Module 3 - Viruses and Fungi

Question 1

Viruses: what are they, why do they not count as living, and what key features are there?

Answer 1

Small, infectious, obligate, intracellular parasites

They rely on host cells to reproduce for them and produce ATP as they cannot reproduce themselves and no known viruses contain mitochondria (or ribosomes either)

May contain RNA as the genome instead of DNA

Question 2

Viral genomes

Answer 2

1) SS or DS DNA
2) SS or DS RNA
3) Use both DNA and RNA (retrovirus - uses reverse transcriptase to convert RNA into DNA(HIV)) (Hepadnavirus - uses an RNA intermediate to replicate its DNA)

Question 3

Virion: structure

Answer 3

Always a nucleic acid surrounded by the capsid (protein coat/shell)

Capsid - polymer of a single/multiple proteins

Some viruses have membranes too

Question 4

What happens when a virus enters a host cell?

Answer 4

Its stable capsid is released and the genome is released and the viral replication redirects the host machinery

Question 5

Helical symmetry

Answer 5

Size - 18-300nm
Length determined by RNA length
Width determined by subunits

Question 6

Icosahedral symmetry

Answer 6

20 faces - most efficient packing in a closed shell

Question 7

Complex viruses

Answer 7

Bacteriophages - icosahedral head, helical tail sheath, tail fibres, infect bacteria only, carry lysozyme for cell entry (breaks peptidoglycan)

Question 8

Virus replication cycle

Answer 8

Induces host cell to synthesise viral components

1 - Attachment
2 - Penetration
3 - Uncoating
4 - Synthesis of nucleic acid and protein
5 - Assembly of subunits and genome packaging
6 - Mature virion released

Question 9

Viral growth curve: the steps

Answer 9

1- Adsorption into the cell
2 - Eclipse (nucleic acid/coat separated)
3 - Latent period (synthesis begins)
4 - Maturation (viruses matured)
5 - Assembly and release

Question 10

Fungi: what are they, when did they arrive on land, and what are the other key features?

Answer 10

Organoheterotrophic (saprotrophs) eukaryotes with chitin as the cell wall that are mostly aerobic and haploid

430,000 million years ago

• 100,000 species are identified out of an estimated 1.5 million
• 70s mitochondria ribosomes

Question 11

6 Major phyla of fungi

Answer 11

*Microsporidia (unicellular animal/protist parasites)
*Chytridiomycota (flagellated and motile spores)
*Zygomycota (Sexual/asexual phases - mould)
*Glomeromycota (Symbiotic with plants)
*Ascomycota (Cell division as budding)
*Basidiomycota (Classic mushroom)

Question 12

Microsporidia

Answer 12

Parasitic fungi - only on eukaryotes/protists

Question 13

Chytridiomycota: what are they, what key features do they have, and where are they found?

Answer 13

Most primitive aquatic fungi with flagellated asexual/sexual spores

Has a large thallus from which rhizoids emerge and contain motile zoospores

• Present in the rumen of cattle as an important part of the digestive process
• Causes fatal disease in amphibians

Question 14

Zygomycota: what are they, what key features do they have, and where are they found?

Answer 14

Terrestrial asexual fungi with non-motile spores

Have a large sexual zygospore

Organic material that isn’t alive, for example, they’re a large part of causing food to spoil

Question 15

Glomeromycota

Answer 15

Symbiosis with plants

Question 16

Ascomycota: what are they, what key features do they have, and where are they found?

Answer 16

Fungi with large fruiting bodies that also produce aerial-borne chains of asexual conidiospores

The most diverse group (includes Penicillium and Aspergillus)

Found as plant pathogens/food spoilage

Question 17

Basidiomycota: what are they, what key features do they have, and where are they found?

Answer 17

The club-shaped “mushrooms and toadstools” group with Basidia lining the gills/pores of the mushroom usually with four sexual basidiospores borne on a basidium

Most advanced group

Many form mycorrhizal associations with trees

Question 18

The use of hyphae in fungi

Answer 18

Apical growth of hyphae enables fungi to extend into fresh zones of the substrate

Enzymes are released from hyphae and diffuse into the surrounding media.

Released nutrients and minerals form from the decomposition of organic matter for e.g. are taken up into the hyphae

Question 19

What are the two types of hyphae and what does each do?

Answer 19

• Septate - fungal hyphae are divided into separate cells by perforated septa. Tiny holes in the septa allow the flow of nutrients between cells.
• Coenocytic - No septa, large cells with multiple nuclei.

Question 20

Chitin: what is it?

Answer 20

Polymer of N-acetylglucosamine

Question 21

Fungal hyphae growth: what is it, how does it occur, and what does the growth give the fungi?

Answer 21

Only occurs at the tip of hyphae in the apical region (can be rapid – up to 40mm/min)

There is continuous movement of materials into the tip from older regions of the hyphae also the growth is powered by actin polymerisation and cytoplasmic expansion forces

Apical growth gives penetrating power- enables penetration of plant cells, insect cuticles, and hard materials such as wood

Question 22

Hyphal growth: what is the actual process?

Answer 22

• Secretory vesicles derived from the Golgi are transported to the top
• Vesicles fuse with the plasma membrane at the tip and release their contents: enzymes involved in cell wall biosynthesis; glucan synthase; chitin synthase.
• Enzyme activators GTP (glucan synthase) and protease (chitin synthase)
• Enzymes involved in cell wall lysis
• Preformed wall polymers such as mannoproteins

Question 23

The three regions of hyphae

Answer 23

Apical region - bigger actin cytoskeleton, more actin microfilaments and microtubules, less cell wall crosslinking. ~50nm thick

Subapical region - less of an actin skeleton, with a rigified cell wall with more crosslinking

Vacuolation zone - higher cell wall cross-linking, smaller actin cytoskeleton, most organelles are here