Ch 21 - Viruses, Viroids, Prions Flashcards

A set of practice Q&A flashcards covering virus structure, replication, life cycles, classification, and pathogens (phages, animal/plant viruses, prions, viroids), based on the lecture notes.

What type of nucleic acid do viruses contain and what covers their nucleic acid?

Viruses contain a single type of nucleic acid (DNA or RNA) and have an outer protein coat called a capsid.

Are viruses alive, and what are some characteristics that distinguish them from cells?

Viruses are acellular, much smaller than most bacteria, can’t reproduce on their own, can’t carry out metabolic functions, and don’t process energy.

What is a capsid and what are capsomeres?

The capsid is the protein shell enclosing the nucleic acid, built from subunits called capsomeres.

Do all viruses have envelopes? If not, what are envelopes and when are they present?

Envelopes are lipid membranes derived from the host cell; they are present in some animal viruses and surround the capsid during part of the viral life cycle.

Name the major capsid shapes and give an example for each.

Isometric (spherical) – e.g., Adenoviruses; Filamentous – e.g., Tobacco mosaic virus (TMV); Head-and-tail with a tail – e.g., Bacteriophages; Enveloped forms also exist.

What is a bacteriophage (phage) and how is its structure specialized for infecting bacteria?

A bacteriophage infects bacteria and typically has a complex structure with an elongated head enclosing DNA and a tail piece for attachment and DNA injection.

List the general stages of viral replication in order.

Attachment, Entry and uncoating, Replication of genome and transcription, Translation of viral proteins, Self-assembly of new virions, Exit from the cell (often by budding).

What are the lytic and lysogenic cycles and which types of phages use them?

Lytic cycle results in host cell lysis to release progeny; Lysogenic cycle replicates phage DNA without destroying the host. Virulent phages use only the lytic cycle; temperate phages can use both.

Describe the key steps of the Lytic cycle.

Phage attaches to the host, injects DNA, degrades host DNA, replicates phage genome and synthesizes proteins, assembles phage particles, and lyses the cell to release progeny.

What is a prophage and what happens during lysogeny?

A prophage is phage DNA integrated into the bacterial chromosome; as the host divides, it copies the prophage and passes it to daughter cells; sometimes induction triggers the lytic cycle.

What is a provirus and how does it differ from a prophage?

A provirus is viral DNA integrated into the host genome of an animal cell (e.g., retroviruses); a prophage is phage DNA integrated into a bacterial chromosome.

What does the Baltimore classification system categorize, and what are its main groups related to mRNA production?

It classifies viruses by how they produce mRNA. Major groups include dsDNA (I), ssDNA (II), dsRNA (III), (+) ssRNA (IV), (-) ssRNA (V), ssRNA-RT (VI), and dsDNA-RT (VII).

How does an enveloped animal virus typically enter a cell and acquire its envelope?

The virus enters by endocytosis or fusion; replication occurs inside the cell, and during exocytosis the viral envelope is re-formed from host membranes as new virions leave.

Give an example of a retrovirus and explain its unique replication cycle.

HIV is a retrovirus; it uses reverse transcriptase to convert its RNA genome into DNA, which integrates into the host genome as a provirus and can produce new virions.

What are some ways vaccines can be formulated to combat viral diseases?

Active but weakened (live-attenuated) viruses, inactivated viruses, viral subcomponents, and mRNA vaccines; some vaccines also boost the immune system of already infected individuals.

What are the two main modes of transmission for plant viruses, and what is a common feature of their genomes?

Horizontal and vertical transmission; most plant viruses have RNA genomes.

What are prions and how do they cause disease?

Prions are infectious misfolded proteins that convert other proteins to the wrong shape, leading to neurodegenerative diseases with brain vacuolization.

What are viroids and what is their basic nature?

Viroids are small, naked circular RNA molecules that infect plants.

What is a phage and what organism do they infect?

A bacteriophage (phage) is a virus that infects bacteria.

What is a capsid, and what is a capsomere?

The capsid is the protein shell enclosing the viral genome; it is built from subunits called capsomeres.

What distinguishes an enveloped virus from a non-enveloped (naked) virus?

Enveloped viruses have a lipid envelope derived from host membranes around the capsid; non-enveloped viruses lack this envelope.

What is the structural feature of head-and-tail viruses and which host do they typically infect?

Head-and-tail viruses have a polyhedral head containing DNA and a tail for attachment and injection; they typically infect bacteria (bacteriophages).

Who is Wendell Stanley and what did he discover related to viruses?

Wendell Stanley crystallized the tobacco mosaic virus (TMV), providing evidence that viruses can be crystallized and are not cells.