Microbiology – Chapter 6: Viruses and Prions

Question-and-answer flashcards covering core concepts, structures, replication strategies, classification, diagnostics, treatment, and prion biology from Chapter 6: Viruses and Prions.

What scientific field is dedicated to the study of viruses?

Virology

Approximately how large are most viruses?

About 20–400 nanometres in diameter

How are viruses classified in terms of lifestyle and cell dependence?

They are acellular obligate intracellular pathogens

Are viruses considered living organisms?

No, they are not considered alive

Roughly how many mammal-infecting viral species have been described to date?

Over 5,000

Where do most human-infecting viruses naturally reside before spilling over?

In other animals (zoonotic reservoirs)

What name is given to a single, infectious viral particle?

A virion

What is the protein shell that surrounds and protects a viral genome called?

The capsid

What repeating protein units make up a viral capsid?

Capsomeres

What are the two main capsid shapes found in animal viruses?

Helical and icosahedral

How are capsids that are neither helical nor icosahedral classified?

As complex capsids

What specialised structures help many bacteriophages inject their genome into host cells?

Tail fibres, sheath, baseplate, and pins

What key structural feature distinguishes enveloped from naked viruses?

A lipid-based envelope surrounding the capsid

How do enveloped viruses typically acquire their envelope?

By budding from the host cell membrane

How are most naked viruses released from host cells?

By lysing (bursting) the host cell

Are bacteriophages ever enveloped?

No, they are always naked

What are viral spikes (peplomers) made of?

Glycoproteins

What is the primary function of viral spikes?

Attachment to and entry into specific host cells

Which two spike proteins define influenza A viruses?

Hemagglutinin (HA) and neuraminidase (NA)

Approximately how many genes do most viruses possess?

Fewer than 300

What kinds of nucleic acids can serve as viral genomes?

DNA or RNA

List four ways viral genomes can be structurally organised.

Single- or double-stranded, segmented or single, circular or linear

What is the overarching goal of any virus inside a host cell?

To make viral proteins so new virions can be assembled

How do double-stranded DNA viruses generate mRNA?

They use host RNA polymerase to transcribe their DNA

What must single-stranded DNA viruses do before transcription?

Be converted to double-stranded DNA

How is an ssRNA(+) genome used once inside the host cell?

It acts directly as mRNA for translation

How do ssRNA(–) viruses produce mRNA?

They use viral RNA-dependent RNA polymerase to make complementary mRNA

Which enzyme converts retroviral RNA into DNA?

Reverse transcriptase

Why do RNA viruses mutate faster than DNA viruses?

RNA polymerases lack proofreading functions

What is an attenuated viral strain?

A strain with reduced infectivity, often used in vaccines

What is viral reassortment?

Exchange of genome segments when two viruses co-infect the same cell

Define antigenic drift in influenza viruses.

Minor, gradual mutations in HA and NA spike genes

Define antigenic shift in influenza viruses.

Major genetic reassortment creating new HA/NA combinations

Why is antigenic shift a public-health concern?

It can spark pandemics because populations lack prior immunity

List four criteria ICTV uses to group viruses.

(1) Type of nucleic acid, (2) capsid symmetry, (3) envelope presence, (4) genome architecture

What does host range describe?

The set of species a virus can infect

What does tropism describe?

The specific tissues or cell types a virus infects

Give an example of a virus with a narrow host range.

Measles virus (infects only humans)

Name a virus approximately 30 nm in diameter.

Rhinovirus (or poliovirus)

Which virus discovered in 2014 reaches about 1,500 nm in length?

Pithovirus

What is the highest taxonomic level assigned to viruses?

Phylum

What suffix identifies a viral order name?

-virales

What suffix identifies a viral family name?

-viridae

What phrase describes how viruses use cellular machinery?

They hijack host cell machinery

Name the five stages of the bacteriophage lytic cycle.

Attachment, penetration, replication, assembly, release

What is a temperate phage?

A phage capable of entering a lysogenic cycle

What is a prophage?

A phage genome integrated into the bacterial chromosome

How can prophages increase bacterial virulence?

By providing new pathogenicity factors such as toxins

List the six general steps of animal virus replication.

Attachment, penetration, uncoating, replication, assembly, release

How do enveloped animal viruses typically exit host cells?

By budding from the plasma membrane

What is a provirus?

An animal virus genome permanently integrated into host DNA

Differentiate chronic from latent viral infections.

Chronic releases virions continuously; latent has dormancy with periodic flare-ups

Name three herpesviruses that establish latent infections.

HSV-1, HSV-2, and varicella-zoster virus (HHV-3)

What is an oncogenic virus?

A virus capable of causing cancer in its host

Which DNA virus family is linked to cervical cancer?

Human papillomaviruses (HPVs) in the Papillomaviridae family

How do oncogenic viruses promote cancer?

By triggering uncontrolled cell division or blocking cell-death signals

Which laboratory method quantifies bacteriophages by clear zones?

Plaque assay

What unit expresses the number of plaques in a sample?

Plaque-forming units (PFU)

Why are embryonated eggs useful in virology?

They provide a living system for cultivating certain animal viruses

In diagnostics, what does test specificity mean?

The ability to detect only the target virus (no false positives)

In diagnostics, what does test sensitivity mean?

The ability to detect very low levels of the target (no false negatives)

What does an ELISA detect?

Viral antigens or antibodies indicated by a colour change

Why might ELISAs fail after an antigenic shift?

New viral antigens may no longer be recognised by existing antibodies

Which molecular method amplifies viral genetic material for detection?

Polymerase chain reaction (PCR)

Why is antiviral drug design especially challenging?

Viruses are intracellular and offer few unique drug targets

What is post-exposure prophylaxis (PEP)?

Injectable antibodies given after exposure to block viral entry

Which topical drug blocks HHV-1 entry into cells?

Docosanol

Name a nucleoside analog effective against herpes simplex viruses.

Acyclovir

Which drug blocks influenza A and B virion release?

Oseltamivir (Tamiflu) or zanamivir (Relenza)

What are prions?

Infectious misfolded proteins lacking nucleic acids

Which acquired human TSE is linked to contaminated beef?

Variant Creutzfeldt-Jakob disease (vCJD)

List three neurodegenerative diseases that show prion-like protein misfolding.

Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis (ALS)