Chapter 19: Viruses

Question 1

What is a virus?

Answer 1

An infectious particle (pathogen) made of genes packaged in a protein coat.

NOT ALIVE

Question 2

Why are viruses considered nonliving?

Answer 2

They require a host cell in order to reproduce themselves and carry out their functions.

Question 3

True or false:

Viruses can be composed of either double/single stranded DNA and double/single stranded RNA.

Answer 3

True

Question 4

What is a capsid?

Answer 4

Protein coat that protects the virus.

Comes in different shapes; round, spaceship etc.

Question 5

If you get an infection, is it beneficial or harmful to the virus?

Answer 5

Beneficial - it will help the virus enhance its abilities

However, this will harm you

Question 6

What does a virus use a host cell for?

Answer 6

Reproduction:

• DNA Replication
• Transcribe and Translate

Question 7

Explain the process of how a virus infects a cell.

Answer 7

Step 1
- Virus binds to the cell with receptors

Step 2
- Viral DNA/RNA and capsid proteins enter the cell

Step 3
- DNA is replicated and mRNA is made to help the capsid proteins replicate

Step 4

• Replicated DNA and capsid proteins self-assemble into newly formed viruses
• New viruses exit the cell

Question 8

Which of the following is a likely explanation for why the influenza virus can infect respiratory epithelial cells but no epithelial cells on the surface of your skin?

A. Only respiratory epithelial cells come into contact with the virus
B. Respiratory cell membranes are permeable to the virus while skin cell membranes are not
C. Respiratory cells contain a receptor that the virus can bind to while skin cell membranes do not
D. Immune cells can protect skin cells but not respiratory cells

Answer 8

C

Question 9

What is a viral envelope?

Answer 9

A layer of glycoproteins that disguise the virus to look like a cell
(Circular layer)

Question 10

How do replicated viruses get their glycoprotein envelope when they exit the host cell?

Answer 10

Glycoproteins are stored within the rough ER of the cell and are embedded into the cell membrane for the viruses to use to build an envelope

Question 11

A viral envelope protein is synthesized:

A. On a free ribosome in the cytoplasm
B. In the same way as a secreted cellular protein
C. In the same way as a membrane cellular protein

Answer 11

C

Question 12

What are the two cycles of viral reproduction?

Answer 12

Lytic cycle

Lysogenic cycle

Question 13

What is the lytic cycle? Explain it’s process.

Answer 13

The process of which the virus causes the cell to explode.

Step 1
- Virus binds to cell and injects its DNA content (e.g. Phage)

Step 2
- Viral DNA circularizes in order to hide amongst the cells own DNA

Step 3
- Viral DNA and proteins are made into new viruses

Step 4
- There are too many viruses for the cell to handle, so it lyses, releasing the viruses.

Question 14

What is the lysogenic cycle? Explain it’s process.

Answer 14

The process of which the virus reproduces and spreads before it does the lytic cycle.

Step 1
- Virus binds to cell and injects its DNA content (e.g. Phage)

Step 2
- Viral DNA inserts itself into the cell’s own DNA sequence (now called a prophage)

Step 3
- The prophage (along with the cell’s own DNA) gets copied as it does mitosis and is spread amongst the copied cells as well

Step 4
- Each of the cell’s daughters will push the prophage out of its DNA (it will stand alone in the cell as circularized viral DNA)

Step 5

• Each daughter cell will now do the lytic cycle
  
  • make more viruses and explode

Question 15

When a virus does the lysogenic cycle, will all of the daughter cells explode?

Answer 15

Yes, but not all at once.

Question 16

If a virus enters a damaged cell, it most likely does the lytic cycle, why do you think this is?

Answer 16

If the cell is damaged, it is probably stuck at one of its checkpoints and cannot do mitosis. The damage is probably too harmful to repair, so the cell is trying to figure out what to do.
The virus, however, is very impatient and would rather explode the cell and move onto a healthy one.

Question 17

What is a retrovirus?

Answer 17

An RNA virus who’s single stranded RNA is used as a template to form DNA in order to do normal transcription within the cell

Question 18

What is reverse transcription?

Answer 18

When a virus converts its RNA to DNA using the enzyme, reverse transcriptase.

Question 19

Where does the enzyme, reverse transcriptase, come from? The host cell or the virus?

Answer 19

The virus

• the cell has no need for reverse transcription, so why would it have it?

Question 20

Explain how reverse transcriptase changes RNA to DNA in the host cell.

Answer 20

Step 1
- Virus binds to cell and injects its single stranded RNA, reverse transcriptase, and capsid.

Step 2

• Reverse transcriptase starts to transcribe the RNA into DNA
• An RNA-DNA hybrid is made in between (one strand is RNA, other is DNA)

Step 3
- The hybrid will separate and DNA polymerase with replicate the single stranded DNA strand

Step 4
- A double stranded DNA is fully made and ready to be transcribed by the cell

Question 21

HIV is a retrovirus. Explain how it affects a host cell.

Answer 21

• HIV binds to the receptors, releases its RNA, capsid, and reverse transcriptase.
• After undergoing reverse transcription, the DNA will hide itself by sitting as a provirus in the cell’s nuclear DNA.
• The provirus will take part in mitosis (lysogenic cycle)
• DNA will be transcribed into RNA, leave the nucleus, become a protein, and leave the cell as a new virus (viral shedding)
• The problem with HIV is that it will NEVER do lyses

Question 22

How does HIV affect the immune system?

Answer 22

• HIV binds to the receptors of a CD4 cell
• replicates its DNA
• CD4 (immune) cell fights with the HIV but loses
• New HIV viruses leave the cell and continue to damage more CD4 cells
• HIV infected the immune system