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.