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Botany Question 13 – AP-EAMCET 2025

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Arrange the chromosomal events of meiosis in a sequence. I. Disappearing of nucleolus II. Appearance of recombining nodule on chromatids III. Terminalisation of Chiasmata IV. Pairing of chromosomes V. Dissolution of synaptonemal complex

Recall the sequential events that occur during Prophase I of meiosis, specifically focusing on the substages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.

Step 1: Understand the Stages of Meiosis I Prophase✦ Active

Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. Meiosis I is divided into Prophase I, Metaphase I, Anaphase I, and Telophase I. Prophase I is a complex and extended stage, further subdivided into five substages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis. Each substage is characterized by specific chromosomal events.

💡 Teacher's Secret Hint

Remembering the mnemonic 'Leptos Zips Pachy Dips Diakinesis' can help recall the order of Prophase I substages.

Step 2: Identify Key Events in Each Substage○ Expand

Let's map the given events to the substages of Prophase I: * **Leptotene**: Chromosomes condense and become visible. * **Zygotene**: Homologous chromosomes begin to pair up (synapsis) forming bivalents. The synaptonemal complex starts to form. This corresponds to event **IV. Pairing of chromosomes**. * **Pachytene**: Chromosomes become shorter and thicker. Crossing over occurs between non-sister chromatids of homologous chromosomes. Recombining nodules, sites of crossing over, become visible. This corresponds to event **II. Appearance of recombining nodule on chromatids**. * **Diplotene**: The synaptonemal complex dissolves, and homologous chromosomes begin to separate, but remain attached at points called chiasmata (where crossing over occurred). This corresponds to event **V. Dissolution of synaptonemal complex**. * **Diakinesis**: Chiasmata move towards the ends of the chromosomes (terminalisation). The nuclear envelope breaks down, and the nucleolus disappears. This corresponds to events **III. Terminalisation of Chiasmata** and **I. Disappearing of nucleolus**.

💡 Teacher's Secret Hint

Note that the disappearance of the nucleolus and nuclear envelope are late Prophase I events, often occurring concurrently with or just after chiasmata terminalisation.

Step 3: Arrange the Events in Chronological Order○ Expand

Based on the events identified in Step 2, the chronological sequence is:

IV. Pairing of chromosomes (Zygotene)II. Appearance of recombining nodule on chromatids (Pachytene)V. Dissolution of synaptonemal complex (Diplotene)III. Terminalisation of Chiasmata (Diakinesis)I. Disappearing of nucleolus (Diakinesis)

Thus, the correct sequence is IV, II, V, III, I.

💡 Teacher's Secret Hint

Pay close attention to the order of events within Diplotene and Diakinesis, as they often involve multiple concurrent changes.

Step 4: Select the Correct Option○ Expand

Comparing the derived sequence (IV, II, V, III, I) with the given options, option 4 matches this sequence.

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