Molecular Biology and Biotechnology- ICAR JRF/SRF 2024

• Pollen Dimorphism in Androgenesis: During anther and microspore culture, microspores differentiate into two physiological populations:
• S-grains (Sporophytic pollen grains): Smaller, poorly vacuolated, starched microspores with non-committed cytoplasm that respond preferentially by dividing symmetrically to initiate haploid embryogenesis.
• P-grains (Pollen grains): Normal gametophytic microspores committed to terminal pollen maturation that fail to undergo androgenic embryogenesis.

Correct option: (1) S-grains

• Rice Chromosome Number: Cultivated rice (Oryza sativa) is a diploid species with $2n = 2x = 24$ chromosomes.
• Meiotic S-phase & Chromatid Count: Prior to entering Prophase I of Meiosis I, the cell completes interphase DNA replication. Consequently, each of the 24 chromosomes consists of two identical sister chromatids joined at the centromere.
• Calculation:

Correct option: (3) 48

• Hammerhead Ribozyme Mechanism: The hammerhead ribozyme is a small catalytic RNA motif comprising three base-paired helical stems surrounding a conserved core.
• Cleavage Chemistry: It catalyzes a site-specific phosphodiester bond transesterification wherein the 2'-hydroxyl oxygen attacks the adjacent 3'-phosphorus in an in-line $S_N2$ nucleophilic substitution, producing a 2',3'-cyclic phosphate and a 5'-hydroxyl terminus without requiring a protein cofactor.
• Conclusion: Option 2 correctly identifies its cleavage mechanism.

Correct option: (2) It cleaves RNA sequences using a water molecule in a mechanism similar to protein enzymes.