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Briefly explain the events… (Class 12 Biology 2026, Q31a)

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BiologyCBSE Class 12 · Board exam 2026, set 57/5/1 · 5 marks · Long answer

CBSE Class 12 Biology 2026: question 31(a)

Set in the CBSE Class 12 board exam of 2026, 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 answerHuman Reproduction

Briefly explain the events of fertilisation and implantation in an adult human female.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 57/5/1.

Hints

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

Hint 1

Start from where in the fallopian tube the sperm meets the ovum, then follow the sperm's entry step by step.

Hint 2

Think about how polyspermy is prevented and what the oocyte does after sperm entry.

Hint 3

For implantation, recall which layer of the blastocyst sticks to the uterine wall.

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. Fertilisation: It takes place in the ampullary (ampullary-isthmic junction) region of the fallopian tube, where the ovum and the sperm reach together.

    ½ mark

  2. The secretions of the acrosome help the sperm dissolve the zona pellucida and plasma membrane and enter the ovum.

    ½ mark

  3. When a sperm contacts the zona pellucida, it induces changes in the membrane that block the entry of additional sperms. This ensures that only one sperm fertilises the ovum (monospermy).

    ½ mark

  4. Entry of the sperm induces the secondary oocyte to complete meiosis II, forming the ootid (ovum) and a second polar body.

    ½ mark

  5. The haploid nucleus of the sperm fuses with the haploid nucleus of the ovum.

    ½ mark

  6. This fusion forms a diploid zygote (2n). The sex of the baby is decided at this stage by the X or Y chromosome of the sperm.

    ½ mark

  7. Implantation: The zygote divides mitotically (cleavage) to form a morula and then a blastocyst, which moves to the uterus. In the blastocyst, the outer trophoblast layer gets attached to the endometrium of the uterus.

    1 mark

  8. The uterine cells divide rapidly and cover the blastocyst. As a result, the blastocyst becomes embedded in the endometrium. This embedding is implantation, and it leads to pregnancy.

    1 mark

Answer: Fertilisation in the ampulla: acrosomal entry, block to polyspermy, completion of meiosis II, fusion of nuclei to form a diploid zygote. Implantation: trophoblast attaches to the endometrium and uterine cells cover the blastocyst, embedding it.

Where the marks are

  • Site of fertilisation – ampulla / ampullary-isthmic junction (½ mark)
  • Acrosome secretions help sperm entry (½ mark)
  • Changes in zona pellucida block other sperms (½ mark)
  • Completion of meiosis II by the secondary oocyte (½ mark)
  • Fusion of sperm and ovum nuclei (½ mark)
  • Formation of diploid zygote (½ mark)
  • Trophoblast attaches to endometrium (1 mark)
  • Uterine cells cover the blastocyst and embed it in the endometrium (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Human Reproduction, one at a time, marked against the answer key and marking points.Back to the full paper2026 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.