L2 Viruses, Fungi & Parasites

Question 1

Why are viruses not a living organism?

Answer 1

• Consist only of DNA or RNA and protein
• Are incapable of independent reproduction
• Are smaller than any cell
• Have no cell membrane
• Do not have ribosomes
• Have few enzymes for metabolism

Question 2

Key components of a virus

Answer 2

• Infect bacterial, plant and animal cells
• Are large, inert macromolecules consisting of nucleic acid (DNA
or RNA) and protein
• No cell wall, ribosomes, mitochondria or nucleus -cannot produce energy or synthesize proteins
•Incapable of independent replication and require living host - “obligate intracellular parasites”

Question 3

Name the 6 aspects of the viral lifecycle

Answer 3

1. Attachment to host cell surface
2. Penetration into host cell cytoplasm
3. Uncoating to release viral genome
4. Replication of viral genomes, mRNA and proteins
5. Assembly of viral components
6. Maturation and release of progeny viruses through host
   membranes

Question 4

4 pathogenic consequences of viral infection

Answer 4

1. acute infection: e.g. common cold, virus eventually dies off
2. productive chronic infection: slow release of virus particles in host, long-term persistent infection
3. non-productive latent infection: genome persists in host cell but is not active all the time, there is recurrent/reactivation of infection over time, some cases genome never goes away
4. transformation: severe, infection turns into tumour or cancer cell

Question 5

DNA viral diseases

Answer 5

Adenovirus: respiratory. tumour-causing virus
Papillomavirus (HPV): warts, cervical cancer
Herpesvirus: Herpes, chicken pox, cold/genital sores

Question 6

RNA viral diseases

Answer 6

Coronavirus: Severe Acute Respiratory Syndrome (SARS)
Retrovirus: (HIV/AIDS), leukemia

Question 7

Structure of fungus

Answer 7

1. thick carbohydrate cell wall
   - protection from the environment
   - acquisition of nutrients
   - cell recognition & mating
2. plasma membrane similar to eukaryotes (chitin layer)

Question 8

3 types of fungal infections and medical examples

Answer 8

1. superficial mycoses
2. Subcutaneous mycoses
3. Systemic mycoses

Aspergillus spp.
Candida spp. (yeast)

Question 9

3 types of parasites and medical examples

Answer 9

1. Helminths (worms) : Pinworm
2. Protozoa: Plasmodium spp.
3. Arthropods

Question 10

Describe the 3 basic fungal structures

Answer 10

Moulds: multicellular, hyphae, form into mycelium
Yeast: single cell fungi that multiply by budding & division
Dimorphic fungi: most common, moulds at room temp but yeast at body temp.

Question 11

Why are fungal infections rare compared to viral infections?

Answer 11

Our immune system can easily recognise fungal cells due to their larger size.
Are difficult to diagnose.
Antifungal therapy is complex (issues relating to toxicity/side effects to our cells)