Cels 32

What are viruses?

Acellular obligate intracellular parasites that require a host cell to reproduce.

Why are viruses called a “borrowed life”?

They have no metabolism of their own and rely completely on host cells.

What does obligate intracellular parasite mean?

An organism that must live and reproduce inside a host cell.

What types of organisms can viruses infect?

Animals, plants, bacteria, fungi, and algae.

What is a host organism?

The organism infected by a virus.

What is a host cell?

The cell in which a virus replicates.

What is the approximate size range of viruses?

10–400 nm.

What are the two essential components of a virus?

Genetic material and a capsid.

What is the viral genome made of?

Either DNA or RNA.

Can viruses contain both DNA and RNA?

No, viruses contain either DNA or RNA, never both.

What is a capsid?

A protein coat that surrounds and protects the viral genome.

What are capsomers (capsomeres)?

Repeating protein subunits that make up the capsid.

What are the three major types of capsid symmetry?

Helical, icosahedral, and complex.

What type of capsid symmetry does Tobacco Mosaic Virus (TMV) have?

Helical.

What type of capsid symmetry does adenovirus have?

Icosahedral.

What type of capsid symmetry do bacteriophages have?

Complex symmetry.

What is an icosahedron?

A 20-faced polyhedron with triangular faces.

What is a helical capsid?

A spiral arrangement of capsomers around the genome.

What is a complex virus?

A virus with multiple structural symmetries, such as bacteriophages.

What is the difference between naked and enveloped viruses?

Naked viruses lack a lipid envelope while enveloped viruses possess one.

What is the viral envelope made of?

Lipids.

What is the main function of the viral envelope?

Facilitates attachment and entry into host cells.

Why are enveloped viruses generally more fragile than naked viruses?

The lipid envelope is easily disrupted.

What structures on enveloped viruses help bind host cells?

Glycoprotein spikes.

What is the function of viral glycoprotein spikes?

Attachment to host receptors and cell entry.

What are bacteriophages?

Viruses that infect bacteria.

What is the lytic cycle?

A viral replication cycle that destroys the host cell to release new viruses.

What are the six steps of viral infection?

Attach, penetrate, uncoat, genome replication/gene expression, assembly, and release.

What occurs during viral attachment?

The virus binds to a specific receptor on the host cell.

Why are receptors important in viral infection?

A virus cannot infect a cell without binding a receptor.

What occurs during penetration?

The virus or viral genome enters the host cell.

What occurs during uncoating?

The viral capsid is removed and the genome is released.

What occurs during viral gene expression and genome replication?

The virus replicates its genome and synthesizes viral proteins.

What occurs during assembly?

New viral particles are assembled.

What occurs during release?

New viruses leave the host cell.

What are common viral release mechanisms?

Lysis, budding, and exocytosis.

What is burst size?

The number of viruses released from one infected cell.

What is the approximate burst size of bacteriophages?

100–200 viral particles.

What is SARS-CoV-2?

An enveloped positive-sense single-stranded RNA virus.

What type of genome does SARS-CoV-2 have?

Linear +ssRNA non-segmented genome.

What does +ssRNA mean?

Positive-sense single-stranded RNA that can function directly as mRNA.

What receptor does SARS-CoV-2 bind to?

ACE2 receptor.

What viral protein binds the ACE2 receptor?

The spike protein.

Why is the spike protein important?

It mediates attachment and entry into host cells.

Why are vaccines designed to target the SARS-CoV-2 spike protein?

It is essential for viral attachment and infection.

Where does SARS-CoV-2 replicate?

In the cytoplasm.

What enzyme does SARS-CoV-2 use to copy its RNA genome?

RNA-dependent RNA polymerase.

Why does SARS-CoV-2 generate variants frequently?

Its RNA polymerase has limited proofreading ability.

What is HIV?

An enveloped RNA retrovirus that causes AIDS.

What cells does HIV primarily infect?

CD4 T cells.

What is a retrovirus?

An RNA virus that converts its RNA genome into DNA.

What enzyme converts HIV RNA into DNA?

Reverse transcriptase.

What is reverse transcriptase?

An enzyme that synthesizes DNA from an RNA template.

What is a provirus?

Viral DNA integrated into the host genome.

What happens after HIV DNA integrates into the host genome?

It becomes a provirus and can direct production of new viruses.

What is the role of viral integrase in HIV?

It inserts viral DNA into the host genome.

What is the difference between DNA and RNA viruses?

DNA viruses use DNA genomes while RNA viruses use RNA genomes.

What are the four possible forms of viral nucleic acid?

ssDNA, dsDNA, ssRNA, and dsRNA.

What is a segmented viral genome?

A genome divided into separate nucleic acid segments.

Give an example of a segmented virus.

Influenza virus.

Give an example of a circular viral genome.

Hepatitis B virus.

Give an example of a linear viral genome.

Poxvirus.

Why are viruses considered acellular?

They are not made of cells and lack cellular machinery.

Why must viruses hijack host machinery?

They cannot perform metabolism or reproduce independently.

What are the major structural components of an enveloped virus?

Genome, capsid, envelope, and glycoprotein spikes.