Viruses

Question 1

Virus

Answer 1

An infectious particle consisting of nucleic acids encoded in a protein coat

Question 2

Why do viruses “straddle” the definition of life?

Answer 2

1) They can’t replicate on their own: Require a host cell to replicate

2) Also can’t perform metabolic processes on their own

Question 3

Viral Genome Possibilities

Answer 3

1) Single-Stranded RNA
2) Double-Stranded DNA

3) Double-Stranded RNA
4) Single-Stranded DNA

Question 4

Virus genome can be…

Answer 4

Continuous or segmented: can be one molecule or multiple molecules

Question 5

Viruses with segmented genome…

Answer 5

tend to have higher rates of mutation as during replication, recombination of the genome can occur

Question 6

Capsid

Answer 6

The protein shell enclosing a viral genome

Question 7

What makes up the capsid?

Answer 7

Capsomeres

–> made of a large number of small protein subunits (usually just the same one or few repeating over and over again)

Question 8

Capsomere

Answer 8

The protein subunits that make up the capsid

Question 9

Most animal viruses also contain a…

Answer 9

Viral envelope (AKA membranous envelope)

Question 10

Viral Envelope

Answer 10

Derived from a host cell’s membrane –> Cloaks the viral capsid and gives the virus the cell’s membrane proteins

Question 11

Bacteriophage

Answer 11

Viruses that infect bacterial cells

Question 12

First virus found

Answer 12

Tobacco Mosaic Virus

Question 13

Viruses are “Obligate Intracellular Parasites” which means…

Answer 13

They can only replicate in host cells

–> Lack metabolic enzymes and the equipment needed for making proteins (Ex: they lack ribosomes)

Question 14

Why are viruses called “selfish genes”?

Answer 14

Because they only exist to replicate themselves and continue on no matter what

Question 15

Host Range

Answer 15

The range of species/cells/hosts a virus is able to infect

Question 16

What determines a virus’s host range?

Answer 16

The interaction between viral surface proteins and host cell membrane receptors

–> Interact in a “lock and key” or “handshake” model

Question 17

Variation of host range

Answer 17

Some species can have:

1) Narrow host range = can only infect one species/cell type

2) Broad host range = can infect multiple species/cell types

Question 18

Generic reproductive process of viruses (simple DNA virus)

Answer 18

1) Virus enters cells
2) Virus gets uncoated: releases capsid proteins and viral DNA

3) Some of the viral DNA goes to be transcribed and the rest goes to be replicated to produce more viral DNA

4) mRNA produced from transcription of viral DNA then gets translated to produce more capsid proteins

5) Capsid proteins + replicated viral DNA spontaneously self-assemble

6) New virus is formed and leaves the cell in some manner

Question 19

Reproductive Cycles of Viruses

Answer 19

1) Lytic Cycle

2) Lysogenic Cycle

Question 20

Lytic Phase

Answer 20

Ends with cell LYSIS (cell dies/is destroyed)

–> Causes massive release of viral progeny which can then go on to infect neighboring cells

Question 21

Lysogenic Phase

Answer 21

“latent” viral stage; the viral genome integrates into host cell’s genome

–> The genomes coexist with the viral DNA getting replicated alongside the host cell’s DNA

–> Does NOT destroy the host cell

Question 22

What causes a switch between lytic and lysogenic cycles?

Answer 22

It depends: It’s usually some environmental change (like a certain chemical) that triggers the switch

Question 23

Lysogenic Phase: Process (in bacteria)

Answer 23

1) Virus infects host cell with genome

2) Viral genome circularizes

3) Viral genes integrate with the host cell’s chromosomes by recombination

4) Cell reproduces normally with viral DNA in it
–> Cell divisions occur = new cells with viral DNA in them

——– Process can end here if the virus never leaves lysogenic phase

5) In some cases, the viral DNA will EXCISE from the host cell genome and enter the LYTIC cycle

Question 24

Lytic Phase: Process (in bacteria)

Answer 24

1) Virus infects host cell with genome

2) Viral genome circularizes

3) New virus production begins: Production of both capsid proteins (and any other viral proteins) and viral genome

4) Cell lyses and releases a ton of viral progeny

–> cycle begins again in another cell

Question 25

How is the viral envelope produced?

Answer 25

Through viruses hijacking the endo-membrane system to produce viral glycoproteins that then get secreted onto the plasma membrane

–> Then when new viral progeny bud out of the cell they are enveloped by the membrane containing the viral glycoproteins

Question 26

Animal Viruses Reproduction Process

Answer 26

1) Viral glycoproteins on virus surface bind to cell membrane receptors

2) Virus enters cell by fusing with PM or endocytosis

3) Enzymes digest the capsid and release the genome

4) Viral DNA goes to replicate + get transcribed

5) mRNA gets translated into capsid proteins and preliminary proteins targeted to the ER

6) Virus uses endo-membrane system to then produce glycoproteins in the ER

7) Viral glycoproteins secreted to the PM surface through transport vesicles that fuse with the PM

8) Capsid proteins and viral genome self-assemble

9) Newly formed viruses bud out of the cell, getting enveloped by PM with the viral glycoproteins on it

Question 27

Enveloped viruses acquire…

Answer 27

HOST-derived membranes enriched in VIRAL proteins to form their external envelope

Question 28

3 possible genome functions for single-stranded RNA viruses:

Answer 28

1) Single-stranded RNA that acts as mRNA

2) Single-stranded RNA that acts as a TEMPLATE for mRNA production

3) Single-stranded RNA that acts as a TEMPLATE for DNA production (Retrovirus)

Question 29

Single-stranded RNA that acts as mRNA

Answer 29

The viral genome is just directly transcribed (since it is acting as an mRNA already)

Question 30

Single-stranded RNA that acts as TEMPLATE for RNA

Answer 30

RNA genome gets transcribed into a complementary RNA strand (mRNA) that then undergoes translation

–> Requires special enzyme!

Question 31

Enzyme for RNA(template) —> RNA(synthesis)

Answer 31

RNA Dependent RNA Polymerase (RdRP)

Question 32

Retrovirus

Answer 32

A “backwards” virus –> Single stranded RNA acts as a TEMPLATE for DNA synthesis

–> Requires a special enzyme!

Question 33

Enzyme for RNA(template) —> DNA(synthesis)

Answer 33

Reverse Transcriptase

Question 34

Reverse Transcriptase

Answer 34

Transcribes an RNA template into a DNA strand

Question 35

What happens to the DNA produced from a retrovirus?

Answer 35

It gets inserted into the host cell’s genome as a “provirus”

Question 36

Provirus

Answer 36

A virus genome that is integrated into the host cell’s genome

Question 37

Where is reverse transcriptase kept in the virus?

Answer 37

Packaged with the viral genome within the capsid

Question 38

Retrovirus reproduction process

Answer 38

1) Virus gets into cell and viral capsid is removed

2) Reverse transcriptase catalyzes the synthesis of a DNA strand from the viral RNA template

–> Once complete, a single DNA strand is produced and the RNA is hydrolyzed (degraded)

3) Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first one

4) The produced double stranded DNA enters the nucleus

5) DNA incorporates into host genome as a provirus

6) Proviral genes are transcribed into:
1. mRNA for translation
2. RNA for the new viral progeny genome

7) Viral proteins produced:
1. capsid proteins
2. Reverse transcriptase
3. glycoproteins

8) Self assembly of the virus

9) Virus buds off and exits cell

Question 39

3 main sources of viral emergence:

Answer 39

1) Mutations creating new strains
2) Zoonosis mutations
3) Dissemination

Question 40

Mutations creating new viral strains

Answer 40

The mutation of an existing virus creates new viruses that can spread more easily

–> Can create new viral strains with enough genetic differences from earlier strains that allows for the EVASION OF IMMUNITY (people with acquired immunity from earlier strains are no longer immune to the newer one)

Question 41

Why do RNA viruses tend to mutate more than others?

Answer 41

Because replication of the viral RNA genome lacks proof reading mechanisms = greater mutation rate

Question 42

Zoonosis

Answer 42

When a virus undergoes mutation that BROADENS its host range

–> Allows existing viruses to “hop” from one species to another

Question 43

Dissemination

Answer 43

The spread of a viral disease from a small-isolated population

–> Is more of an issue now that we are able to move more freely around the world = greater spreading

Question 44

Methods of combatting viral infections

Answer 44

1) Vaccines
2) Anti-viral drugs

Question 45

Vaccines

Answer 45

Harmless viral variants or derivates that stimulates the immune system to mount defense against the actual harmful pathogen

–> a PREVENTATIVE measure

Question 46

Vaccines cannot…

Answer 46

treat a viral infection once it has started

Question 47

Anti-Viral Drugs

Answer 47

Combat viruses by interfering with virus-specific processes: usually by the interference of viral nucleic acid synthesis (stopping viral reproduction)

–> an ACTIVE measure; tries to TREAT an infection that has already begun