Q.64 Study the following diagram depicting the plant cell cycle and match the following
| Stages of Cell Cycle | Type of Cyclin |
|---|---|
| P. Late G1 phase | 1. Cyclin B |
| Q. Beginning of S-phase | 2. Cyclin E |
| R. Prior to mitotic phase | 3. S-Cyclin |
| S. Early G1 phase | 4. Cyclin D |
Introduction
The plant cell cycle consists of four major phases: G₁, S, G₂ and M.
Progression through these stages is controlled by regulatory proteins called cyclins.
Each cyclin appears at a specific phase and activates CDKs to push the cell into the next stage.
Understanding the correct matching of cyclins with cell cycle stages is a frequently asked question in competitive biology exams.
Types of Cyclin
- 1. Cyclin B
- 2. Cyclin E
- 3. S-Cyclin
- 4. Cyclin D
Concept: Role of Each Cyclin
Early G₁ Phase → Cyclin D
Cyclin D is the first cyclin produced after mitosis. It responds to growth signals and helps the cell
progress through the early G₁ phase.
Late G₁ Phase → Cyclin E
Cyclin E helps the cell cross the G₁/S checkpoint (restriction point) and prepares it for DNA replication.
Beginning of S Phase → S-Cyclin
S-Cyclin initiates DNA replication and is essential for synthesis of new DNA strands.
Prior to Mitosis (G₂/M) → Cyclin B
Cyclin B forms MPF (Maturation Promoting Factor) and triggers the onset of mitosis.
Correct Matching
| Stage | Phase | Cyclin | Number |
|---|---|---|---|
| P | Late G₁ | Cyclin E | 2 |
| Q | Beginning of S | S-Cyclin | 3 |
| R | Prior to Mitosis | Cyclin B | 1 |
| S | Early G₁ | Cyclin D | 4 |
Final Answer
Option (B) → P-2, Q-3, R-1, S-4
Why Other Options Are Incorrect?
Option A
Cyclin D is incorrectly placed in late G₁ and Cyclin E in early G₁.
Option C
Cyclin B cannot function in G₁ and S-Cyclin cannot act before mitosis.
Option D
The sequence of cyclin activation is completely mismatched.
Quick Memory Trick
Remember the order: D → E → S → B
(G₁ → G₁/S → S → M)
Mnemonic: Dear Students Be Ready
Key Points to Remember
- Cyclin D → Early G₁
- Cyclin E → Late G₁
- S-Cyclin → DNA replication
- Cyclin B → Mitosis
