Virus

Question 1

Structure of a virion

Answer 1

• Spike/Attachment proteins
• DNA/RNA
• Envelope (absent in non-enveloped virus)
• Capsid

Question 2

What are viruses?

Answer 2

• Obligate intracellular parasite
• Lack nucleic acid replication and protein synthesis machineries
• Very small in size

Question 3

Where do enveloped viruses get their envelope?

Answer 3

The lipid envelope is derived from host’s cell membrane

Question 4

Why does each virus only infect specific hosts?

Answer 4

Specificity of viral attachment proteins to receptors on the host cell membrane

Question 5

How do viruses replicate?

Answer 5

• Viruses lack the biosynthetic machinery trait that is necessary for replication
• In order to replicate, it must infect a suitable living host first

Question 6

Process of bacteriophage replication in prokaryote

Answer 6

1. Attachment proteins from bacteriophage recognizes receptors on bacterial cell wall. Enzyme in phage tail weaken bacterial cell wall and viral DNA moves to bacterial cytoplasm
2. Breakdown of bacterial chromosome by phage-specific enzyme
3. Transcription: Phage DNA is transcribed to mRNA using host cell’s machinery
4. Replication of phage chromosome, using bacterial materials and phage enzymes
5. Expression of phage genes to produce phage structural components
6. Assembly of progeny phage particles
7. Release of progeny phages by lysis of bacterial cell wall

Question 7

Process of virus replication in eukaryotes

Answer 7

1. Virion attaches to the host cell by specific binding of its receptors to the attachment receptors
2. Virion is engulfed into a vesicle and enters the host cell
3. Envelope (if any) and viral capsid of the virus are removed and the genetic material of the virus is freed into the cytoplasm/nucleus
4. Under the control of viral genes, the host cell synthesizes the basic components of the viral genome and viral protein
5. Basic components are assembled into a virion. Viral genetic material is packaged within the capsid. Genetic material is inserted into the host cell membrane for viral envelope (applies to enveloped virus only)
6. Enveloped virus released from the host membrane, carrying away an envelope with attachment proteins. Complete virion is able to infect another cell

Question 8

Difference between virion and virus

Answer 8

Virus:
The infectious agent that requires a living host cell for it to multiply

Virion:
A form of a virus that is capable of surviving outside of the host cell and infecting new host cells

Question 9

Difference in RNA viruses and DNA viruses

Answer 9

DNA virus:

• Genetic material is double stranded (DNA)
• Buds from the nucleus

RNA virus:

• Genetic material is single stranded (RNA)
• Buds from the plasma membrane

Question 10

How are viruses classified?

Answer 10

By capsid structure, genome type and the presence of an envelope

Question 11

Structure of bacteriophage

Answer 11

• Head
• Collar
• Core
• Helical sheath
• Tail pins
• Hexagonal base plate