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Trace the events from… (Class 12 Biology 2026, Q31a)

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

CBSE Class 12 Biology 2026: question 31(a)

Set in the CBSE Class 12 board exam of 2026, paper 57/1/1, 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

Trace the events from fertilization till implantation of blastocyst in human female. Also mention the site where fertilization takes place.

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

Hints

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

Hint 1

Begin with what the sperm triggers in the secondary oocyte.

Hint 2

Name the embryo stages in order as cell number increases.

Hint 3

Implantation involves the outer layer of the blastocyst and the uterine lining.

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

    ½ mark

  2. The haploid nucleus of the sperm fuses with the haploid nucleus of the ovum, forming a diploid zygote.

    ½ mark

  3. The zygote moves through the isthmus of the oviduct towards the uterus.

    ½ mark

  4. On the way it undergoes mitotic divisions (cleavage), forming 2, 4, 8 and 16 daughter cells called blastomeres.

    ½ mark

  5. The embryo with 8–16 blastomeres is called a morula.

    ½ mark

  6. The morula keeps dividing and transforms into a blastocyst as it moves into the uterus.

    ½ mark

  7. The blastomeres of the blastocyst are arranged into an outer layer, the trophoblast, and an inner cell mass attached to it.

    ½ mark

  8. The trophoblast attaches to the endometrium. The uterine cells then divide rapidly and cover the blastocyst, so it becomes embedded in the endometrium. This is implantation, which leads to pregnancy.

    ½ mark

  9. Site of fertilization: the ampullary region of the fallopian tube (ampullary–isthmic junction).

    1 mark

Answer: Sperm entry, completion of meiosis II (ootid), fusion of nuclei (zygote), cleavage while moving through the isthmus (blastomeres), morula (8–16 cells), blastocyst (trophoblast + inner cell mass), attachment of trophoblast to endometrium and embedding = implantation. Fertilization occurs in the ampullary region of the fallopian tube.

Where the marks are

  • Completion of meiosis II after sperm entry forming the ootid (½ mark)
  • Fusion of nuclei forming the zygote (½ mark)
  • Movement through the isthmus towards the uterus (½ mark)
  • Cleavage into blastomeres (½ mark)
  • Morula at 8–16 cells (½ mark)
  • Blastocyst formation (½ mark)
  • Trophoblast and inner cell mass (½ mark)
  • Trophoblast attaches and gets embedded in the endometrium: implantation (½ mark)
  • Site: ampullary region of the fallopian tube (1 mark)

Revise the chapter

  • Chapter: Human Reproduction (notes, formulas, where marks are lost and every past question)
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 57/1/1: 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.