CBSE Class 12 Biology: Reproduction in Organisms — Important Questions with Answers 2026

Short Answer Questions (2-3 Marks)

• Q: Describe the structure of a human ovum and sperm.
  Ans: Ovum: Largest human cell (~120 μm). Spherical, non-motile. Surrounded by zona pellucida (glycoprotein layer) and corona radiata (follicular cells). Contains haploid nucleus (23 chromosomes), cytoplasm rich in yolk. Released during ovulation. Sperm: Smallest human cell (~60 μm long). Has head (acrosome + haploid nucleus), middle piece (mitochondria for energy), and tail (flagellum for motility). About 300 million per ejaculation. Only one fertilises the ovum.
• Q: Explain the process of fertilisation and implantation in humans.
  Ans: Fertilisation: Occurs in ampullary-isthmic junction of fallopian tube. Sperm undergoes capacitation in female tract. Acrosomal enzymes dissolve zona pellucida. One sperm fuses with ovum → zygote (2n). Polyspermy is prevented by zona reaction. Implantation: Zygote undergoes cleavage → morula → blastocyst. Blastocyst has trophoblast (outer) and inner cell mass. Trophoblast attaches to endometrium (~7 days after fertilisation). Chorionic villi develop → placenta forms.
• Q: Describe the menstrual cycle with hormonal regulation.
  Ans: 28-day cycle, 4 phases: (1) Menstrual phase (Day 1-5): Endometrium sheds; progesterone and estrogen are low. (2) Follicular/proliferative phase (Day 6-13): FSH stimulates follicle growth; estrogen rises, rebuilds endometrium. (3) Ovulatory phase (Day 14): LH surge causes ovulation; mature ovum released from Graafian follicle. (4) Luteal/secretory phase (Day 15-28): Corpus luteum secretes progesterone; endometrium thickens, becomes secretory. If no fertilisation → corpus luteum degenerates → progesterone drops → menstruation.

Exam Tips for This Chapter

• Revise all definitions and laws from Reproduction in Organisms — commonly asked as 1-2 mark questions
• Practice diagrams related to Reproduction in Organisms — neat labelled diagrams carry 2-3 marks
• For numericals, always show formula → substitution → answer with correct units
• Previous year analysis shows Reproduction in Organisms carries significant marks in the board exam