Ch 11 Viruses

What are the main characteristics of viruses?

They are extremely small (about 20–1,500 nm ), acellular, contain DNA or RNA , have a protein capsid, may have an envelope, lack their own metabolism, and are obligate intracellular parasites that use host cells to replicate.

How do viruses differ from living cells?

Viruses are not cellular, do not carry out independent metabolism, and cannot reproduce independently. They must use a suitable host cell to make new viruses.

What are the three major components or features of a virus?

Genome: DNA or RNA containing genetic information.

Capsid: protein coat protecting the genome.

Envelope: optional lipid bilayer surrounding the capsid.

How can viral genomes differ?

A genome may be DNA or RNA; single-stranded (ss) or double-stranded (ds); linear, circular, or segmented. RNA viruses may have positive-sense (+) or negative-sense (−) RNA.

what is a capsid, and what are capsomeres?

The capsid is the protein shell surrounding and protecting the viral genome.

Capsomeres are the protein subunits that make up the capsid; they can interact and self-assemble.

What does the viral capsid protect the genome against?

Environmental damage such as UV light and desiccation, as well as host enzymes.

What are the main viral shapes?

Helical: capsomeres form a spiral around a central axis.

Icosahedral: a roughly spherical shape with 20 triangular faces.

Complex: combines structural features, as in bacteriophages. Spherical: rounded, as in influenza.

What is the difference between a naked virus and an enveloped virus?

A naked virus has a capsid without an envelope. An enveloped virus has a lipid bilayer outside its capsid, usually obtained from a host-cell membrane.

What are viral envelope spikes, and what is their function?

They are viral proteins embedded in or projecting from the envelope. They help the virus recognize and attach to specific host-cell receptors.

How does an envelope affect a virus's environmental stability and transmission?

Enveloped viruses generally need to remain moist and are often transmitted through body fluids. Naked viruses lack the lipid envelope and are generally more resistant to drying and environmental conditions.

What are viral host range and viral specificity?

Host range is the range of hosts a virus can infect. Specificity refers to the particular host cells or tissues it can infect; many viruses are restricted to certain hosts and cell types.

How do viruses recognize and attach to host cells?

Viral attachment proteins or spikes bind to compatible receptors on host cells. The presence of the appropriate receptor helps determine whether infection can occur.

What is the hypotheses virus-first for the origin of viruses?

viruses originated before true cells as self-replicating units.

What is the hypothesis regressive/reduction for the origin of viruses?

viruses evolved from cellular organisms that lost genetic information.

what is the hypothesis progressive/escape for the origin of viruses?

mobile genetic elements gained the ability to leave cells and enter other cells

what is the hypothesis combination/chimeric for the origin of hypothesis

proposes that viruses arose through a mixture of these processes.

How are viruses classified?

Using characteristics such as genome type, single- versus double-stranded nucleic acid, RNA sense, morphology, envelope status, host range, and replication strategy.

Viral classification does not strictly follow the traditional Linnaean system

What are positive-sense and negative-sense single-stranded RNA?

Positive-sense (+) RNA can act directly as mRNA and be translated by host ribosomes.

Negative-sense (−) RNA is complementary to mRNA and must first be copied into positive-sense RNA by a viral RNA-dependent RNA polymerase.

What are examples of positive-sense RNA virus families in the notes?

Picornaviridae: polio and common cold viruses. Retroviridae: HIV. Coronaviridae: SARS-CoV and SARS-CoV-2.

What are examples of negative-sense RNA virus families in the notes?

Orthomyxoviridae: influenza viruses. Filoviridae: Ebola virus. These viruses are enveloped and commonly have helical nucleocapsids.

What is a double-stranded RNA virus, and what example is in the notes?

Reoviridae is a double-stranded RNA virus family. The notes identify rotavirus, associated with gastrointestinal and respiratory infections.

What are examples of double-stranded DNA virus families in the notes?

Herpesviridae: herpes, chickenpox, and shingles. Poxviridae: smallpox and cowpox. Papillomaviridae: human papillomavirus (HPV).

What is an example of a single-stranded DNA virus family in the notes?

Parvoviridae, associated in the notes with fifth disease.

What is a bacteriophage?

A virus that infects bacteria. A typical complex bacteriophage has an icosahedral head containing DNA, a tail, and tail fibers that attach to bacterial receptors.

What are the five steps of the bacteriophage lytic cycle?

1. Attachment: tail fibers bind to host receptors

. 2. Penetration: the tail contracts and injects viral DNA.

3. Replication: viral nucleic acid and proteins are produced.

4. Assembly: new phage particles are assembled.

5. Release: the host cell lyses and releases new viruses.

What happens during bacteriophage attachment and penetration?

Tail fibers bind to specific bacterial receptors, determining host specificity. The tail contracts and injects the viral genome into the bacterium; the capsid generally remains outside.

What happens during bacteriophage replication and assembly?

Viral enzymes and host machinery are used to make viral components. Viral genes may redirect or shut down host DNA and protein synthesis. New heads and tails are assembled, and the heads are filled with viral DNA.

How are new bacteriophages released in the lytic cycle?

Phage lysozyme weakens the bacterial cell wall, causing lysis. The bacterial cell breaks open and releases the newly assembled phages.

What is the difference between virulent and temperate bacteriophages?

Virulent phages follow the lytic cycle and destroy the host cell. Temperate phages can enter the lysogenic cycle, allowing viral DNA to persist in the host without immediately destroying it.

What is the lysogenic cycle, and what is a prophage?

During lysogeny, phage DNA integrates into bacterial DNA and is copied as the host cell divides. The integrated phage DNA is called a prophage. Host stress can trigger it to leave the host genome and enter the lytic cycle.

How does replication of an animal virus differ from bacteriophage replication?

Animal viruses generally attach through capsid proteins or envelope spikes, enter the host cell, and uncoat to release their genome. They replicate and assemble new particles, then exit through mechanisms such as budding or cell lysis.

What is tissue tropism?

The tendency of a virus to infect particular cell types or tissues in a host. It depends partly on whether cells have compatible receptors and support viral replication.

Where does animal-virus replication or assembly occur?

The location depends on the virus. The notes describe many DNA-virus replication and transcription steps occurring in the nucleus, with assembly often in the nucleus. Enveloped viruses may acquire their envelopes by budding through the plasma membrane or nuclear envelope.

How do positive-sense, negative-sense, and double-stranded RNA viruses produce proteins?

Positive-sense RNA can function directly as mRNA. Negative-sense RNA must first be copied into positive-sense RNA by viral RNA polymerase. Double-stranded RNA viruses produce mRNA from their genome segments so host ribosomes can translate viral proteins.

What makes retroviruses different from other RNA viruses?

They use reverse transcriptase to make DNA from their RNA genome. The viral DNA enters the host nucleus and integrates into host DNA as a provirus. HIV is an example.

why can't viruses grow on ordinary microbiological growth media?

They require living host cells to replicate. Bacteriophages can be grown with bacteria; animal viruses can be cultivated in systems such as cell cultures or fertilized chicken eggs.

What is a plaque in virus cultivation?

A clear area in a lawn of bacteria where bacterial cells have been lysed by bacteriophages. Plaques can be counted as plaque-forming units to estimate infectious virus.

What are cytopathic effects (CPE)?

Observable changes in virus-infected host cells that can help identify a virus. Examples in the notes include inclusion bodies, syncytia (large multinucleated cells), and shrunken nuclei.

What are satellite viruses or satellite nucleic acids?

Small infectious agents or nucleic acid molecules that require an unrelated helper virus for replication. Many are associated with plant viruses; hepatitis D virus requires hepatitis B virus as a helper.

What is a virophage?

A small virus that infects a giant virus, such as a mimivirus, and interferes with the giant virus's replication. Sputnik is the example in the notes.

What is a prion, and how does it cause disease?

-infectious protein without nucleic acid.

An abnormally folded protein induces normal versions of the protein to misfold, leading to progressive neurological disease and spongiform changes in nervous tissue. Examples include scrapie, bovine spongiform encephalopathy, Creutzfeldt-Jakob disease, and kuru.

What is a viroid, and how does it differ from a virus?

A viroid is a small, circular, single-stranded RNA molecule that infects plants. It has no capsid or envelope and does not encode proteins, unlike a typical virus.

How do viruses, viroids, prions, satellites, and virophages differ?

Viruses contain DNA or RNA and a protein capsid, sometimes with an envelope.

Viroids are small naked RNA molecules. Prions are infectious proteins without nucleic acid.

Satellites require a helper virus to replicate.

Virophages infect giant viruses and can interfere with their replication.

How can viruses contribute to cancer?

Some viruses interfere with tumor-suppressor genes or activate proto-oncogenes, disrupting normal control of cell growth. HPV is associated with cervical, anal, penile, and oropharyngeal cancers.

What is the difference between a proto-oncogene and a tumor-suppressor gene in viral cancer development?

Proto-oncogenes normally help regulate cell growth; inappropriate activation can promote uncontrolled growth. Tumor-suppressor genes normally restrain cell division or help prevent abnormal growth; their disruption removes these controls.

A newly discovered infectious agent contains RNA inside a protein capsid and replicates only inside host cells. Is it most likely a virus, viroid, or prion?

A virus, because it contains RNA and a protein capsid and depends on host cells for replication. A viroid lacks a capsid, while a prion is made of infectious protein rather than RNA.