Viruses Chap 19

What are viruses made of?

Viruses are infectious particles consisting of genes enclosed in a protein coat.

Can viruses reproduce or carry out metabolism on their own?

No, they require a host cell to replicate and carry out metabolism.

Why are viruses considered to have "borrowed life"?

They exist in a state between living organisms and chemicals since they depend on a host for replication.

What is the Tobacco Mosaic Virus (TMV), and why is it significant?

TMV was the first virus discovered, stunting tobacco plant growth and causing mosaic coloration.

Who confirmed the existence of viruses, and how?

Wendell Stanley (1935) crystallized the TMV, proving that viruses exist as distinct infectious particles.

Main components of a virus

Nucleic acid (DNA or RNA), a protein coat (capsid), and sometimes a lipid envelope.

What are capsomeres?

Protein subunits that make up the capsid.

Two main structural types of viruses

Helical and icosahedral viruses.

Purpose of a viral envelope

Helps the virus infect host cells and is derived from the host cell membrane.

Types of genomes viruses can have

DNA or RNA, single-stranded or double-stranded, linear or circular.

Number of genes in a viral genome

Between 3 and 2,000 genes.

What is an obligate intracellular parasite?

A virus that can only replicate inside a host cell.

What is a host range?

The specific set of species a virus can infect.

General steps of viral replication

Virus attaches to the host cell. Viral genetic material enters the cell. Host cell replicates viral genome and produces viral proteins. New viral particles assemble. Viruses are released to infect new cells.

Two types of bacteriophage replication cycles

Lytic cycle and lysogenic cycle.

What happens in the lytic cycle?

The virus replicates inside the host, eventually lysing (breaking open) the cell to release new viruses.

What happens in the lysogenic cycle?

The viral DNA integrates into the host genome as a prophage and replicates with the host cell until activated.

What is a temperate phage?

A bacteriophage that can use both lytic and lysogenic cycles.

How do bacteria defend against viruses?

Mutation of surface receptors to prevent viral attachment

Use of restriction enzymes to cut viral DNA. The CRISPR-Cas system, which stores viral DNA to recognize and destroy future infections.

What is the CRISPR-Cas system?

A bacterial immune mechanism that records and targets viral DNA for destruction.

Two key factors in classifying animal viruses

DNA or RNA genome/ presence or absence of a viral envelope.

How do viral envelopes help viruses infect host cells?

The envelope contains glycoproteins that bind to host cell receptors.

What are retroviruses?

RNA viruses that use reverse transcriptase to convert their RNA into DNA (e.g., HIV).

What is a provirus?

A viral DNA segment that permanently integrates into the host genome.

Possible origins of viral genomes

Plasmids and transposons, which are mobile genetic elements.

Why are giant viruses controversial?

They have large genomes with genes related to DNA repair and protein synthesis, challenging traditional virus definitions.

How do viruses cause disease?

Destroying host cells directly.Causing the release of toxic lysosomal enzymes.Producing harmful viral proteins.

How do vaccines work?

They expose the immune system to harmless forms of pathogens to build immunity.

Can antibiotics treat viruses?

No, antibiotics target bacteria, not viruses.

How do antiviral drugs work?

They inhibit viral replication by blocking DNA/RNA synthesis or virus assembly.

What are emerging viruses?

Newly appearing viruses that can cause outbreaks (e.g., HIV, Ebola, Zika).

Common causes of emerging viral diseases

Mutation of existing viruses.Spread from isolated populations.Transmission from animals to humans.

How are flu strains classified?

By their H (hemagglutinin) and N (neuraminidase) surface proteins.

What is a pandemic?

A global outbreak of a viral disease (e.g., H1N1 in 2009).

How does climate change affect viral spread?

Warmer temperatures may expand mosquito populations that carry viruses.

Common effects of plant viruses

Leaf spots, stunted growth, damaged roots and flowers.

Two main transmission routes for plant viruses

Horizontal transmission – Infection through external sources.Vertical transmission – Inherited from parent plants.

What are prions?

Infectious proteins that cause neurodegenerative diseases.

Examples of prion diseases

Mad cow disease, scrapie, and Creutzfeldt-Jakob disease.

How do prions replicate?

By inducing normal proteins to misfold and form aggregates.

Why are prions difficult to destroy?

They are extremely stable and resistant to heat and chemicals.