VIRAL LIFE CYCLES (LECTURE 10)

Question 1

Describe the virus structure.

Answer 1

A virus is a viral genome enclosed in a protective coat.

Question 2

Describe the viral genome.

Answer 2

A single or double stranded DNA or RNA, either linear or circular.

Question 3

Describe the protective coat of a virus. (3)

Answer 3

Also known as the capsid, it is either rod-shaped or polyhedral and is comprised of protein subunits called capsomeres.

Question 4

What third element of the virus structure can be found in some viruses?

Answer 4

An envelope.

Question 5

Describe the virus envelope. (3)

Answer 5

It is a membrane that covers the capsid and is derived from the host membrane. It contains viral proteins and glycoproteins.

Question 6

Define a virus in terms of how it reproduces.

Answer 6

Viruses are obligate intracellular parasites.

Question 7

Viruses are ____ specific.

Answer 7

Viruses are host specific (the host being the cell they infect, not the organism to which these cells belong).

Question 8

What cells does the influenza virus infect?

Answer 8

The influenza virus infects cells of the respiratory tract.

Question 9

How do viruses infect only specific cells?

Answer 9

By specific interactions between the virus’s glycoproteins and host-cell proteins.

Question 10

List the 5 steps of virus infection.

Answer 10

(1) Virus recognizes cell (2) Genome enters cell (3) Viral DNA is copied and proteins are produced (4) Self-assembly (5) New viruses are released.

Question 11

How does a virus recognize a potential host cell?

Answer 11

Through protein-protein interaction between the membrane proteins on the target cell and the viral proteins.

Question 12

When the viral genome enters the host cell, is it DNA or RNA?

Answer 12

Either.

Question 13

How is viral DNA copied once within the host cell, and how are new viral proteins produced? (5)

Answer 13

The host machinery is used, as in the host’s enzymes, ribosomes, tRNA, amino acids, ATP.

Question 14

Describe the self-assembly step of viral infection.

Answer 14

Self-assembly refers to the formation of viral particles (genome + capsid).

Question 15

In what ways are new viruses released from a host cell?

Answer 15

Either via cell lysis or budding.

Question 16

What type of virus is released from the host through cell lysis?

Answer 16

Non-enveloped viruses.

Question 17

What type of virus is released from the host through budding?

Answer 17

Enveloped viruses.

Question 18

How do viral glycoproteins get into the viral envelope if the envelope is a piece of the host cell’s membrane?

Answer 18

Viral glycoproteins have signal peptides - synthesized at ER and inserted into ER membrane.

Question 19

What are the 2 types of bacteriophage life cycles?

Answer 19

The lytic life cycle and the lysogenic life cycle.

Question 20

What type of cells do bacteriophages infect?

Answer 20

Bacteria.

Question 21

Give an example of a bacteriophage with the lytic life cycle. What bacteria does it infect?

Answer 21

T4 bacteriophage, which infects E. Coli.

Question 22

List the steps of the lytic life cycle. (4)

Answer 22

(1) Phage attaches to the cell surface and injects DNA into cell. (2) Enzymes destroy host cell’s DNA. (3) Phage components are produced by host cell. (4) Host cell lysis releases new phage particles.

Question 23

Why do bacterial enzymes destroy the bacteria’s DNA after being infected by bacteriophage (lytic life cycle)?

Answer 23

The host cell’s transcription and translation machinery will transcribe and translate viral genome - first proteins produced will destroy host’s DNA.

Question 24

When phage components are produced by a host cell infected by a bacteriophage (lytic life cycle), how do these components assemble?

Answer 24

Many copies of the phage genome and capsid proteins are made and then the phage components spontaneously assemble.

Question 25

How does the host cell lyse when infected with bacteriophage (lytic life cycle)?

Answer 25

Lysozyme produced from the viral genome digests the bacterial cell wall - the cell will then undergo lysis du to osmosis.

Question 26

What is the primary difference between the lysogenic life cycle and the lytic life cycle?

Answer 26

In the lysogenic life cycle, there is replication of the viral genome without destroying the host cell.

Question 27

Provide an example of a bacteriophage characterized by the lysogenic life cycle.

Answer 27

Lambda phage, which is both lytic and lysogenic.

Question 28

List the steps of the lysogenic life cycle. (4)

Answer 28

(1) Phage attaches to cell surface and injects DNA into cell. (2) Phage DNA circularizes - can enter lytic or lysogenic cycle. (3) If it enters lysogenic cycle, DNA integrates into the bacterial chromosome by genetic recombination. (4) Prophage exits the bacterial chromosome to initiate lytic cycle.

Question 29

What is prophage?

Answer 29

Incorporated phage genome (into bacterial chromosome) is a prophage.

Question 30

Are prophage genes expressed?

Answer 30

Only one prophage gene is expressed - it codes for a repressor protein and inactivates the other prophage genes (sleeping virus).

Question 31

Does prophage replicate? How?

Answer 31

Prophage replicates as part of the host DNA.

Question 32

What triggers prophage to exit the bacterial chromosome and to initiate the lytic cycle?

Answer 32

Can be triggered by radiation or a chemical trigger.

Question 33

What is the primary bacterial defense against bacteriophages?

Answer 33

Restriction endonucleases.

Question 34

How do restriction endonucleases protect against bacteriophages?

Answer 34

Restriction endonucleases cut foreign DNA that enters the cell.

Question 35

Why don't restriction endonucleases cut bacterial DNA?

Answer 35

Bacterial DNA is protected from degradation by methylation.

Question 36

What are retroviruses?

Answer 36

Enveloped viruses with a single stranded RNA genome.

Question 37

What function does the single stranded RNA genome (in retroviruses) serve?

Answer 37

It serves as a template for DNA synthesis by the enzyme reverse transcriptase.

Question 38

Does the reverse transcriptase that synthesizes DNA from the viral genome of a retrovirus belong to the host?

Answer 38

No, it is a viral enzyme.

Question 39

Provide an example of a retrovirus.

Answer 39

Human Immunodeficiency Virus (HIV).

Question 40

List the steps of the retroviral life cycle.

Answer 40

(1) Virus enters the cell (2) Reverse transcriptase reverse transcribes the RNA viral genome (3) DNA is inserted into host chromosome as a provirus (4) Viral RNA and viral proteins are produced from provirus (5) Virus is assembled and can leave the cell.

Question 41

How do retroviruses leave a host cell after being assembled?

Answer 41

It buds from the cell.

Question 42

Characterize the SARS-CoV-2 virus (COVID).

Answer 42

Enveloped virus with a single stranded RNA genome.

Question 43

Are prions viruses?

Answer 43

No.

Question 44

What are prions?

Answer 44

Prions are a misfolded form of a protein normally found in brain cells.

Question 45

Why are prions dangerous?

Answer 45

Once in a cell, they convert the normal form of the protein to the misfolded prion form. Prions thus form aggregates that disrupt normal cell functions in the brain.

Question 46

How quickly can prions affect someone?

Answer 46

Prions act slowly (10+ year incubation).

Question 47

Can we destroy prions?

Answer 47

Not really, they are virtually indestructible.

Question 48

Provide 4 diseases caused by prions.

Answer 48

Creutzfeldt-Jakob disease, mad cow disease, scrapies and kuru.