Lecture 12 – Viruses, Viroids & Prions

Vocabulary flashcards covering key terms, structures, classifications, replication processes, and special infectious agents from Lecture 12 on viruses, viroids, and prions.

Virus

Acytoplasmic, obligate intracellular infectious particle consisting of nucleic acid surrounded by a protein coat; shows both living and non-living characteristics.

Virion

The complete infectious viral particle, assembled from pre-made components inside a host cell.

Obligate intracellular parasite

An organism (e.g., virus) that can reproduce only inside a living host cell, hijacking its metabolic machinery.

Ultramicroscopic

Too small to be seen with a light microscope; typical of viruses (20 nm–1 µm).

Capsid

Protein shell composed of capsomeres that encloses and protects the viral genome.

Capsomere

Individual protein subunit of a viral capsid.

Nucleocapsid

Genome plus capsid; term for a non-enveloped (naked) virus particle.

Envelope (viral)

Host-derived lipid bilayer surrounding some viruses, embedded with viral glycoprotein spikes.

Spike (viral)

Surface glycoprotein projecting from capsid or envelope, mediating attachment to host-cell receptors.

Matrix protein

Layer of proteins between envelope and capsid that stabilises enveloped virions.

Helical capsid

Rod-shaped capsid with capsomeres arranged in a spiral around genome (e.g., rabies virus).

Icosahedral capsid

Capsid with 20 triangular faces and 12 corners, enclosing maximum volume with minimal proteins (e.g., adenovirus).

Complex virus

Virus with elaborate, non-helical/icosahedral structure, often with tails or extra layers (e.g., bacteriophage, poxvirus).

Baltimore classification

System grouping viruses into seven classes based on genome type and pathway to mRNA synthesis.

Group I virus

Double-stranded DNA virus; uses host RNA polymerase for mRNA (e.g., herpesvirus).

Group II virus

Single-stranded DNA virus; converts to dsDNA before transcription (e.g., parvovirus).

Group III virus

Double-stranded RNA virus; carries RNA-dependent RNA polymerase for mRNA (e.g., rotavirus).

Group IV virus

Positive-sense ssRNA virus; genome acts directly as mRNA (e.g., poliovirus, coronaviruses).

Group V virus

Negative-sense ssRNA virus; carries RNA-dependent RNA polymerase to make +RNA (e.g., influenza).

Group VI virus

Retrovirus; ssRNA genome reverse-transcribed to DNA and integrated (e.g., HIV).

Group VII virus

dsDNA virus with reverse transcriptase; replicates via RNA intermediate (e.g., hepatitis B).

ICTV

International Committee on Taxonomy of Viruses; official authority naming viral orders, families, genera and species.

-viridae

Suffix designating a viral family name (e.g., Retroviridae).

-virus (genus)

Suffix designating a viral genus name (e.g., Lentivirus).

Polythetic species

ICTV concept: viral species defined by multiple shared traits; not all traits must be present in every member.

Enteric virus

Virus transmitted by fecal–oral route, replicating mainly in intestinal tract (e.g., rotavirus).

Respiratory virus

Virus acquired by inhalation of droplets, replicating in respiratory tract (e.g., rhinovirus).

Arbovirus

Arthropod-borne virus transmitted through bites of mosquitoes, ticks, etc. (e.g., West Nile virus).

Zoonotic virus

Virus transmitted from animals to humans where humans are usually dead-end hosts (e.g., rabies).

Host range

Spectrum of host species a virus can infect; determined largely by receptor compatibility.

Tissue tropism

Preference of a virus for specific cell types or tissues (e.g., HBV for hepatocytes).

Rabies virus

Broad host-range virus infecting almost all warm-blooded animals; neurotropic.

Hepatitis B virus

Narrow host-range DNA virus infecting human liver cells; can cause liver cancer.

gp120

HIV envelope glycoprotein that binds CD4 and co-receptor CCR5/CXCR4 on T-cells.

CD4 receptor

Primary cellular receptor for HIV entry, located on helper T-cells and macrophages.

Bacteriophage

Virus that infects bacteria; often composed of icosahedral head and tail apparatus.

Lytic cycle

Phage life cycle culminating in host lysis and release of progeny virions.

Lysogenic cycle

Temperate phage integration into host genome as prophage, replicating passively with host DNA.

Prophage

Phage genome integrated into bacterial chromosome during lysogeny.

Induction (phage)

Switch of a temperate prophage from lysogenic to lytic cycle, often triggered by stress.

Lysogenic conversion

Acquisition of new bacterial traits (e.g., toxin genes) encoded by integrated prophage DNA.

Adsorption (animal virus)

Attachment of viral spikes or capsid proteins to specific host-cell receptors.

Fusion (viral entry)

Enveloped virus enters host cell by merging its envelope with plasma membrane.

Endocytosis (viral entry)

Host cell engulfs virus into an endocytic vesicle; used by both enveloped and naked viruses.

Uncoating

Removal of viral capsid to expose genome inside host cell.

Budding

Release of enveloped virions through host membrane, acquiring lipid envelope in process.

DNA-dependent DNA polymerase

Enzyme that copies DNA from a DNA template; used by DNA viruses for genome replication.

RNA-dependent RNA polymerase

Viral enzyme (replicase/transcriptase) that copies RNA from RNA template; absent in host cells.

Reverse transcriptase

RNA-dependent DNA polymerase of retroviruses; synthesizes DNA from RNA template.

Integrase

Retroviral enzyme that inserts viral cDNA into host chromosome.

Viroid

Small, circular ssRNA molecule (no capsid) that infects plants, causing diseases like potato spindle tuber.

Prion

Infectious misfolded protein (PrPSc) causing transmissible spongiform encephalopathies.

PrPC

Normal cellular prion protein with alpha-helical structure, non-infectious.

PrPSc

Pathogenic beta-sheet-rich prion isoform resistant to proteases and capable of templating misfolding.

Transmissible spongiform encephalopathy (TSE)

Neurodegenerative disease group caused by prions, characterized by spongy brain tissue (e.g., CJD, BSE).

Amyloid plaque (prion)

Aggregates of PrPSc accumulating in brain, disrupting neuronal function.

Oncogenic virus

Virus capable of integrating into host DNA and inducing cancer (e.g., HPV, EBV).

Latent virus

Virus that remains dormant within host cells, periodically reactivating (e.g., HSV, VZV).

Pandoravirus

Giant virus up to 1 µm in length, larger than some bacteria.

Mimivirus

Giant dsDNA virus ~400 nm in diameter; notable for size and complex genome.

Circovirus

One of the smallest viruses, 17–20 nm in diameter, circular ssDNA genome.