CH. 6 Viruses, Viroids, and Prions

Question 1

What is a virus? Describe and define what a virus is.

Answer 1

Viruses are acellular, obligate intracellular parasites that are made out of nucleic acids (DNA/RNA) in a capsid.

*must infect a host self to replicate
*subverts the cell’s machinery and directs it to produce viral particles
*size range: 0.02 - 0.90 micrometers

Question 2

What is a virion?

Answer 2

A virion is a virus particle that possesses a nucleic acid with a surrounding capsid

Question 3

How do viruses benefit ecosystems? List/describe these

Answer 3

• Limit host population densities
• Recycle nutrients
• Increase host diversity
• Gene transfer

Question 4

Describe the viral shunt in marine ecosystems – how is this beneficial?

Answer 4

Viral Shunt: a viral infection of hosts converts them to detritus, rich in organic and inorganic molecules for microbes to recycle for ecosystems

Question 5

Describe the virus life cycle contributions of the host cell?

Answer 5

Viruses subvert the cell’s machinery and direct it to produce viral particles using the host’s DNA polymerase, RNA polymerase, ribosomes, tRNA, metabolism, and nucleotides

*This varies between viruses

Question 6

What is viral infectivity and what serves as the basis for this?

Answer 6

Viral infectivity depends on the virus-host recognition and its interaction between viral surface proteins and host cell source molecules

Question 7

What is meant by host range? Provide examples.

Answer 7

A virus infects a particular group of host species:

Ex.
Broad -> rabies
Narrow range -> HIV (only humans)

Question 8

What is meant by tropism? What is the basis for this?

Answer 8

Tropism is the ability of a virus to infect different cellular types

Question 9

How do viral genomes differ among viruses?

Answer 9

Small viruses have less than 10 genes. Large viruses have over 100 genes

Virus genomes could be segmented/non-segmented and have antisense or sensing RNA strands

Question 10

Describe and characterize the structure of a virus.

Answer 10

Symmetrical Viruses: have an icosahedral or filamentous capsid and envelope deriving from the host that surrounds the capsid (ex. hepatitis A and herpesvirus). The capsid is a long tube of protein with a genome is coiled inside. It also has a tail attached to the icosahedral head for additional genome delivery

Asymmetrical Viruses: lacks capsid symmetry (ex. influenza)

Question 11

How do animal/plant viruses differ from bacterial viruses (i.e., phage).

Answer 11

Phages differ from animal viruses by having to penetrate into the host’s cell wall while animal viruses enter by endocytosis or membrane fusion

Animal/Plant viruses replication occurs in the nucleus of the host but in bacterial viruses, it occurs in the cytoplasm

Question 12

What are: glycoprotein spikes? envelope? What is their function?

Answer 12

Glycoprotein spikes function in host recognition & attachment or other function

The envelope derives from the host membrane to surround the virus capsid

Question 13

Describe, then compare and contrast viroids and prions. How are these different from viruses?

Answer 13

Viroids are RNA molecules without capsids that infect plants

Prions are proteins that infect animals (have NO nucleic acid component). They are often known as a misfolded form of a normal brain cell protein

Question 14

What are the criteria to classify viruses according to the ICTV? What is the Baltimore classification system based on?

Answer 14

The International Committee on Taxonomy of Viruses (ICTV) is based on: genome, capsid symmetry, envelope, host range, and virion size

The Baltimore Classification System is based on: the genome (RNA or DNA) & route used to express mRNA

Question 15

What are the different variations of single‐stranded RNA viruses?

Answer 15

• Sense RNA
• Antisense RNA
• Retrovirus

Question 16

What is sense & antisense RNA; how do they differ? How do they differ in terms of replication & transcription of their genomes? What about double‐stranded RNA viruses?

Answer 16

Sense RNA (+ssRNA) has its genomes act as an mRNA and can be translated into viral proteins (infectious genetic material)

Antisense RNA (- ssRNA) CANNOT be directly translated to make viral proteins. It has to convert into an +ssRNA strand by RDRP (RNA-dependent RNA polymerase) first (noninfectious genetic material)

Double-stranded RNA (ds RNA) is converted into +ssRNA by RDRP to then translate into viral proteins

Question 17

What are the basic steps of any viral life cycle?

Answer 17

1) Host recognition & attachment - virus recognition of the host cell surface proteins for binding using viral proteins

2) Genome entry - the capsid and genome enter the host cell

3) Synthesis & virion assembly - genome replication, protein synthesis; assembly of virions

4) Exit & transmission - release from host cell for progeny infect more host cells

Question 18

Describe the life cycles of bacteriophages. What are the two types of cycles? Describe each and the types of viruses that undergo each cycle.

Answer 18

The lytic cycle is when the phage quickly replicate killing the host’s cell (lytic burst) to then produce new viral particles for more infection. T even phages does replicate this way

The lysogenic cycle is when the phage’s DNA (prophage) is inserted into the bacterial chromosome, so it is allowed to be copied and passed on along with the cell’s own DNA. It usually remains dormant and does not necessarily kill the host cell. However, it can reactivate back to a lytic cycle based on environmental cues. Lambda and temperate phages does this

Question 19

Describe what is meant by slow-release exit from a host? What type of phage does this?

Answer 19

Slow release exit meaning the phage particles reproduce and exit without harming the host cell where the host cells grow slowly but does not die

The M13 filamentous phage (ssDNA) does this

Question 20

Describe bacterial defenses to viral infection.

Answer 20

Genetic resistance is the alteration of receptor proteins

Restriction endonucleases is the cleave viral DNA sequences lacking methylation

CRISPR is when the host cleaves phage DNA, inserting fragment into chromosome as a “spacer”

Question 21

What is uncoating of a virus? What are the different mechanisms of this?

Answer 21

The uncoating of the virus is when the virus genome is released from its capsid

1) uncoating at the cell membrane
2) uncoating within endosomes
3) uncoating at the nuclear membrane

Question 22

Describe/differentiate these animal virus infections: Human Papilloma virus; Picornavirus; and Retrovirus

Answer 22

Human Papillomavirus is a double-stranded DNA virus where transcription occurs in the nucleus and translation occurs outside the nucleus.

Picornavirus virus is a positive single-stranded RNA virus where it possesses an RDRP to transcribe their mRNA

Retrovirus is a positive single-stranded virus that uses reverse transcriptase to copy its genomic sequence into DNA for insertion in the host chromosome

Question 23

What is RNA‐dependent RNA polymerase? Why would a virus possess this? What type of virus would have this?

Answer 23

RNA-dependent RNA-polymerase is used to transcribe RNA viruses’ mRNA

ONLY non-retrovirus RNA viruses contains this

Question 24

What is reverse transcriptase? How does it function? What viruses possess this?

Answer 24

Reverse transcriptase is an enzyme that catalyzes the formation of DNA from an RNA template in reverse transcription

It functions as the uncoating to release RNA genome and viral enzymes to copy viral RNA to form dsDNA

Retrovirus (RNA) does this

Question 25

What is budding? How does this differ from lysis?

Answer 25

Budding is when the virus develops an envelope from the host cell membrane and then detach from the host without killing them

Lysis is the same process BUT it kills the host cell

Question 26

Describe animal cell defenses to viral infection.

Answer 26

Genetic resistance where the hosts continually experience mutations

The immune system where interferon (IF) is produced by virus-infected cell, so it can produce anti-viral proteins in non-infected cells for protection. Adaptive immunity can also be develop by having antibodies to the virus

RNA interference the host expressed RNA’s that are complimentary to viral genome sequence to shut down expression of viral genes (within eukaryotes & archaea)