Q.32 Arrange the following in order of their action –

• (A) G₁ cyclin CDK complex
• (B) APC–Cdh1
• (C) S phase cyclin CDK complex
• (D) APC–Cdc20

Choose the correct sequence answer from the options given below:

1. (B), (C), (A), (D)
2. (A), (C), (D), (B)
3. (B), (A), (D), (C)
4. (A), (D), (B), (C)

   APC–Cdc20 (D) acts in metaphase-anaphase transition, followed by APC–Cdh1 (B) in late mitosis/G1, with G1 cyclin-CDK (A) initiating G1/S and S-phase cyclin-CDK (C) driving DNA synthesis. The correct sequence is (A), (C), (D), (B), reflecting standard cell cycle progression.

   Step Sequence

   Cell cycle regulators activate sequentially: G1 cyclin-CDK promotes G1 progression, S-phase cyclin-CDK triggers DNA replication, APC-Cdc20 enables anaphase, and APC-Cdh1 maintains G1 stability.

   • G1 cyclin CDK complex (A): Active in G1 phase, phosphorylates Rb to release E2F for S-phase entry.

   • S phase cyclin CDK complex (C): S-phase cyclins (Cyclin A/E) with CDK2 drive DNA synthesis.

   • APC–Cdc20 (D): Activates in prometaphase (post-S/G2/M), degrades Cyclin B/securin for anaphase onset.

   • APC–Cdh1 (B): Activates post-mitosis (late M/G1), degrades Cdc20/Cyclins to prevent premature S-phase.

   Option Analysis

   • (B), (C), (A), (D): Wrong; APC-Cdh1 (G1) can’t precede G1/S cyclins (A,C).​

   • (A), (C), (D), (B): Correct; G1 → S → M (APC-Cdc20) → late M/G1 (APC-Cdh1).

   • (B), (A), (D), (C): Wrong; APC-Cdh1 before G1/S cyclins disrupts timing.​

   • (A), (D), (B), (C): Wrong; APC-Cdc20 (M-phase) can’t precede S-phase CDK; skips S-phase.​

   Introduction to Arrange Order of Action in Cell Cycle

   Arrange the following in order of their action cell cycle regulators like G1 cyclin CDK complex, S phase cyclin CDK, APC-Cdc20, and APC-Cdh1 follows G1 → S → M → G1 sequence, essential for GATE Life Sciences cell biology prep.​

   Cell Cycle Phases Overview

   G1: Growth (A active). S: DNA replication (C). G2/M: Mitosis preparation. M: Division (D then B).​

   Detailed Regulator Timeline

   • G1 cyclin-CDK (A): Rb phosphorylation → S-entry.

   • S cyclin-CDK (C): DNA synthesis initiation.

   • APC-Cdc20 (D): Securin degradation → sister chromatid separation.

   • APC-Cdh1 (B): Cyclin degradation → G1 stability.

   ​
   Cell cycle diagram showing sequential activation of CDKs and APC complexes.

   Why Sequence Matters

   Disrupted order causes genomic instability; APC-Cdc20 before S-phase halts progression.​

   GATE Exam Tips

   Memorize: Cyclins/CDKs early (G1/S), APC late (M/G1). Option (A,C,D,B) is standard.​