Introduction to viruses

Question 1

What are viruses?

Answer 1

Acellular microorganisms that cannot survive without a host

Question 2

Why can viruses not survive without a host?

Answer 2

Have no metabolic ability of their own

• Can’t make proteins or DNA, or carry out metabolic processes
• Needs to use host cell in order to have these processes - rely completely on the machinery of the infected cell to multiply

Question 3

Do antibiotics have effects on viruses?

Answer 3

Antibiotics have no effect on viruses as they target specific bacteria components

Question 4

How big are viruses?

Answer 4

Virus size can vary from 10 - 400 nm (much smaller than bacteria)

Question 5

What is the composition of a virus?

Answer 5

Genetic material - made from either DNA or RNA

Capside - a protein coat that surrounds and protects the genetic material

And in some cases
Envelope of lipids that surrounds the protein coat when they are outside the cell

Question 6

What are the two main types of viruses?

Answer 6

Naked (non-envelope) and enveloped virus

Question 7

What is a viral capsid?

Answer 7

Capsids are made of multiple units of the same protein building block known as capsomers
Capsomers = subunit of the capsid arranged in a precise and highly repetitive pattern around the nucleic acid

Question 8

What are the three structures of a viral capsid?

Answer 8

• Helical (e.g. TMV)
• Icosahedral (e.g. Adenovirus)
• Complex (e.g. bacteriophage) (the head has icosahedral symmetry and the tail is helical)

Question 9

What is the viral genome made up of?

Answer 9

Nucleic acid: RNA or DNA 
Could be:
- Linear (e.g. Poxvirus)
- Circular (e.g. Hepatitis B)
- Segmented (e.g. influenza virus)
All possible forms of RNA and DNA are found in viruses: single and double stranded DNA and RNA

Question 10

What are bacteriophages?

Answer 10

Viruses that infect and replicate in bacteria

• Can be attacked by multiple viruses
• Heralded as a potential treatment for diseases as they kill bacteria

Question 11

What are the two types of bacteriophage infections in bacteria?

Answer 11

Lytic and Lysogenic

Question 12

What is involved in the Lytic cycle of bacteriophage infection?

Answer 12

1. Bacteriophage attaches to host cell - tail fibre recognises specific receptors on the bacterial cell surface
2. The phage injects its DNA into the bacterial cell (host genome is destroyed)
3. Synthesis of viral genomes and proteins (virus takes over host machinery and used this to replicate its genome) - first proteins transcribed are ones that degrade the host genome
4. Virus assembles inside the bacterial cell
5. The bacteria bursts (100-200 virus in the cell, it will burst - the burst size)
   Cycle takes 20-30 mins

Question 13

What is involved with the lysogenic cycle of infection?

Answer 13

Same process as lytic, except the viral genome gets integrated into the host genome instead of taking it over (called a prophage)
Host bacterium is called lysogen or lysogenic bacterium

• Phage genome is ‘silent’ transcription of prophage genes inhibited - does not kill the host
• Sometimes the prophage genes become unstable and the genes can come out of the genome. They will then enter the lytic cycle

Question 14

What is HIV?

Answer 14

HIV is a virus that leads to the disease AIDS
The persons immune system is attacked, and they become very susceptible to opportunistic infections e.g. fungi, cancer, bacterial

Question 15

What is the host cell that HIV virus enters?

Answer 15

CD4 T-cell (white blood cells)

Question 16

What is the genome of HIV virus?

Answer 16

RNA genome (9 genes) two identical single strand copies of each.

Question 17

What are some componants of the HIV virus?

Answer 17

• Surrounded by envelope made of glycoproteins (a enveloped virus)
• Protein called reverse transcriptase inside the viral envelope as well as the RNA

Question 18

What are the steps of the replications of RNA enveloped virus (HIV)

Answer 18

1. HIV fuses with host cell membrane and digestion of capsid (recognised receptors on the cell membrane) - virus is now inside the cell
2. Viral genome is inside the cell as well as the reverse transcriptase. Take the RNA and converts it to DNA - do this in order to insert its DNA into the host cells DNA. Does this twice, so that the DNA is double stranded
3. Made DNA is transported to the nucleus and is integrated into the host DNA - host is now infected for life, no way of getting virus DNA out of the host cell genome.
4. Viral DNA is then replicated and synthesised back into RNA and can now make new viruses. Capsid is reformed and the new virus buds out of the host cell.
5. New virus can now go on and infect other host cells.