PYQ Bank — UPSC Civil Services

Statement 1 is correct: guide RNA directs Cas9 to specific DNA sequences for precise editing.

Statement 2 is incorrect: CRISPR-Cas9 can be used in both somatic and germline cells, though germline editing raises ethical concerns.

Statement 3 is correct: CRISPR-Cas9 is derived from bacterial immune mechanisms that bacteria use against viruses.

Therefore, only statements 1 and 3 are correct.

Option C is incorrect because dark matter has NOT been directly observed or photographed, despite decades of research.

Its existence is inferred through indirect gravitational effects on visible matter and radiation.

Options A, B, and D are all correct: dark matter comprises ~85% of matter in the universe, its existence was inferred from galaxy rotation curves, and it forms halos around galaxies affecting their dynamics.

Dark matter remains one of the greatest unsolved mysteries in physics.

Statements 1, 2, and 3 correctly describe mechanisms of antibiotic resistance: bacteria can enzymatically inactivate antibiotics, acquire resistance genes through horizontal gene transfer from other bacteria, and mutations conferring resistance are selected for under antibiotic pressure.

Statement 4 is incorrect because antibiotic exposure does not directly cause permanent genetic changes; rather, it selects for pre-existing or newly mutated resistant variants.

Understanding these mechanisms is crucial for developing strategies to combat antibiotic resistance, a major global health threat.