Viruses, Viroids, and Prions – Vocabulary Review

Vocabulary flashcards covering major terms and concepts from the lecture on viruses, viroids, and prions.

Obligatory intracellular parasite

An organism (e.g., virus) that must invade and use a living host cell’s machinery to multiply.

Virion

A complete, fully developed viral particle composed of nucleic acid, a capsid, and (in some viruses) an envelope with spikes.

Capsid

The protein coat surrounding a virus’s nucleic acid; composed of capsomeres.

Envelope (viral)

A lipid-containing membrane that surrounds some viral capsids and is derived from the host cell’s plasma membrane.

Spikes

Glycoprotein projections on some viral envelopes that aid in attachment to host cells and are targets for antibodies.

Host range

The spectrum of host species and specific cell types a virus can infect, determined by attachment sites and cellular factors.

Bacteriophage

A virus that infects bacteria; often abbreviated phage.

Animal virus

A virus that infects eukaryotic animal cells; entry and exit mechanisms differ from bacteriophages.

Helical virus

A virus whose capsid forms a hollow, cylindrical, helical structure around the nucleic acid (e.g., rabies virus).

Polyhedral virus

A many-sided virus; most have an icosahedral (20-triangle) capsid (e.g., poliovirus).

Complex virus

A virus with a complicated structure, such as a bacteriophage with head, tail, sheath, and tail fibers.

Lytic cycle

Bacteriophage replication pathway that ends with lysis and death of the host cell, releasing new virions.

Lysogenic cycle

Phage replication pathway in which phage DNA integrates into host DNA and remains latent as a prophage.

Prophage

Phage DNA that has integrated into the bacterial chromosome during lysogeny.

Specialized transduction

Transfer of specific bacterial genes from one cell to another via a lysogenic phage carrying adjacent host DNA.

Attachment (viral)

The first step of viral infection in which virion proteins bind to specific receptor sites on the host cell surface.

Penetration (phage)

Step in bacteriophage infection where the tail sheath contracts, injecting phage DNA into the bacterial cell.

Uncoating (animal virus)

The separation of viral nucleic acid from its capsid after entry, mediated by viral or host enzymes.

Biosynthesis (virus)

Stage of viral replication where viral nucleic acid directs synthesis of viral components by host machinery.

Maturation

Assembly of viral nucleic acid and capsid proteins into complete virions.

Release (virus)

Exit of new virions from the host cell by lysis (nonenveloped viruses) or budding (enveloped viruses).

RNA-dependent RNA polymerase

Viral enzyme that copies RNA from an RNA template; required for RNA virus replication in cytoplasm.

Reverse transcriptase

Enzyme used by retroviruses to synthesize DNA from single-stranded RNA.

Retroviridae

Family of enveloped RNA viruses (e.g., HIV) that convert RNA to DNA and integrate as a provirus in the host genome.

Plaque-forming unit (PFU)

A quantitative measure of infectious virus; each clear plaque on a bacterial lawn arises from one virion.

Cytopathic effect (CPE)

Visible structural changes or deterioration in host cells caused by viral infection in cell culture.

Viroid

A small, circular piece of naked RNA that infects plants and lacks a protein coat (e.g., causes potato spindle tuber).

Virusoid

A viroid enclosed in a protein coat that causes disease only when the host cell is coinfected with a helper virus.

Prion

An infectious protein (no nucleic acid) that causes neurodegenerative diseases such as Creutzfeldt-Jakob disease.

PrPᶜ (cellular prion protein)

Normal host glycoprotein found on cell surfaces; can be converted into the infectious form.

PrPˢᶜ (scrapie protein)

Misfolded, infectious form of PrP that accumulates in brain tissue, forming plaques.

Spongiform encephalopathy

Family of progressive, fatal neurodegenerative diseases caused by prions (e.g., mad cow disease).

Sense (+) RNA strand

Single-stranded viral RNA that can act directly as mRNA for protein synthesis.

Antisense (−) RNA strand

Single-stranded viral RNA complementary to mRNA; must be copied into a + strand before translation.

Receptor-mediated endocytosis

Animal virus entry mechanism where binding to receptors triggers engulfment of the virion into a vesicle.

Membrane fusion (viral entry)

Entry method where the viral envelope fuses with the host plasma membrane, releasing the capsid into cytoplasm.

Budding

Process by which enveloped viruses acquire their envelope and exit the host cell without immediately lysing it.

Embryonated egg inoculation

Laboratory technique for cultivating viruses by injecting them into specific sites of a fertilized chicken egg.

Primary cell line

Freshly isolated animal cells cultured for virus growth; exhibit the cytopathic effect and die after a few divisions.