Viruses

Question 1

Define Virus

Answer 1

Infectious, obligate (relies on host) intracellular parasite that has DNA or RNA (genetic material) and a protein coat (capsid)

Question 2

Basic facts about viruses

Answer 2

• sub-microscopic
• Depends completely on living host for ATP, amino acids and nucleotides
• Infects all types of living cells
• estimated 10^31 viruses on Earth

Question 3

Are viruses living?

Answer 3

• Energy processing: NO
• Growth and development: NO
• Response: NO
• Regulation: NO
• Reproduction: Sort of

Question 4

Discovery of viruses

Answer 4

1. Extracted sap from tobacco plant infected with mosaic disease
2. Sap passed through filter to remove bacteria
3. Sap transferred to healthy plant
   Results: Healthy plant infected with mosaic disease
   *Viruses cannot be produced in test tubes because they need a living host cell to survive

Question 5

Structure of viruses

Answer 5

ALL viruses have:
- Nucleic acid (DNA or RNA) and a protein coat (capsid), sometimes with a membrane (viral envelope)
- Capsid: encloses viral genome, made of capsomeres

Question 6

Types of viruses

Answer 6

1. Helical capsid: RNA, e.g. tobacco mosaic virus
2. Polyhedral (with glycoproteins): DNA, e.g. Adenovirus (used to deliver genetic materials to target cells)
3. Bacteriophage (heads that enclose DNA, protein tails with fibers)
4. Membranous envelope with glycoproteins - RNA, e.g. Zika virus, Corona virus, Influenza
   *SARS uses glycoproteins to bind to ACE-2 receptors on many cells

Question 7

Basic viral life cycle

Answer 7

1. Virus enters host cell and releases nucleic acids + capsid proteins
2. Host enzymes replicate the viral genome
3. Host enzymes transcribe the viral genome to mRNA which is host ribosomes use to make more capsid proteins
4. Viral genome and capsid proteins self assemble into new viruses and exit the host

Question 8

Life cycle of an enveloped virus (retrovirus)

Answer 8

1. Retroviruses have enzymes called reverse transcriptase which change RNA to DNA
2. The new DNA is incorporated into the host cell (provirus)

Question 9

Life cycle of a phage

Answer 9

Two possible reproductive mechanisms:
1. Lytic cycle (produces more phages + destroys host cell)
2. Lysogenic cycle (produces more phages + does not destroy host cell)
*Lytic cycle will be induced if the prophage exits the bacterial chromosome

Question 10

Extra information

Answer 10

• Viruses may shuffle genes in host cell which may make virus less pathogenic
• Different variants of a virus can infect different animals