ICAR AIEEA PG 2024 - Plant Biotechnology

Attenuation is a prokaryotic gene regulatory mechanism that regulates transcription elongation and premature termination after transcription initiation has already occurred. It operates via ribosome stalling during translation of a short leader peptide containing regulatory amino acid codons, which directs alternative secondary stem-loop structures (antiterminator vs. terminator hairpin) in the nascent mRNA. Because attenuation halts transcription after initiation, the statement that it acts 'before the initiation of mRNA synthesis' is incorrect.

Mycoplasmas (class Mollicutes) are the smallest known self-replicating organisms. Key features include the absolute lack of a peptidoglycan cell wall and resistance to beta-lactam antibiotics. Crucially, most Mycoplasma species are not obligate intracellular parasites and can indeed be readily cultured in vitro in specialized cell-free artificial media enriched with serum/sterols, producing characteristic 'fried-egg' morphology colonies. Therefore, stating that 'they cannot be cultured in laboratory' is false.

Crick's Wobble Hypothesis describes non-standard base pairing between the 3' base of the codon and the 5' wobble base (position 34) of the tRNA anticodon. Post-transcriptional enzymatic modifications of the wobble base (such as conversion of adenosine to inosine by adenosine deaminase, or methylation and thiolation of uridines) fundamentally alter the hydrogen-bonding geometry and drastically dictate which codon bases can pair with the anticodon. Therefore, asserting that modifications in the anticodon do not affect wobble pairing is factually incorrect.

The heat shock method is the standard protocol for artificial bacterial transformation (uptake of recombinant plasmid DNA). Competent E. coli cells prepared with divalent cations (such as $CaCl_2$) on ice are briefly subjected to a sudden thermal pulse at $42^\circ\text{C}$ for 45–90 seconds followed by immediate chilling on ice. This rapid temperature shift creates a transient thermal imbalance that increases membrane permeability and opens dynamic pores through which negatively charged DNA molecules enter the cell.