C)
Coronaviridae	Single, +	Coronavirus (SARS, common cold)
Paramyxoviridae	Single, -	Paramyxovirus (measles, mumps)
Orthomyxoviridae	Single, - segmented	Influenzavirus (influenza)
Retroviridae	Single, +	Lentivirus (HIV)

Table of RNA virus families

Viral Replication Cycles

Bacteriophage Replication: Lytic Cycle

The lytic cycle involves five stages: attachment, entry, synthesis, assembly, and release. This process results in the destruction (lysis) of the host cell and the release of new virions. - Burst size: Number of virions released per cell. - Burst time: Time taken for lysis and release. Diagram of lytic and lysogenic cycles

Lysogenic Replication Cycle (Lysogeny)

In lysogeny, temperate phages integrate their genome into the host chromosome as a prophage, allowing the host to reproduce normally for generations before lysis. Lysogenic conversion can alter the phenotype of the host bacterium.

Replication of Animal Viruses

Animal viruses follow similar replication pathways but differ due to the presence of envelopes, eukaryotic cell structure, and lack of cell walls. Attachment is mediated by glycoprotein spikes or other molecules. Entry occurs via direct penetration, membrane fusion, or endocytosis. Mechanisms of entry for animal viruses

Synthesis of DNA and RNA Viruses

- DNA Viruses: Often enter the nucleus; viral genome replicated in nucleus, proteins synthesized in cytoplasm. - RNA Viruses: Replicate in cytoplasm; positive-sense RNA acts as mRNA, negative-sense RNA requires conversion. - Retroviruses: Use reverse transcriptase to synthesize DNA from RNA.

Assembly and Release

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

Latency of Animal Viruses

Some animal viruses remain dormant (latent) in host cells for years, with no viral activity. Incorporation of provirus into host DNA is permanent. Examples include HIV, herpes, and hepatitis B.

Role of Viruses in Cancer

Viruses can contribute to cancer by carrying oncogenes, promoting host oncogenes, or interfering with tumor repression. They are implicated in 20–25% of human cancers, including Burkitt's lymphoma, Hodgkin's disease, Kaposi's sarcoma, and cervical cancer.

Culturing Viruses in the Laboratory

Viruses require host cells for growth and are cultured using mature organisms, embryonated eggs, or 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. Potato tubers affected by viroid infection

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 pathogenic form.

Prion Diseases

Prion diseases, or spongiform encephalopathies, are characterized by large vacuoles in brain tissue, giving a spongy appearance. Examples include BSE, scrapie, kuru, CWD, and vCJD. Transmission occurs via ingestion, transplantation, or contact with infected tissues. There is no standard treatment; prions are resistant to normal sterilization and are destroyed by incineration or autoclaving in concentrated sodium hydroxide. Brain tissue affected by prion disease Additional info: Prion diseases are a major concern in medical and veterinary fields due to their resistance to conventional sterilization and their severe neurological effects. ----------------------------------------