Q1

Source: CSIR NET Nov 2020, Session I

Question:
If the blastomeres of a 4-celled sea urchin embryo are isolated, each blastomere can form a pluteus larva. This is an example of:

A. Autonomous specification
B. Conditional specification
C. Determination
D. Mosaic development

Key Concept

The cells are not locked into a specific fate yet.

Statement Analysis

A. Autonomous specification

• Cell fate is pre-determined early
• Each cell follows a fixed developmental path
• When isolated, it forms only its assigned part, not the whole organism
  

B. Conditional specification

• Cells are flexible
• Fate depends on interaction with neighbouring cells.
• When isolated, each cell can form a complete organism
• Seen in echinoderms and vertebrates (including humans)
  

C. Determination

• Not a type of specification
• A stage where fate becomes irreversible
• Even if moved, the cell sticks to its destiny

D. Mosaic development

• Development is rigid and pre-programmed
• Each cell contributes to a specific structure only
• Removing a cell results in missing body parts
• Like a mosaic/ jigzaw, each piece has a fixed role

Final Takeaway

If a single cell can form a whole organism, it is conditional specification
So, correct answer– b. conditional specification

Q2

Source: CSIR NET Nov 2020, Session II

Question:
Which one of the following statements regarding amphibian development is correct?

A. The Nieuwkoop centre is formed on the dorsal side of the embryo due to accumulation of β-catenin, which helps activate the siamois and twin genes
B. The ectodermal cells form neural tissues in the presence of BMP molecules
C. Brain formation requires activation of both Wnt and BMP pathways
D. There is a gradient of nodal-related protein across the ectoderm with low concentration on the dorsal side

Key Concept

How does the embryo establish its top (dorsal) and bottom (ventral)?

Concept Breakdown

After fertilization:

• Cortical rotation occurs
  • Leads to accumulation of β-catenin on one side
  • This side becomes the dorsal side
• Nieuwkoop Centre Formation
  • Forms on the dorsal side
  • Driven by β-catenin accumulation
  • Activates genes:
    • siamois
    • twin
• Leads to formation of the Spemann organizer
  

Role of the Organizer

• Blocks BMP signaling (so statement b is incorrect)
• BMP present – it will form epidermis (skin)
• BMP inhibited – it will form neural tissue
• Brain Formation requires:
  • Low BMP
  • Low Wnt signaling
• High Wnt are required for formation of posterior structures (spinal cord) (so statement c is incorrect)
• Nodal proteins are involved in mesoderm formation ( so statement d is incorrect)

Correct answer- option a. The Nieuwkoop centre is formed on the dorsal side of the embryo due to accumulation of β-catenin, which helps activate the siamois and twin genes

Exam Insights (Quick Recall)

• Flexible cells = Conditional specification
• β-catenin = dorsal side = Nieuwkoop centre
• BMP present = no neural tissue
• Brain = low BMP + low Wnt