A2.3

Question 1

Viruses

Components of a Virus

(Non-living particle that infects cells & reproduces inside them)

Answer 1

• Small size and no growth
• Nucleic acid as genetic material that gets inserted into host cell
• Protein capsid (outer coat)
• No cytoplasm/organelles
• Spikes (protein spikes to help invading cells)

Question 2

Viruses

Capsid?

Answer 2

• Protein coat containing genetic material
• Can be outermost later, viruses may have envelope (made of plasma membrane & helps with recognition)

Question 3

Viruses

Prophage?

Answer 3

• The combined bit of viral DNA and host DNA
• Arises during the lysogenic viral life cycle, whereby viral DNA in intergrated with DNA of host cell
• When cell division occurs –> whole prophage is replicated

Question 4

Viruses

Genetic Material of Viruses

Answer 4

• Viruses have genetic material and often have DNA as well
• Can also have RNA as their genetic material
• Others may transcribe it into mRNA and retroviruses reverse transcribe it into DNA

Question 5

Viruses

Structure of Bacteriophrage Lambda

Answer 5

• Viruses invading E.Coli bacteria
• Has a capsid and DNA as genetic material
• Has a tail sheath and base plate with tail fibres that help attach to host cell
• Tail sheath has proteins to help get through host’s membrane

Question 6

Viruses

Structure of Coronavirus

Answer 6

• Spherical shape
• Single stranged RNA as genetic material
• An envelope in addition to a capsid that’s attached closely to the RNA
• Spike protiens on the envelope

Question 7

Viruses

Structure of HIV

Human Immonodeficieny Virus attacking helper T cells

Answer 7

• A capsid coat & outer envelope
• Two identical RNA strangs that are copied into DNA by reverse transcriptase
• Protein and carbohydrate spikes

Question 8

Viruses

Sequence of a Lytic Life Cycle

To be contrasted with Lysogenic

Answer 8

1. Virus attaches to the host cell
2. The virus inserts its DNA into the host
3. The host cell uses the DNA to make viral parts
4. Viral parts assemble into viruses
5. New viruses lyse (break out) the host cell and are released.

Question 9

Viruses

Distinctiveness of a Lysogenic Life Cycle

Answer 9

• Lysogenic cycle is longer that eventually leads to a lytic stage
• First involves the intergration of viral DNA into host DNA
• Slowly spreads as cell reproduces
• If viral DNA is released from host host (i.e through UV exposure/chemicals) the lytics cycle is initiated.

Question 10

Viruses

Convergent Evolution of Viruses

Answer 10

• Wide variety of viruses suggest a common ancestor is unlikely
• Hence, likely evolved separately - any similarities likely result of convergent evoltion whereby the features were so adaptive that they independantly evolved from them

Question 11

Viruses

Theories on the Origin of the Viruses

Answer 11

1. Viruses existed before cells and that cells evolved from viruses.
2. Regressive hypothesis –> viruses once were cells but lost important functional components and ended up viruses.

Question 12

Viruses

Rapid Evolution of Influenza

you “Sneeze” when you get the flu (S)

Answer 12

• Rapidly evolves by a process called antigentic shift
• Occurs when two or more different viruses invade the same cell and then recombine genetic material leading to changes in short time frames

Question 13

Viruses

Rapid Evolution of Influenza

Answer 13

• Rapidly evolves through antigentic drift
• Result of smaller genetic changes (mutations within the genetic material)
• RNA based viruses don’t proofread so mutations happen, enough mutations accumulate and lead to structural changes