BISC 120 - Lecture 17: Viruses

Vocabulary practice flashcards covering viral structure, history, origin hypotheses, host ranges, replication cycles, and bacterial antiviral defense systems from BISC 120 Lecture 17.

Tobacco mosaic disease

Viral disease of tobacco that stunts growth and causes a mosaic coloration on infected leaves.

Late 1800s - tobacco mosaic experiment

The infectious agent passed through filters that trapped bacteria.

Late 1800s - virus hypothesis

Researchers proposed an infectious particle smaller than a bacterium.

Virus vs. bacterium on nutrient media

Unlike bacteria, the tobacco mosaic agent could not be cultivated on nutrient media such as agar.

1930s - tobacco mosaic virus

The infectious particle was purified and crystallized, confirming tobacco mosaic virus.

Virus

A small infectious particle made of DNA or RNA inside a protein capsid, sometimes with a membrane envelope.

Viruses and living cells

Viruses are not living cells and do not fit the lecture's definition of living organisms.

Viral genome

A virus contains nucleic acid as either DNA or RNA.

Capsid

Protein coat surrounding viral nucleic acid.

Viral envelope

Optional membrane envelope surrounding some viruses.

Ancient virus origin hypothesis

Viruses arose very early, around the same time as protocells.

Ancient-origin idea

Selfish genetic elements may have parasitized independently replicating protocells early in life's history.

Evidence for ancient-virus origin

Viruses infect all 3 domains, and many cell genomes contain integrated viral genetic information.

Cell-first origin hypothesis

Viruses evolved after cells from cellular components, especially nucleic acids, that could exit cells.

Most favored virus-origin hypothesis

The lecture says the cell-first origin is considered most likely by most researchers.

Multiple origins of viruses

The cell-first hypothesis proposes viruses evolved several times rather than from one single origin.

Evidence for cell-first origin

Viruses only replicate in cells, vary greatly in structure/genomes, and cells contain mobile genetic elements.

Plasmids

Mobile genetic elements found in bacteria; cited as support for the cell-first origin hypothesis.

Obligate intracellular parasite

An organism or particle that must be inside a host cell to replicate; viruses fit this description.

Host resources used by viruses

Viruses use host enzymes, ribosomes, ATP, and other molecules.

Host range

The range of host species or cell types a virus can infect.

Broad host range

A virus can infect several host species; West Nile virus is the lecture example.

West Nile virus

Example of broad host range; can infect humans, birds, and insects.

Narrow host range

A virus infects a limited host range; measles virus is the lecture example.

Measles virus

Example of a narrow host range; infects humans only.

Virus-host specificity

Viral surface molecules bind specific host-cell molecules in a lock-and-key interaction.

Lytic cycle

Viral replication cycle that ends with lysis of the host cell.

Lysis

Breaking open of the host cell, releasing viral particles.

Lysogenic cycle

Viral DNA remains in the host without immediately killing it and may later enter the lytic cycle.

Prophage

Phage DNA integrated into host DNA during the lysogenic cycle.

Lytic vs. lysogenic

Lytic kills the host through lysis; lysogenic includes a prophage stage and can later become lytic.

Bacteriophage

A virus that infects bacteria.

Restriction enzymes

Bacterial enzymes that can cut up infecting viral or phage DNA.

CRISPR

Clustered regularly interspaced short palindromic repeats used in bacterial antiviral defense.

CRISPR spacer DNA

CRISPR regions can contain pieces of infecting phage DNA between repeat sequences.

CRISPR RNA

RNA transcribed from CRISPR sequences helps recognize matching viral DNA.

Cas protein

Enzyme that cuts the specific viral DNA sequence recognized through CRISPR RNA.

CRISPR-Cas defense

Stored phage sequence -> CRISPR RNA -> Cas protein targets and cuts matching viral DNA.

CRISPR technology

Bacterial CRISPR-Cas antiviral machinery has been adapted for gene editing.

Lecture 17 date sequence

Late 1800s: virus hypothesized; 1930s: tobacco mosaic virus purified and crystallized.