CBSE Important Questions Class 12 Biology Chapter 2 – Human Reproduction

Class 12 Biology Chapter 2 Human Reproduction is the second chapter of the NCERT Class 12 Biology textbook. It describes the male and female reproductive systems, the formation of sperms and ova by meiosis (gametogenesis), the menstrual cycle and the hormones that control it (FSH, LH, estrogen and progesterone), fertilisation in the fallopian tube, implantation, the role of the placenta, pregnancy, childbirth and lactation.

These CBSE important questions for Class 12 Biology Chapter 2 cover the ovarian hormone graph and the uterine changes in each phase of the cycle, a labelled sectional view of the female reproductive system, the placenta, meiosis in gametogenesis, the role of the prostate gland, Leydig cells and the vas deferens, colostrum, why the testes lie in the scrotum, and how the zona pellucida prevents polyspermy. The set has multiple-choice, short-answer and long-answer questions with answers for students, parents and teachers.

Q1. Study the graph given below.

Graph of ovarian hormone levels (estrogen and progesterone) over the 28-day cycle

(a) Read the graph given above and correlate the uterine events that take place according to the hormone level on
(i) 6–15 days
(ii) 16–25 days
(iii) 26–28 days (if the ovum is not fertilised)
(b) Specify the source of the hormones mentioned in the graph.

Answer: (a) (i) 6–15 days (follicular phase): The gonadotropic hormones LH and FSH are released from the pituitary. As a result, the primary follicle in the ovary matures fully to become the Graafian follicle and secretes estrogen. Under the action of estrogen, the endometrium of the uterus regenerates through proliferation.
(ii) 16–25 days (luteal phase): The remaining part of the Graafian follicle is transformed into the corpus luteum. The corpus luteum secretes a large amount of progesterone, which is essential for the maintenance of the endometrium.
(iii) 26–28 days: In the absence of fertilisation, the corpus luteum degenerates and the levels of progesterone, LH and FSH fall. The endometrium breaks down completely, which leads to menstruation.
(b) Source of the hormones:

Hormone Source
Estrogen Ovarian (Graafian) follicle
Progesterone Corpus luteum

Q2. Draw the sectional view of the human female reproductive system. Discuss the role of the placenta in brief.

Sectional view of the human female reproductive system showing ovary, fallopian tube, uterus, endometrium, cervix and vagina

Answer: The placenta is the tissue that acts as a temporary association between the foetal and maternal tissues.
1. It provides nourishment to the growing foetus.
2. It acts as a respiratory organ by supplying oxygen to the developing foetus and carrying away carbon dioxide from the foetal blood into the maternal blood.
3. It helps in the removal of nitrogenous wastes such as urea from the foetal blood into the maternal blood, which can then be eliminated by the mother's kidneys.
4. It produces some hormones as well, such as human chorionic gonadotropin (hCG), progesterone and estrogen.

Q3. Name the cell division responsible for gametogenesis. Give one morphological difference between the two types of gametes in a heterogametic organism.

Answer: Meiosis is the cell division responsible for gametogenesis. It is a reductional division that halves the chromosome number, so the gametes are haploid (n). When the male and female gametes fuse during fertilisation, the diploid (2n) number is restored in the zygote.
Morphological difference: The male gamete is usually small and has a flagellum (motile), while the female gamete is usually big and non-flagellated (non-motile).

Q4. “Successful gamete transfer and fusion of gametes is essential for the most critical event in sexual reproduction.”
i. Give the technical term for the fusion of gametes.
ii. What are the events that occur during gamete transfer in plants?
iii. What is formed as a result of fusion of gametes/syngamy?
iv. How will you categorise syngamy based on the fusion of gametes inside or outside the female body?
v. What would happen if syngamy does not occur?

Answer: i. Fertilisation (syngamy).
ii. In plants, pollen grains are the carriers of male gametes and ovules have the egg. The pollen grains have to reach the stigma before fertilisation. The process of pollination transfers pollen grains to the stigma. The pollen grains germinate on the stigma, and the pollen tubes carrying the male gametes reach the ovule and release the male gametes.
iii. Fusion of gametes (syngamy) results in the formation of a diploid zygote.
iv. If syngamy occurs outside the body of the organism, i.e. in an external medium such as water, it is called external fertilisation. If syngamy occurs inside the body of the female organism, it is called internal fertilisation.
v. In sexual reproduction, meiosis halves the diploid (2n) number of chromosomes to the haploid number (n) in the gametes. The two gametes (egg and sperm) fuse at fertilisation and the chromosome number is restored, giving the new individual two sets of chromosomes, one from each parent. If syngamy does not occur, the two haploid gametes will not fuse, the chromosome number will not be restored, and no new individual will form.

Q5. Justify the significance of meiosis at the time of gametogenesis.

Answer: Meiosis is a fundamental process during sexual reproduction. It ensures two essential outcomes:
a. Reduction division: During meiosis, each gamete receives a haploid set (n) of chromosomes. The diploid number (2n) is restored when two gametes fuse. If gametes contained the same number of chromosomes as body cells, the chromosome number would double in the zygote and an abnormal organism would develop.
b. Rearrangement of genes: Crossing-over between maternal and paternal homologous chromosomes during meiosis shuffles the genes. This ensures that the offspring are genetically different from one another and helps in maintaining genetic diversity.

Q6. The fusion of male and female gametes is known as
(a) Insemination
(b) Fertilization
(c) Implantation
(d) Parturition

Answer: (b) Fertilization
The sperm and the egg are haploid (n) gametes. When they unite, they form a diploid (2n) zygote. This fusion is called fertilisation. Insemination is the introduction of sperm into the female tract, implantation is the attachment of the embryo to the uterine wall, and parturition is childbirth.

Q7. In males and females, the sexual characteristics are of two types – primary and secondary. The development of the secondary sexual characteristics in females is controlled by
(a) oxytocin.
(b) estrogen.
(c) testosterone.
(d) progesterone.

Answer: (b) estrogen.
Estrogen is the main hormone that starts and controls the development of female secondary sexual characteristics at puberty, such as breast development, widening of the hips and growth of pubic hair. Progesterone mainly prepares the uterus for pregnancy, oxytocin causes uterine contractions during childbirth and milk ejection, and testosterone is the male sex hormone.

Q8. How does colostrum provide initial protection against diseases to newborn infants? Give one reason.

Answer: Colostrum is the early milk secreted by the mammary glands during the first few days after childbirth. It contains antibodies called immunoglobulins such as IgA, IgG and IgM. These antibodies pass to the newborn and provide passive immunity, which helps protect the infant against diseases.

Q9. Explain the significance of the testes being located in the scrotum, outside the abdominal cavity, in human males.

Answer: The testes are located in the scrotum outside the abdominal cavity to maintain a temperature that is 2–2.5°C lower than the normal internal body temperature. This lower temperature is essential for spermatogenesis, the formation of sperms, which takes place in the seminiferous tubules. The enzymes and cellular processes involved work best at about 34.5–35°C. The higher temperature inside the abdomen would impair sperm production.

Q10. The sperms cannot survive in the vagina if the gland R does not pour X into the semen.
a. What do R and X denote?
b. What would happen if the cells (Y) present in the interstitial tissue of T are mutated?
c. What would happen if ‘S’ is blocked and all other parts of the male reproductive system are functional?

Diagram of the human male reproductive system with parts labelled P, Q, R, S and T

Answer: a. In the given image, R denotes the prostate gland, which pours an alkaline secretion (X) into the semen. This secretion neutralises the acidic environment of the vagina and enables the sperms to survive.
b. Leydig or interstitial cells (Y) are present in the interstitial tissue of the testes (T). These cells secrete androgens (mainly testosterone), which play a key role in spermatogenesis and maintain the secondary and accessory sexual characteristics in males. If the Leydig cells are mutated, the sexual characteristics in males will not express and sperm formation will not occur, which would lead to infertility.
c. If ‘S’ (vas deferens) is blocked, the sperms cannot travel from the testes to the urethra. As a result, the ejaculate would contain no sperms (azoospermia), causing male infertility.

Q11. The following table shows the variation in hormone levels during different phases of the human menstrual cycle in a woman. Analyse the findings to answer the questions that follow:

Day of Cycle FSH LH Oestrogen Progesterone
Days 1–5 High Low Low Low
Days 13–15 High Surge Peak Rising
Days 16–28 Low Low Moderate High

a) Interpret the hormonal data and explain the physiological changes expected in the ovary and uterus during days 13–15.
b) Why does progesterone increase in the latter part of the cycle, and what structure is responsible for this change?
c) What would happen to the endometrium if progesterone levels dropped immediately after ovulation?

Answer: a) During days 13–15, the LH surge along with high FSH and peak oestrogen triggers ovulation – the release of the secondary oocyte from the Graafian follicle. In the uterus, the rising oestrogen level leads to thickening of the endometrial lining, preparing it for possible implantation.
b) Progesterone increases in the latter part of the cycle (days 16–28) because, after ovulation, the ruptured follicle changes into the corpus luteum. The corpus luteum secretes progesterone, which maintains the endometrium and supports implantation if fertilisation occurs.
c) If progesterone dropped immediately after ovulation, the endometrial lining would not be maintained and would degenerate. This would prevent implantation and result in menstruation, ending the cycle early.

Q12. During a biology class, a teacher explains that fertilisation occurs in the ampullary region of the fallopian tube and involves the fusion of a sperm with an ovum. She also talks about the role of the zona pellucida in preventing polyspermy, that is, the entry of multiple sperms into one ovum. Based on the case study, which of the following events ensures that only one sperm fertilises the ovum?
(a) The motility of the sperms to reach the ampullary region.
(b) The changes in the zona pellucida that block the entry of additional sperms after the first sperm penetrates.
(c) The completion of the second meiotic division of the secondary oocyte.
(d) The fusion of the haploid nuclei of the sperm and ovum to form a diploid zygote.

Answer: (b) The changes in the zona pellucida that block the entry of additional sperms after the first sperm penetrates.
When the first sperm enters the zona pellucida, it triggers the cortical reaction in the secondary oocyte. Cortical granules release enzymes that harden the zona pellucida and destroy the sperm-binding sites. This stops other sperms from entering the ovum, so polyspermy is prevented.

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