Viruses Overview

What does “acellular” mean in the context of viruses?

Viruses consist of a genome (DNA or RNA) and a protein capsid, sometimes with a lipid envelope, but lack organelles, cytoplasm, or metabolism.

Why are viruses considered obligate intracellular parasites?

They cannot reproduce or carry out metabolism outside a host cell.

What viral enzymes might be carried by viruses?

Reverse transcriptase and neuraminidase

What are the three main virus shapes

Icosahedral (e.g., adenovirus), helical (e.g., TMV), and complex (e.g., bacteriophages).

What types of viral genomes exist?

ds/ss DNA or RNA, linear or circular, positive or negative sense.

What’s the difference between enveloped and naked viruses?

A: Enveloped viruses have a lipid coat from the host and viral spikes (fragile); naked viruses consist only of a capsid (heat/drying resistant).

What is the structure of Tobacco Mosaic Virus?

Helical shape, RNA genome, capsomere of capsid

What is the structure of Adenovirus?

Polyhedral shape, DNA genome, glycoprotein, capsomere.

What is the structure of Influenza Virus?

Spherical shape, RNA genome, membranous envelope, glycoprotein, capsid

What is the structure of a Bacteriophage?

Complex shape with a head (DNA) and tail fibres

What is the progressive (escape) hypothesis?

Viruses originated from mobile genetic elements like retrotransposons.

What is the regressive (reduction) hypothesis?

Viruses evolved from parasitic cells that lost genes (e.g., Mimivirus).

What is the convergent hypothesis?

Viral traits like capsids and parasitism evolved independently multiple times

What does the universal genetic code suggest about virus evolution?

Viruses evolved after cellular life

Why do viruses evolve rapidly?

Short generation times, high mutation rates (especially RNA), and strong selection pressures

What are two genetic mixing mechanisms in viruses?

Recombination (e.g., HIV) and reassortment (e.g., influenza).

What is the first step of the lytic cycle?

Attachment — the phage binds to the surface of the host cell.

What happens during penetration in the lytic cycle?

Only the viral nucleic acid enters the host cell, not the entire virus.

What occurs during biosynthesis?

Phage DNA replicates and phage proteins are synthesized

What is the maturation step in the lytic cycle?

New phage particles are assembled inside the host

What is the final step of the lytic cycle?

Lysis — the host cell bursts, releasing new phages.

What is lysogeny?

The viral genome integrates into the host’s DNA and remains dormant.

What triggers the lysogenic cycle to enter the lytic pathway

An environmental event causes induction, activating the lytic cycle

How does the lysogenic cycle differ from the lytic cycle

It involves viral DNA integration and dormancy instead of immediate replication and lysis

What happens during bacterial reproduction in the lysogenic cycle

Integrated viral DNA integration and dormancy instead of immediate replication and lysis

What ha