Skip to content

Study the figures given… (Class 12 Biology 2025, Q32b)

questionbuddy.in/q/cbse-12-biology-2025-57-7-1-q32b-figures-showing-initial-stages-formation

BiologyCBSE Class 12 · Board exam 2025, set 57/7/1 · 5 marks · Long answer

CBSE Class 12 Biology 2025: question 32(b)

Set in the CBSE Class 12 board exam of 2025, paper 57/7/1, section E, question 32(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q32(b) · 5 marks · Long answerSexual Reproduction in Flowering Plants

Study the figures given below showing initial stages in the formation of female gametophyte and answer the questions that follow.
(i) Identify (P) and (Q).
(ii) I. What kind of division does cell (P) undergo to form (Q) ?
II. How many (Q) cells form the embryo sac ? What is the name given to such kind of development ?
III. How many free nuclear mitotic divisions will the functional megaspore undergo to form the embryo sac ?
IV. Describe the structure of a mature female gametophyte.

Figure in the paper: Three stages of ovule nucellus with micropylar end marked at top; first shows cell (P) in nucellus, second shows nucellus with enlarged cell, third shows a group of cells labelled (Q) at the micropylar end. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 57/7/1.

Hints

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

Hint 1

Think about megasporogenesis: which diploid cell in the nucellus gives rise to haploid spores?

Hint 2

Of the four spores formed, how many survive? Count how many mitotic rounds are needed to reach 8 nuclei.

Hint 3

For the mature embryo sac, describe it end by end: 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) (P) is the megaspore mother cell (MMC), a large diploid cell in the nucellus near the micropylar end. (Q) is the group of four haploid megaspores (a megaspore tetrad).

    1 mark

  2. (ii) I. The MMC (P) undergoes meiosis (reduction division) to form the four haploid megaspores (Q).

    ½ mark

  3. (ii) II. Only one of the four megaspores is functional and forms the embryo sac; the other three degenerate. This is called monosporic development.

    1 mark

  4. (ii) III. The functional megaspore undergoes three successive free-nuclear mitotic divisions. These give 2, then 4, then 8 nuclei.

    ½ mark

  5. (ii) IV. Structure of the mature female gametophyte (embryo sac):

    • Micropylar end: three cells form the egg apparatus, made of one egg cell and two synergids.
    ½ mark
    • The synergids carry special cellular thickenings at their micropylar tips called the filiform apparatus, which guides the pollen tube.
    ½ mark
    • Chalazal end: three cells called the antipodals.
    ½ mark
    • Centre: a large central cell with two polar nuclei. The mature embryo sac is therefore 8-nucleate and 7-celled.
    ½ mark

Answer: (i) P – megaspore mother cell; Q – four megaspores (tetrad). (ii) I. Meiosis; II. Only one megaspore, monosporic development; III. Three free-nuclear mitotic divisions; IV. 7-celled, 8-nucleate embryo sac: egg apparatus (egg + 2 synergids with filiform apparatus) at micropylar end, 3 antipodals at chalazal end, central cell with 2 polar nuclei.

Where the marks are

  • Identifies P as megaspore mother cell and Q as megaspores (½ + ½ mark)
  • States that P divides by meiosis (½ mark)
  • States that only one megaspore forms the embryo sac (½ mark)
  • Names this monosporic development (½ mark)
  • States three free-nuclear mitotic divisions (½ mark)
  • Describes the egg apparatus (egg + two synergids) with filiform apparatus at the micropylar end (½ + ½ mark)
  • Describes three antipodals at the chalazal end (½ mark)
  • Describes the central cell with two polar nuclei, giving an 8-nucleate, 7-celled embryo sac (½ mark)

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 paper2025 board paper, set 57/7/1: 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.