Unit 2- Viruses

Question 1

What 3 components make up a virus?

Answer 1

• nucleic acid
• a protein coat
• a lipid envelope

Question 2

Do Viruses have RNA or DNA?

Answer 2

Viruses can have double and single strands of RNA or DNA

Question 3

Describe the structure of a protein coat:

Answer 3

• it is also called a capsid
• made of subunit capsomeres
• simple geometrical shapes such as helix or icosahedrons

Question 4

How does the lipid protein stay attached to the capsid? And where does the lipid envelope come from?

Answer 4

• the lipid envelope is derived from a host cell membrane, so the virus doesn’t make it
• the lipid envelope contains matrix proteins that link to glycoproteins in the capsid

Question 5

What is a virus particle outside of a cell called?

Answer 5

A vision

Question 6

Name the kind of nucleic acid the following bacteria have: Lambda phage, TMV, Ebola, HIV

Answer 6

Lambda phage- ds DNA
TMV- ss RNA (+ve sense strand)
Ebola- ss RNA (-ve sense strand)
HIV- ss RNA (retrovirus)

Question 7

Structure of a lambda phage:

Answer 7

• icosahedral head containing ds DNA
• helical sheath
• base plate
• tails
• considered complex

Question 8

Structure of the TMV:

Answer 8

• coil of single stranded RNA
• surrounded by a helical capsid
• very small

Question 9

Structure of Ebola virus:

Answer 9

• very large
• negative sense single strand RNA
• surrounded by a large flexible helical capsid
• enclosed in a lipid envelope

Question 10

Structure of HIV:

Answer 10

• Single stranded RNA
• surrounded by icosahedral capsid
• surrounded by a sphere of proteins attached to a lipid envelope
• contains the enzyme reverse transcriptase

Question 11

How do we classify viruses?

Answer 11

By type of nucleic acid

Question 12

Summary of how a virus infects a host cell:

Answer 12

• virus nucleic acid in injected into host cell
• capsid remains outside the cell
• the host cells ribosomes, enzymes, nucleotides, amino acids etc are used to synthesise new viral nucleic acid and proteins

Question 13

What is meant by a positive sense strand?

Answer 13

It can be directly translated into the proteins

Question 14

How does TMV replicate?

Answer 14

• TMV infects plant cells through damaged cell walls and contains positive sense RNA into the host
• the viral RNA is uncoated
• the RNA sequence can be translated directly by ribosomes to synthesise viral proteins
• the virus particles can also spread through the plant via plasmodesmata

Question 15

How does phage lambda replicate?

Answer 15

• Lambda phage attaches to host cell and injects its DNA into host cell cytoplasm
• the viral DNA is integrated into hosts bacterium DNA becoming a PROPHAGE
• prophage means bacterial DNA with viral DNA (like a vector)
• prophage remains latent and is replicated every time the host cell divides
• that was the lysogenic phase
• due to an environmental signal the viral DNA starts being expressed into phage proteins which assemble into phage capsid
• the cell bursts by lysis
• this is the lytic phase

Question 16

How does Ebola replicate:

Answer 16

• the virus attaches to the host cell, the 2 membranes fuse, releasing capsid into cell cytoplasm
• the viral RNA is uncoated and the viral enzyme RNA polymerase synthesises many copies of mRNA from the negative sense viral RNA, using the host cell nucleotides
• the mRNA is translated to make viral proteins which assemble into virus capsids
• the virus particles are released by budding, collecting their lipid envelope from the cell membrane

Question 17

How is HIV replicated?

Answer 17

• the virus attaches to host cell and the 2 membranes fuse, releasing the capsid into the membrane
• the viral enzyme reverse transcriptase synthesises double stranded DNA from the single stranded RNA
• the DNA is incorporated into the hosts DNA, this is called provirus
• the provirus remains latent for years and is replicated every time the cell divides
• an environmental trigger causes the viral DNA to express viral proteins which assemble into a viral capsid
• the virus particles are released by budding, using the cell membrane to make their lipid envelope