GATE 2020 BT Q.33: solution and answer

Official answer: BSource: Official answer key, GATE 2020 BT (IIT Delhi)

1. Carl Woese's landmark work (1977) that led to the discovery of Archaea and the three-domain system of life (Bacteria, Archaea, Eukarya) was based on comparing ribosomal RNA sequences across organisms, not protein sequences.

2. Specifically, Woese used the small-subunit ribosomal RNA (16S rRNA in prokaryotes, homologous to 18S rRNA in eukaryotes), because it is universally present in all cellular life, present in high copy number, of manageable and comparable length across organisms, and contains both highly conserved regions (for aligning distantly related sequences) and variable regions (for distinguishing closely related lineages).

3. Option (A), rRNA of the large ribosomal subunit (23S rRNA), is a real molecule used in some phylogenetic studies but is not the one Woese pioneered for founding prokaryotic taxonomy — incorrect.

4. Option (B), rRNA of the small ribosomal subunit (16S rRNA), matches the historically and scientifically correct choice — correct.

5. Options (C) and (D) refer to ribosomal proteins rather than ribosomal RNA; Woese's classification method was based on nucleic acid sequence comparison, not protein sequences, so both are incorrect.

6. Conclusion: the small-subunit rRNA (16S rRNA) is the molecule Woese used, consistent with its continued use today in 16S rRNA sequencing for bacterial and archaeal identification and phylogeny. Final answer: (B).