A2.3 Viruses

Question 1

Describe the few features shared by all viruses

Answer 1

1. nucleic acid (DNA/RNA) as genetic material (single/double stranded)
2. capsid made of protein, some with an envelope outside
3. no cytoplasm
4. few or no enzymes

Question 2

Impt features of bacteriophage lambda

Answer 2

1. Capsid head that protects double stranded DNA
2. Tail fibres attach virus to host cell
3. Tail sheath consists of proteins that contract to drive the tail tube through the host cell’s outer membrane
4. DNA that is injected through the tail of the host cell

Question 3

Impt features of coronaviruses

Answer 3

1. spherical shape
2. single stranded RNA
3. envelope outside of capsid, with protein spikes

Question 4

Impt features of HIV

Answer 4

1. envelope outside of capsid, with spikes made of carbohydrates and protein
2. Has two identical strands of RNA
3. “retrovirus” - makes a DNA copy from its RNA code by reverse transciptase
4. Host cell - white blood cell

Question 5

What do viruses depend on host cells for?

Answer 5

1. energy supply
2. nutrition
3. protein synthesis
4. all other life functions

Question 6

Lytic cycle (bacteriophage lambda)

Answer 6

Attachment: phage attaches to host cell

Penetration: phage penetrates host cell and injects DNA

Biosynthesis: phage DNA directs synthesis of viral components by the host cell

Maturation: viral components are assembled into virions by host cell.

Release: host cell lyses and virions are released.

Question 7

lysogenic cycle

Answer 7

1. phage attaches to host cell and injects DNA
2. phage DNA circularizes and enters lysogenic cycle
3. phage DNA integrates within the bacterial chromosome by recombination, forming a prophage.
4. lysogenic bacterium reproduces normally, where viral DNA is replicated as well.
5. due to UV light/spontaneous events, prophage separates from bacterial DNA and lytic cycle begins.

Question 8

virus first hypothesis

Answer 8

viruses existed before cells, where the ancestors of modern viruses could have provided the raw material for the first cells.

Strength: virus genomes have genes not present in cells
Weakness: all modern viruses can only replicate with cells

Question 9

escape hypothesis

Answer 9

virus evolved from sections of DNA/RNA that escaped from cells

Strength: modern bacterial cells exchange genetic material, suggesting a possible escape mechanism for genetic material
Weakness: most genes and proteins found in viruses are not found in cells.

Question 10

Regressive hypothesis

Answer 10

viruses were once small cells that parasitised larger cells. Genes not required for parasitism were lost over time

Strength: giant viruses exist and have similar genetic material to parasitic bacteria
Weakness: Smallest cellular parasites do not resemble viruses in any way

Question 11

How has obligate parasitism shown by viruses been a major factor in convergent evolution (essay)

Answer 11

All viruses are obligate parasites, and are totally dependent on their host cell for energy production, protein synthesis, nutrition, and to carry out all necessary functions of life. Hence viruses do not need mitochondria for energy production or ribosomes to synthesise proteins.
Viruses only have one type of genetic material (DNA or RNA) as they rely on the genetic material of the host cell for replication
Because all viruses exist in the host cell, they all have similar structures and processes.
Hence, as seen in convergent evolution, similar environments give rise to similar structures and processes.

Question 12

Reasons for rapid evolution of viruses

Answer 12

1. High replication rate - increased chance of random mutation occurring
2. No proofreading mechanism during replication - more likely for a mutation. (especially for RNA viruses like HIV and influenza)
3. Immune systems select against non-mutated viruses as they recognise the capsids, but select for mutated viruses as they do not recognise the capsids

Question 13

Rapid evolution in HIV and Influenza

Answer 13

1. Large population size, short generation time, high mutation rates
2. have single stranded RNA (only HIV)- need to produce DNA for gene replication. This extra step increases chances of mutation - major source of variation
3. DNA viruses do not have this step - lower rate of mutation

2. Influenza makes RNA copies of its original RNA

Question 14

How does influenza mutate?

Answer 14

Can mutate through both antigenic shift and antigenic drift.
Both processes cause a shape change in the HA and NA surface proteins.

Question 15

Antigenic drift

Answer 15

1. small incremental changes in genetic material over long periods of time
2. these changes accumulate over time, slowly producing variation in surface proteins of the virus, preventing the immune system from recognising
3. HIV undergoes very fast antigenic drift

Question 16

Antigenic shift

Answer 16

1. 2 or more different viral strains infect the same host cell and recombine genetic material, producing MAJOR CHANGE in SHORT periods of time
2. causes rapid and major changes to surface proteins
3. the totally new virus is unrecognisable by the immune system,
4. influenza is capable of antigenic shift

Question 17

Consequences of treating diseases caused by rapidly evolving viruses

Answer 17

1. REV quickly develop resistance to existing drugs and treatment, requiring more research and development of new drugs.
2. Vaccines are helpful in treating antigenic drift (influenza) as changes are small and not rapid.
3. Vaccines are not helpful in treating antigenic shift - changes are not predictable enough