Q.181 Zoology ⭐ 360° Option Elimination Active

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NEET 2021 Zoology Question 181 Solution

Topic: Cell Cycle and Cell Division · Official NTA Booklet Code M1

Question

Which stage of meiotic prophase shows terminalisation of chiasmata as its distinctive feature?

(1)Leptotene
(2)Zygotene
(3)Diakinesis
(4)Pachytene

✓

Verified Against NTA Final Answer Key

Option (3) : Diakinesis

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📋 Given Parameters & Problem Setup

Meiotic Phase Prophase I of Meiosis I

Diagnostic Morphological Event Terminalisation of chiasmata

Question Objective Identify the exact sub-stage exhibiting this hallmark

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The 30-Second Exam Hall Shortcut: ⚡ "Prophase I Sub-stages: LZPD-D Mnemonic → Diakinesis = Terminalisation"

⏱️ Save ~20 seconds

Prophase I sequence: Leptotene (compaction) → Zygotene (synapsis) → Pachytene (crossing over) → Diplotene (chiasmata appear) → Diakinesis (terminalisation of chiasmata). Diakinesis is marked by terminalisation! Select Option (3).

1 Recall Prophase I 5-stage mnemonic: Lazy Zebras Play Deep Down (Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis).

2 Map key diagnostic events: - Zygotene: Synaptonemal complex & synapsis. - Pachytene: Crossing over (recombinase enzyme). - Diplotene: Dissolution of complex, X-shaped chiasmata formed. - Diakinesis: Terminalisation of chiasmata (chiasmata shift to chromosome tips).

3 Question asks for "terminalisation of chiasmata" -> Diakinesis.

4 Select Option (3).

🛡️

Why You Can 100% Trust This Shortcut: NCERT Biology · Cell Cycle and Cell Division (Meiosis I: The final stage of meiotic prophase I is diakinesis. This is marked by terminalisation of chiasmata).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Chronology and Cytological Milestones of Prophase I

Meiosis I Prophase I is an extended, intricate phase subdivided into five successive stages: 1. Leptotene: Chromatin condenses; chromosomes become progressively visible under light microscopy. 2. Zygotene: Homologous chromosomes undergo pairing (synapsis), mediated by the formation of the tripartite nucleoprotein synaptonemal complex to produce bivalents (tetrads). 3. Pachytene: Non-sister chromatids of homologous chromosomes exchange genetic segments (crossing over) at recombination nodules, catalyzed by the enzyme recombinase. 4. Diplotene: Dissolution of the synaptonemal complex occurs; paired homologs separate except at crossover sites, revealing X-shaped chiasmata. 5. Diakinesis: The terminal sub-stage characterized by terminalisation of chiasmata, where cross-over contact points slide toward telomeric ends like unzipping a zipper. The nucleolus disappears, and the nuclear envelope disintegrates.

\text{Prophase I Stages} \implies \begin{cases} \textbf{Leptotene:} & \text{Condensation} \\ \textbf{Zygotene:} & \text{Synapsis (Synaptonemal Complex)} \\ \textbf{Pachytene:} & \text{Crossing Over (Recombinase)} \\ \textbf{Diplotene:} & \text{Chiasmata visible} \\ \textbf{Diakinesis:} & \mathbf{Terminalisation\,of\,Chiasmata} \end{cases}

Step 2

Diagnostic Alignment

NCERT explicitly defines Diakinesis as: "The final stage of meiotic prophase I is diakinesis. This is marked by terminalisation of chiasmata." Thus, Diakinesis is the precise answer.

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4-Option Deep Scan

1 1

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Leptotene is the first stage marked by initial chromosome compaction, long before homologous pairing or crossing over occurs.

⚠️

Student Fallacy (Why candidates pick this trap): Random guessing among stage names.

2 2

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Zygotene is characterized by synapsis and formation of the synaptonemal complex.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing the initiation of pairing (synapsis) with the final resolution of chiasmata.

3 3

✓ 100% Correct

Verdict & Scientific Reason:

Correct. Diakinesis is canonically characterized by the terminalisation of chiasmata.

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Why this is airtight: None. Candidate accurately pairs Diakinesis with terminalisation.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Pachytene is characterized by crossing over and recombination nodule formation.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing the creation of genetic exchange (crossing over in pachytene) with the shifting of chiasmata (diakinesis).

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NTA Trap Alert: Diplotene vs Diakinesis Chiasmata Trap

Chiasmata first become *visible* during Diplotene (due to synaptonemal complex breakdown). However, *terminalisation* of chiasmata (shifting to the ends) occurs in Diakinesis.

🛡️ How to avoid the -1 mark: Remember: Chiasmata appear in Diplotene; Chiasmata terminalise in Diakinesis.

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Core NEET Syllabus Standard

NEET / NCERT Biology · Cell Cycle and Cell Division Standard Benchmark

Core Scientific Principle Tested:

"Terminalisation of chiasmata is the hallmark feature of diakinesis."

Syllabus Focus: Meiosis I Prophase Stages

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Zoology Question 181 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "Prophase I Sub-stages: LZPD-D Mnemonic → Diakinesis = Terminalisation"

Step-by-Step Rigorous Workflow & Mathematical Proof

Chronology and Cytological Milestones of Prophase I

Diagnostic Alignment

360° Option Elimination Engine Key Feature

NTA Trap Alert: Diplotene vs Diakinesis Chiasmata Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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