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Answer the following. (Class 12 Biology 2023, Q31a)

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BiologyCBSE Class 12 · Board exam 2023, set 57/5/1 · 5 marks · Long answer

CBSE Class 12 Biology 2023: question 31(a)

Set in the CBSE Class 12 board exam of 2023, paper 57/5/1, section E, question 31(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(a) · 5 marks · Long answerSexual Reproduction in Flowering Plants

Answer the following.

  1. (i)

    Explain the monosporic development of embryo sac in the ovule of an angiosperm.

    3 marks
  2. (ii)

    Draw a diagram of the mature embryo sac of an angiospermic ovule and label any four parts in it.

    2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 57/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Monosporic means the embryo sac comes from only one megaspore. Ask what happens to the other three.

Hint 2

Count how many mitotic divisions are needed to go from 1 nucleus to 8.

Hint 3

In the diagram, recall which cells lie at the micropylar end, the chalazal end and the centre.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (i) The megaspore mother cell (2n) in the micropylar region of the nucellus divides by meiosis to form a linear tetrad of four haploid (n) megaspores.

    ½ mark

  2. Three megaspores degenerate, and only one functional megaspore remains. The embryo sac develops from this single megaspore, hence 'monosporic' development.

    ½ mark

  3. The nucleus of the functional megaspore divides by mitosis to form two nuclei, which move to opposite poles; this is the 2-nucleate embryo sac.

    ½ mark

  4. Two more successive mitotic divisions give the 4-nucleate and then the 8-nucleate stage. These are free nuclear divisions, meaning nuclear division is not followed by cell wall formation.

    ½ mark

  5. Cell walls then form around six of the nuclei: three cells group at the micropylar end (egg apparatus) and three at the chalazal end (antipodals).

    ½ mark

  6. The remaining two polar nuclei lie in the large central cell. The mature embryo sac is therefore 7-celled and 8-nucleate.

    ½ mark

  7. (ii) Diagram (described): an oval embryo sac. At the micropylar end is the egg apparatus, consisting of one egg cell and two synergids with filiform apparatus. At the chalazal end are three antipodal cells. In the middle is the large central cell with two polar nuclei.

    2 marks

Answer: MMC (2n) undergoes meiosis to form 4 megaspores; 3 degenerate and 1 is functional; three free-nuclear mitotic divisions give 8 nuclei; walls form, giving a 7-celled, 8-nucleate embryo sac (egg, 2 synergids, 3 antipodals, central cell with 2 polar nuclei).

Where the marks are

  • MMC divides by meiosis to form a tetrad of megaspores (½ mark)
  • Three megaspores degenerate, one functional megaspore (½ mark)
  • Mitosis of the functional megaspore nucleus (½ mark)
  • Free nuclear divisions up to the 8-nucleate stage (½ mark)
  • Cell wall formation organising egg apparatus and antipodals (½ mark)
  • 7-celled, 8-nucleate embryo sac with polar nuclei in the central cell (½ mark)
  • Diagram with any four correct labels (½ × 4 = 2 marks)

Revise the chapter

Practise this chapterBoard-style questions from Sexual Reproduction in Flowering Plants, one at a time, marked against the answer key and marking points.Back to the full paper2023 board paper, set 57/5/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.