Lecture 1- Viruses

Question 1

What features of viruses make them good model organisms?

Answer 1

• Small genomes
• Reproduce quickly
• Usually haploid- (one copy of a gene makes genetic analysis easier.)

Question 2

The first virus was discovered in the 1890’s by Dmitri Ivanovsky. What virus did he discover and how was it discovered?

Answer 2

The agent that caused Tobacco mosaic disease- the destruction of photosynthetic tissues that devastates the tobacco crops.
He passed leaf extract through a porcelain filter to isolate the disease causing bacteria, however, the disease agent was not retained- the liquid filtrate was still able to cause disease.

Question 3

Viruses are described as what kind of parasite?

Answer 3

Obligate intracellular parasites. They can develop and reproduce only within a specific hosts cell, using its DNA replication and protein synthesis machinery.

Question 4

What are virions?

Answer 4

The virus particle, the minimum unit capable of infecting a cell. (Viruses outside of the host cell).

Question 5

What do virions consist of?

Answer 5

A central core of DNA or RNA surrounded by a capsid of proteins for protection.

Question 6

Why do antibiotic not work against viruses?

Answer 6

Antibiotics target cell walls or ribosomes of bacteria. Viruses do not have cell walls or ribosomal biochemistry.

Question 7

On what 6 characteristics are viruses classified by?

Answer 7

Genome of DNA or RNA
Nucleic acid is double or single-stranded
Simple or complex shape
Virion surrounded by a membrane or not
Type of organism it infects
Manner of infection, such as lytic or lysogenic

Question 8

Define bacteriophage

Answer 8

One of a group of viruses that infect bacteria and ultimately cause their disintegration.

Question 9

What is the first step a bacteriophage takes in their reproductive cycles?

Answer 9

A protein on the capsid of the phage binds to a receptor protein or carbohydrate on the host’s cell wall and injects its nucleic acid using tail assemblies.

Question 10

Define the lytic cycle.

Answer 10

A form of viral reproduction that lyses the host bacterium releasing the new viruses.

Question 11

The phage DNA of a virulent virus is inside a host cell. How does the viral genome effect the host cell?

Answer 11

The cell enters the lytic cycle.
The promoter sequence of the viral genome attracts RNA polymerase.
Early genes code for proteins that shut down host cell transcription and stimulate viral genome replication and transcription.
The host DNA is then digested by viral nuclease enzymes to provide nucleotides.

Question 12

New phage DNA forms during the lytic cycle. What is the next step in this cycle?

Answer 12

The host cell transcribes phage DNA and translates viral RNA, producing phage proteins. Late genes code for proteins of the viral capsid. The phage encoded protein endolysin lyses the cell. The released progeny start the cell again.

Question 13

A lysogenic bacteriophage injects its nucleic acid into a bacteria cell. What effect does this have on the cell?

Answer 13

The phage DNA integrates its self into the bacterial chromosome and becomes a non-infective prophage. The chromosome with its prophage replicates through many cell divisions.

Question 14

How does a cell switch from the lysogenic cycle to the lytic cycle?

Answer 14

The prophage excises its self from the host chromosome when the host becomes stressed or damaged by mutagens. This activates the lytic cycle.

Question 15

What is an advantage of the lysogenic cycle?

Answer 15

Enhances opportunities to produce maximum number of progeny viruses.

Question 16

What can happen if two viruses infect one cell?

Answer 16

It is possible that the viral genomes could cross over, resulting in genetic recombination and new strains being produced.

Question 17

Bacteriophages could be useful to humans. What are these uses?

Answer 17

• Control agents for bacteria-caused diseases.
  Felix D’Herelle noted patients with bacterial dysentry had more phage near bacteria as they recovered then when the bacterial infection was at its peak. Additionally, viruses can be used in gene therapy or as pesticides, but may cause consequences to the eco-system.

Question 18

In what invertebrates are viruses common, and what are these viruses called?

Answer 18

Arthropods, arboroviruses (the insect is the vector, the virus is transmitted through insect bites to vertebrates)

Question 19

Enveloped viruses have a membrane derived from the host cell’s plasma membrane. When is this acquired?

Answer 19

Following viral reproduction, enveloped viruses escape a cell by a budding process- their membrane envelope is from the host cell’s plasma membrane

Question 20

How do animal viruses enter cells?

Answer 20

Endocytosis- either a naked virion, or enveloped virus with glycoproteins that bind to receptors.
Fusion of membrane of host cell and enveloped virus

Question 21

Why is HIV described as a retrovirus?

Answer 21

It has reverse transcriptase- this transcribes RNA into DNA. A DNA provirus is produced that is integrated permanently into the host’s genome. When the proviral DNA is activated, new virions are produced.

Question 22

What 2 ways can viruses spread in plants?

Answer 22

Horizontally, from one plant to another through vectors such as mites. Vertically, from parent to off spring.

In plants, viruses must pass cell wall and plasma membrane. Viruses may also spread through the plasmodesmata.

Question 23

Streak mosaic virus

Answer 23

Tiny mite vector, destruction of photosynthetic tissue, yellow streaks on leaves, reduces grain production.

Question 24

Influenza, an enveloped virus, binds its glycoproteins to receptors on the host cell’s plasma membrane and is taken up by endocytosis. What happens once inside the cell?

Answer 24

The viral membrane and vesicle membrane fuse, the host cell digests the protein capsid and the viral nucelic acid is released.

Question 25

Once viral nucleic acid of influenza is released inside an animal cell, what happens?

Answer 25

Viral RNA acts as a template to make mRNA via viral RNA dependent RNA polymerase. Then the viral mRNA can either be used to make more viral RNA genomes by successive RNA polymerase events, or moves to the ribosome to be translated into viral proteins.

Question 26

After viral mRNA is translated into viral proteins, where do these proteins go?

Answer 26

The envelope glycoproteins are made in the host endoplasmic reitculum and are transported to the plasma membrane via the golgi body. When the virion is assembled, it leaves the cell via budding.