(a) What are prions? How do they differ from viruses? Mention few diseases caused by prions in animals and human beings. (10…

(a) Prions

Prions are infectious proteins, with no nucleic acid, formed when the normal cellular prion protein (PrPᶜ, mainly α-helical) is converted into a misfolded, β-sheet-rich, protease-resistant form (PrPˢᶜ), which converts more PrPᶜ and accumulates as amyloid plaques and vacuoles in the brain.

Differences from viruses: viruses have DNA or RNA in a protein coat and replicate by using host machinery; prions have no genome, propagate by template-directed misfolding, are more resistant to heat, UV, radiation and formaldehyde, produce no immune response or inflammation, and have a long incubation.

Diseases: in animals, scrapie (sheep, goat), bovine spongiform encephalopathy (cattle) and chronic wasting disease (deer); in humans, Creutzfeldt–Jakob disease, variant CJD, kuru, Gerstmann–Sträussler–Scheinker syndrome and fatal familial insomnia.

(b) Parasexuality in fungi

Parasexuality is recombination of genes in fungal mycelium without meiosis and sexual organs (Pontecorvo and Roper, 1952–56, in Aspergillus nidulans).

Mechanism:

1. Heterokaryosis: hyphae of different genotypes fuse (anastomosis), and nuclei of both mix in a common cytoplasm.
2. Karyogamy: rarely two unlike nuclei fuse to give a heterozygous diploid nucleus.
3. Mitotic crossing over: occasionally homologous chromosomes exchange segments during mitosis in the diploid.
4. Haploidisation: the diploid nucleus loses chromosomes stepwise by non-disjunction and returns to haploid, giving recombinant haploids.

Applications: genetic analysis and gene mapping in fungi lacking a sexual stage (Aspergillus, Penicillium, Fusarium), strain improvement for penicillin and enzymes, and breeding of industrial fungi.

(c) Cyanophyceae: features, phylogeny and affinities

Features: prokaryotic blue-green algae with no nucleus, plastids, mitochondria or flagella; cell wall of peptidoglycan with mucilage sheath; pigments chlorophyll a, phycocyanin and phycoerythrin; thylakoids free in the cytoplasm; food reserve cyanophycean starch; 70S ribosomes; reproduction only by fission, fragmentation, hormogonia, akinetes and endospores; heterocysts fix nitrogen.

Phylogeny and affinities: they resemble bacteria in prokaryotic cell structure, peptidoglycan wall and 70S ribosomes, and so are now classed as cyanobacteria. They resemble red algae in chlorophyll a and phycobilins, and the chloroplasts of algae and plants arose from a cyanobacterial endosymbiont (endosymbiotic theory). Fossils (stromatolites) are about 3.5 billion years old, and they began oxygenic photosynthesis.

(d) Distribution of Cycas in India and fern characters

Indian species: Cycas circinalis (Western Ghats, Kerala, Karnataka), C. beddomei (Tirupati and Cuddapah hills, Andhra Pradesh), C. pectinata (North-East, eastern Himalayas), C. sphaerica (Odisha, Andhra Pradesh, Eastern Ghats), C. rumphii and C. zeylanica (Andaman and Nicobar Islands), C. nathorstii and C. annaikalensis (Western Ghats), C. indica (Andaman and Nicobar), with C. revoluta cultivated.

Fern characters: circinate vernation of the young leaf, large pinnately compound leaves with a rachis, scaly (ramenta-like) leaves, multiflagellate motile sperms, archegonia, sporophytic plant with a long-lived gametophyte reduced within the ovule, leaf-like megasporophylls bearing ovules, and sometimes a similar coralloid and mycorrhizal root structure.

(e) Principles of the International Code of Nomenclature and OTUs

The International Code of Botanical Nomenclature (now the International Code of Nomenclature for algae, fungi, and plants) has six principles: botanical nomenclature is independent of zoological nomenclature; the application of names is determined by nomenclatural types; nomenclature depends on priority of publication; each taxon has only one correct name within its rank; scientific names are treated as Latin; and rules are retroactive unless limited.

OTUs: in numerical taxonomy (phenetics, Sneath and Sokal), an OTU is the lowest-ranking taxon or individual studied. Many characters of each OTU are scored and assembled in a data matrix, similarity coefficients are calculated between OTUs, and a cluster method such as UPGMA gives a phenogram showing overall similarity, with equal weight to all characters.