Which of the following statements accurately describes how... | Agrigyan

• Major Cyclin-CDK Complexes of Vertebrates:
• Throughout the eukaryotic cell cycle, distinct cyclin-CDK heterodimers control transitions through specific phase checkpoints:
• Cyclin D (G1 cyclin): Pairs with (IV) CDK4, CDK6 to promote passage through the restriction point in $\text{G}_1$ phase by phosphorylating Rb protein.
• Cyclin E (G1/S cyclin): Pairs with (I) CDK2 to trigger entry into S phase and initiate centrosome duplication.
• Cyclin A (S cyclin): Pairs with (III) CDK2 (and CDK1) to regulate DNA replication progression during S phase.
• Cyclin B (M cyclin): Pairs with (II) CDK1 (Cdc2) to form Maturation Promoting Factor (MPF), triggering mitosis entry.
• Conclusion: Matching: (A)-(IV), (B)-(I), (C)-(III), (D)-(II).

Correct option: (1) (A) - (IV), (B) - (I), (C) - (III), (D) - (II)

Evaluating the Assertion and Reason concerning Hormones and their receptors:

The correct answer is Option 1: Both (A) and (R) are correct and (R) is the correct explanation of (A)..

• Riboswitches in Gene Regulation:
• Riboswitches are structured cis-regulatory RNA elements located predominantly within the $5'$ untranslated region (5' UTR) of bacterial mRNAs.
• Two Functional Domains:

• Aptamer Domain: Directly and selectively binds a specific small metabolite (e.g., TPP, FMN, SAM, lysine, guanine) without requiring protein cofactors.
• Expression Platform: Undergoes conformational structural rearrangement upon metabolite binding, forming alternative terminator/antiterminator hairpins that modulate downstream transcription elongation or translation initiation.
  • Exam Context: Option (4) was designated in the official answer key describing the metabolite-induced conformational change governing downstream gene expression.