Answer the following questions in about 150 words each: (a) What are prions? How do they differ from viruses? Mention few…

Prions and Viral Distinctions Prions are proteinaceous infectious particles composed solely of misfolded PrP^Sc proteins, lacking any nucleic acid. They differ fundamentally from viruses, which are obligate intracellular parasites containing DNA or RNA enclosed in a protein capsid. While viruses replicate by hijacking host machinery to synthesize new genetic material and proteins, prions propagate by inducing the conformational change of normal cellular PrP^C proteins into the pathogenic PrP^Sc form, leading to accumulation and neurodegeneration. This template-directed conversion allows prions to persist in the environment and resist standard sterilization. In animals, prions cause Scrapie in sheep, BSE (mad cow disease) in cattle, and Transmissible Spongiform Encephalopathy in deer. In humans, they cause Creutzfeldt-Jakob Disease (CJD), Kuru, and Fatal Familial Insomnia. Unlike viral infections, prion diseases invariably progress to fatal spongiform encephalopathy without an immune response, as the immune system does not recognize the misfolded host protein as foreign.

Parasexuality in Fungi Parasexuality is a mechanism of genetic recombination in fungi that occurs without meiosis or the formation of specialized sexual structures. It is significant in both imperfect fungi (Deuteromycetes) and perfect fungi (Ascomycetes/Basidiomycetes) that lack active sexual cycles under laboratory conditions. The mechanism involves four stages: first, heterokaryosis, where hyphae of different genotypes fuse to form a heterokaryon; second, nuclear fusion (karyogamy), creating diploid nuclei; third, mitotic crossing over, which generates genetic recombination in the diploid state; and finally, haploidization, where the diploid nucleus returns to a haploid state through mitotic segregation or aneuploidy. This process allows for the exchange of genetic material and the creation of new genotypes. Applications include industrial strain improvement, such as enhancing penicillin yield in Penicillium chrysogenum and improving enzyme production in Aspergillus niger. It is also used in genetic mapping and the study of gene function in fungi where sexual reproduction is difficult to induce.

Cyanophyceae: Features and Phylogeny Cyanophyceae, or blue-green algae, are prokaryotic organisms distinguished by the absence of a true nucleus and membrane-bound organelles. Their cell walls contain peptidoglycan, and photosynthesis occurs in thylakoids, not chloroplasts. They possess chlorophyll a and phycobilins (phycocyanin and phycoerythrin) as accessory pigments. Many filamentous forms, such as Nostoc and Anabaena, exhibit heterocysts, specialized cells for nitrogen fixation, and akinetes for survival. Phylogenetically, Cyanophyceae are not true algae but are classified within the Bacteria domain (Cyanobacteria). They are considered the most primitive photosynthetic organisms, having evolved before eukaryotes. Their affinity with other groups is best understood through endosymbiotic theory: primary chloroplasts in eukaryotic algae and plants originated from engulfed cyanobacteria. Thus, they represent a bridge between prokaryotic bacteria and eukaryotic algae, playing a pivotal role in the oxygenation of Earth's early atmosphere.

Cycas in India and Fern Characters Cycas species are widely distributed across India, with significant diversity in the Western Ghats, Northeast, and Andaman Islands. Notable species include Cycas circinalis (Western Ghats), C. beddomei (Andhra Pradesh), C. pectinata (Northeast), C. rumphii (Andaman), C. nathorstii (Kerala), and C. sphaerica (Sri Lanka/India border). Although Cycas is a gymnosperm, it retains several fern-like characters. These include circinate vernation, where young fronds unroll from a coiled tip. The leaves are large, pinnately compound, with pinnae attached to a central rachis, resembling fern fronds. The leaf traces (vascular bundles) enter the leaf base in a pattern similar to ferns. Additionally, the megasporophylls are not clearly differentiated from foliage leaves in some aspects, and the pollen grains are large and saccate, though the presence of a male cone distinguishes it from true ferns. These characters suggest an evolutionary link between ferns and seed plants.

ICBN Principles and Numerical Taxonomy The International Code of Botanical Nomenclature (ICBN) governs the naming of plants. Key principles include: 1) Priority, where the earliest validly published name is accepted; 2) Typification, where each name is linked to a type specimen; 3) Each taxon can bear only one correct name at a given rank; 4) Scientific names are treated as Latin, regardless of their origin; 5) The name of a taxon is based on a type; and 6) The name of a taxon is independent of its rank. Operational Taxonomic Units (OTUs) are fundamental in numerical taxonomy, a phenetic approach that classifies organisms based on overall similarity rather than evolutionary history. In this method, characters are quantified and scored. OTUs are compared using similarity coefficients, and clusters are formed based on statistical algorithms (e.g., UPGMA). This computer-assisted analysis allows for objective, reproducible classification, minimizing subjective bias. OTUs serve as the basic units for data input, enabling the construction of dendrograms that reflect phenetic relationships. This approach is particularly useful for large datasets and groups with complex morphologies, providing a standardized framework for taxonomic revision.