Q.36 Arrange the events in correct order of sequence:

1. Cleavage
2. Cortical rotation
3. Mid-blastula transition
4. Gastrulation

Choose the correct sequence answer from the options given below:

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

   Correct Answer: (B), (A), (C), (D) — Cortical rotation → Cleavage → Mid-blastula transition → Gastrulation.

   Early embryonic development follows a precise sequence starting with fertilization-induced cortical rotation, followed by rapid cleavage divisions, then mid-blastula transition (MBT) activating zygotic transcription, and finally gastrulation forming germ layers.​

   Event Sequence Explanation

   1. Cortical rotation (B)
   First event post-fertilization in amphibians (e.g., Xenopus). Sperm entry triggers 30° rotation of egg cortex, shifting determinants to form Nieuwkoop center and dorsal organizer.​

   2. Cleavage (A)
   Rapid mitotic divisions without growth create blastomeres, forming morula then blastula. Synchronous until MBT.​

   3. Mid-blastula transition (C)
   ~12th cleavage (2,000+ cells); cell cycles slow, zygotic genome activates (ZGA), maternal RNAs degrade.​

   4. Gastrulation (D)
   Cells rearrange into 3 germ layers (ectoderm, mesoderm, endoderm) via involution, convergence.​

   Option Analysis

   Option 1: (A), (B), (C), (D)
   Wrong: Cleavage cannot precede cortical rotation (determinants needed first).

   Option 2: (C), (B), (A), (D)
   Wrong: MBT follows cleavage; maternal control dominates early.

   Option 3: (B), (A), (C), (D)
   Correct: Natural sequence from fertilization to gastrulation.​

   Option 4: (B), (C), (A), (D)
   Wrong: MBT requires cleavage-generated cell number first.

   Stage	Timing	Key Feature
   Cortical rotation	Fertilization	Asymmetry establishment ​
   Cleavage	1-12 cycles	Blastula formation
   MBT	~2,000 cells	Zygotic transcription
   Gastrulation	Post-MBT	Germ layer formation ​

   Introduction to Cortical Rotation Cleavage Mid-Blastula Transition Gastrulation

   The cortical rotation cleavage mid-blastula transition gastrulation sequence defines early vertebrate embryogenesis, crucial for GATE Life Sciences developmental biology. Correct order: B→A→C→D tests timeline understanding.​

   Cortical Rotation: First Step

   Fertilization rotates egg cortex, localizing β-catenin dorsally for Spemann organizer formation.​

   Cleavage Divisions Follow

   Rapid cleavages partition zygote into blastomeres without size increase, relying on maternal factors.

   Mid-Blastula Transition Marks Change

   ZGA begins; cycles lengthen, enabling patterning gene expression.

   Gastrulation Forms Body Plan

   Involution creates trilaminar disc; all organogenesis derives from these layers.​

   GATE Exam Tip

   Visualize: Fertilization asymmetry → rapid divisions → zygotic control → tissue formation. Practice sequencing PYQs.​