Virus Structure and Function

Question 1

What are capsids?

Answer 1

highly conserved structural motif. protect and help deliver genome. Surrounded by lipid envelope

Question 2

What are the two types of capsids?

Answer 2

Helical capsids - rotational symmetry, form a disk

Icosahedral - 20 triangular faces with 2,3,5 fold axis of symmetry

Question 3

What are the classes of viruses

Answer 3

dsDNA
gapped circular dsDNA
ssDNA
dsRNA
ss(+) RNA
ss(-) RNA
ss(+) RNA w/ DNA intermediate

Question 4

dsDNA viruses

Answer 4

mRNA made using – DNA strand as template using a host RNA Pol II. Genome replication can happen using host DNA polymerase too

Question 5

gapped circular dsDNA

Answer 5

mix of ssDNA and dsDNA. mRNA transcribed using – DNA strand as template using a host RNA Pol II. Can then use reverse transcriptase to replicate genome from the produced mRNAs.

Question 6

ssDNA viruses

Answer 6

mRNA transcribed using – DNA strand as template using host RNA Pol II. Genome is replicated by using a dsDNA intermediate to serve as a template.

Question 7

dsRNA viruses

Answer 7

These viruses have to have RdRp in their capsid to transcribe mRNA off the genome. RdRp also helps with actual genome replication.

Question 8

ss(+)RNA

Answer 8

Translated directly into protein by cellular ribosomes (genome is ‘ribosome ready’). mRNA production via RdRp. Upon infection → ssRNA converted to dsDNA by reverse transcriptase. dsDNA incorporates into host genome → creates a provirus.

Question 9

ss9-) RNA

Answer 9

first copied into + strand mRNA by RdRp. Genome is also replicated by RdRp.

Question 10

ss(+)RNA w/ DNA intermediate

Answer 10

virus copies ssRNA into dsDNA with reverse transcriptase → integrates into host cell → mRNA transcribed using host’s RNA Pol II. Genome replicated with help of RdRp.

Question 11

Describe general replication cycle of a virus

Answer 11

Eclipse period - when virus particles have broken down, no virus is detectable in or outside of cell

Latent period - time it takes from start of infection to the release of new virus particles

Packing - assembly of new viran genome

Egress - release!

Question 12

What occurs during the latent period?

Answer 12

Attachment of virus to a receptor (targets are proteins/carbs, carbs less specific.)

Entry - naked virus endocytosed. Enveloped ciruses fuse w/ host membrane (requires fusion protein)

Both uncoated so the genome is exposed.

Gene expression - DNA viruses, - strand used as template, all gaps must be filled, use host RNA Pol II in the nucleus. (Poxvirus replication in cytoplasm)

In RNA viruses - RdRp helps produce mRNA and replicate RNA genomes.

Question 13

Descripe what occurs during virus packing

Answer 13

• icosahedral → capsid either assembles around the viral genome, or the genome is fed into the capsid.
• helical → viral genome is coated with nucleocapsid protein during synthesis.
• packing generally causes a cytopathic effect and can be demonstrated as cytoplasmic/nuclear inclusions

Question 14

Describe what happens during egress

Answer 14

• naked capsids released by lysis

- enveloped viruses bud either directly from the plasma membrane, or from the Golgi/ER