NCERT Solutions For Class 12 | Biology | Chapter 3 | Human Reproduction

NCERT Solutions for Class 12 Biology Chapter 3 Human Reproduction is the essential study material needed to perfect Human Reproduction topics.

NCERT Class 12 Biology solutions provided here have correct answers to NCERT textbook questions. Solutions curated in a comprehensive manner will help students to understand the subtopics in this chapter in a better way.

IMPORTANT SUB-TOPICS MENTIONED IN THE NCERT CLASS 12 BIOLOGY CHAPTER 3 HUMAN REPRODUCTION:

NCERT Solutions for Class 12 Biology Chapter 3 Human Reproduction has the following sub-topics as given below:

Sr. no	Topics
1	The Male Reproductive System
2	The Female Reproductive System
3	Gametogenesis
4	Menstrual Cycle
5	Fertilisation and Implantation
6	Pregnancy and Embryonic Development
7	Parturition and Lactation

NCERT SOLUTIONS CLASS 12 BIOLOGY CHAPTER 3 HUMAN REPRODUCTION:

1. Fill in the blanks:

• Humans reproduce ___________ (asexually/sexually)
• Humans are ___________ (oviparous, viviparous, ovoviviparous)
• Fertilisation is ___________ in humans (external/internal)
• Male and female gametes are ___________ (diploid/haploid)
• Zygote is ___________ (diploid/haploid)
• The process of release of ovum from a mature follicle is called ___________
• Ovulation is induced by a hormone called ___________
• The fusion of male and female gametes is called ___________
• Fertilisation takes place in ___________
• Zygote divides to form ___________which is implanted in uterus. The structure which provides vascular connection between foetus and uterus is called ___________

Answer –

• Humans reproduce sexually. (asexually/sexually)
• Humans are viviparous. (oviparous, viviparous, ovoviviparous)
• Fertilisation is internal in humans (external/internal)
• Male and female gametes are haploid. (diploid/haploid)
• Zygote is diploid. (diploid/haploid)
• The process of release of ovum from a mature follicle is called ovulation.
• Ovulation is induced by a hormone called luteinising hormone (LH).
• The fusion of male and female gametes is called fertilization.
• Fertilisation takes place in ampullary-isthmic junction.
• Zygote divides to form blastocyst which is implanted in uterus.
• The structure which provides vascular connection between foetus and uterus is called placenta (Umbilical Cord).

2. Draw a labelled diagram of the male reproductive system.

Answer –

3. Draw a labelled diagram of the female reproductive system.

Answer –

4. Write two major functions each of the testis and ovary.

Answer –

FUNCTIONS OF TESTIS:

• Producing sperms or spermatozoa by the process of spermatogenesis in the seminiferous tubules.
• It produces the male hormone namely testosterone which stimulates the development of the testis and secondary sexual characteristics.

FUNCTIONS OF OVARY:

• Through the process of oogenesis, the ovum or egg is produced inside the ovaries.
• It secretes progesterone and oestrogen- the female sex hormone, which stimulates the menstrual cycle and secondary sexual characteristics.

5. Describe the structure of a seminiferous tubule.

Answer – STRUCTURE OF SEMINIFEROUS TUBULES:

• Seminiferous tubules are located in the testicular lobules and are highly coiled structures. The production of sperm occurs inside the seminiferous tubules.
• It is lined by the germinal epithelium. The inner side of the seminiferous tubules is lined by two types of cells namely, male germ cells or spermatogonia and Sertoli cells.
• Spermatogonia undergoes meiotic division to produce spermatids which metamorphosize into sperms.
• Sertoli cells, are elongated and pyramidal & partially envelop the spermatogenic cells. These cells are called nurse cells as they provide nourishment to the germ cells.
• The regions outside the seminiferous tubules called interstitial spaces contain small blood vessels and interstitial cells or Leydig cells which secrete testosterone- a male hormone.

6. What is spermatogenesis? Briefly describe the process of spermatogenesis.

Answer – The process of producing sperm from spermatogonia (male germ cells) in the testis at the age of puberty is called spermatogenesis.
The process of spermatogenesis is as follows:

• The process starts at the age of puberty due to the significant increase of a hypothalamic hormone i.e. Gonadotropin Releasing Hormone (GnRH). The increased levels of GnRH then act at the anterior pituitary gland and stimulate the secretion of two gonadotropins – luteinising hormone (LH) and follicle-stimulating hormone (FSH).
• LH acts on Leydig cells to produce androgens which stimulate the process of spermatogenesis.
• The spermatogonia or male germ cells present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers. Each spermatogonium is diploid and contains 46 chromosomes.
• Some of the spermatogonia called primary spermatocytes periodically undergo meiosis.
• A primary spermatocyte undergoes the first meiotic division to produce two haploid cells called secondary spermatocytes. They have 23 chromosomes.
• The secondary spermatocyte undergoes the second meiotic division to produce four haploid cells called spermatids.
• The spermatids are transformed into spermatozoa or mature motile sperm by a process called spermiogenesis.
• FSH acts on the Sertoli cells and stimulates the production of some factors which are essential for the process of spermiogenesis.
• The sperms have their head embedded in Sertoli cells to get nourishment and are finally released in the lumen of seminiferous tubules by spermiation process.

7. Name the hormones involved in regulation of spermatogenesis.

Answer – Following hormones are involved in the regulation of spermatogenesis:

• Luteinising Hormone (LH) – It triggers Leydig cells to synthesise and secrete androgens.
• Gonadotropin-Releasing Hormone (GnRH) – Triggers or starts spermatogenesis at the age of puberty by acting on the anterior pituitary to secrete LH & FSH.
• Follicle Stimulation Hormone (FSH) – FSH acts on the Sertoli cells and stimulates the secretion of some factors which help in the process of spermiogenesis.
• Androgens – Stimulates the process of spermatogenesis by triggering inhibin production. The functions of male sex accessory ducts and glands are maintained by the testicular hormones (androgens).

8. Define spermiogenesis and spermiation.

Answer –

• Spermiogenesis – The process of transformation of spermatids into mature motile sperm or spermatozoa.
• Spermiation – The process of release of the sperm into the lumen of seminiferous tubules for transport.

9. Draw a labelled diagram of sperm.

Answer –

10. What are the major components of seminal plasma?

Answer – The seminal plasma consists of the secretions of the male accessory glands- Prostate gland, seminal vesicle and Cowper’s gland. It is the fluid which contains sperm during ejaculation. It is rich in fructose, calcium and certain enzymes.

11. What are the major functions of male accessory ducts and glands?

Answer – Functions of male accessory ducts:

• Rete Testis- Stores sperms produces by the seminiferous tubules.
• Vasa Efferentia- This duct conducts sperms from the rete testis to the epididymis.
• Epididymis- This duct nourishes the sperm, and stores it until physiological maturation.
• Vasa Deferentia- This duct conducts sperms from the epididymis to the urethra.
• Functions of male accessory glands:
• Seminal Vesicle- Provides energy to sperm for motility after ejaculation.
• Prostrate Gland- Its secretions nourish and activate the sperms and provide alkalinity to neutralise urine acidity.
• Cowper’s Gland or Bulbourethral Gland- Its secretion helps in the lubrication of the penis to provide easy copulation. Its secretion enhances sperm motility and its survival potential in the genital tract of females by neutralizing the acidic vaginal secretions.

12. What is oogenesis? Give a brief account of oogenesis.

Answer –

• The process of formation of a mature female gamete is called oogenesis.
• It is initiated during the embryonic development period where a million oogonia are formed within each foetal ovary, no more oogonia are formed after birth.
• These oogonia start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage. This stage is called primary oocytes which are surrounded by the granulosa cell layer which is called a primary follicle.
• A number of primary follicles degenerate during the period of birth to puberty. The primary follicle is then surrounded by a layer of more granulosa cells and a theca is formed. This stage is called secondary follicles.
• The secondary follicle is then converted into a tertiary follicle with a fluid-filled cavity called the antrum.
• In the stage of the tertiary follicle, the primary oocyte grows in size and completes its first meiotic division. This results in a secondary oocyte and 1st polar body.
• The tertiary follicle converts into a Graafian follicle or mature follicle, where the secondary oocyte forms a membrane called zona pellucida surrounding it.
• The Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary by the process called ovulation.

13. Draw a labelled diagram of a section through ovary.

Answer –

14. Draw a labelled diagram of a Graafian follicle.

Answer –

15. Name the functions of the following:

• (a) Corpus luteum
• (b) Endometrium
• (c) Acrosome
• (d) Sperm tail
• (e) Fimbriae

Answer –

• Corpus Luteum- It is formed when the Graafian follicle ruptures. The corpus luteum release a large amount of progesterone which is important for the maintenance of the endometrium for implantation.
• Endometrium- It is the innermost lining of the uterus which undergoes cyclic changes during different stages of the menstrual cycle to prepare for embryo implantation.
• Acrosome- It is located in the interior section of the head of the sperm, resembling a cap-like structure. The acrosome is filled with enzymes like hyaluronidase that hydrolyse the outer membrane of the ovum and help fertilisation of the ovum.
• Sperm tail- It is the longest part of the sperm and helps in the motility of the sperm.
• Fimbriae- They are finger-like projections emerging at the end of the infundibulum. They help in the collection of ovum after ovulation.

16. Identify True/False statements. Correct each false statement to make it true.

• (a) Androgens are produced by Sertoli cells. (True/False)
• (b) Spermatozoa get nutrition from sertoli cells. (True/False)
• (c) Leydig cells are found in ovary. (True/ False)
• (d) Leydig cells synthesize androgens. (True/ False)
• (e) Oogenesis takes place in corpus luteum. (True/False)
• (f) Menstrual cycle ceases during pregnancy. (True/False)
• (g) Presence or absence of hymen is not a reliable indicator of virginity or sexual experience. (True/False)

Answer –

• (a) FALSE. Androgens are produced by the Leydig cells present in seminiferous tubules.
• (b) TRUE
• (c) FALSE. Leydig cells are found in seminiferous tubules of the male testis.
• (d) TRUE
• (e) FALSE. Oogenesis occurs in the female ovary.
• (f) TRUE
• (g) TRUE

17. What is menstrual cycle? Which hormones regulate menstrual cycle?

Answer –

• The reproductive cycle of female primates is called the menstrual cycle. In human females, menstruation is repeated at an average interval of about 28/29 days, and the cycle of events starting from one menstruation to the next one is called the menstrual cycle.
• It is the cyclic change in the reproductive tract of female primates.
• The following phases take place during the menstrual cycle:
  • Menstrual Phase– The cycle starts with the menstrual phase when menstrual flow occurs and this lasts for 3-5 days where the endothelium of the uterine wall collapses due to no fertilisation and is expelled out of the vagina in the form of mucus and blood.
  • Follicular Phase– During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle which is triggered by FSH secretion. The gradual increase in LH causes follicles to grow. These follicles secrete oestrogen and simultaneously the endometrium of the uterus regenerates through proliferation.
  • Ovulatory Phase– Both LH and FSH attain a peak level in the middle of the cycle, i.e. about 14 days. The rapid secretion of LH is called LH surge and results in the rupturing of the Graffian follicle, thereby causing the release of an ovum in the fallopian tube.
  • Luteal Phase– During this phase, the remaining parts of the Graafian follicle transform into the corpus luteum. This corpus luteum secretes a large amount of progesterone which is important for the maintenance of the endometrium for implantation after fertilization.
  • In absence of fertilisation corpus luteum degenerates which results in the collapsing of the endometrium. Thus the cycle continues until the woman is pregnant. During pregnancy, the menstrual cycle stops for the gestation period.
• The hormones that regulate the menstrual cycle are Follicle Stimulating Hormone (FSH), Luteinizing Hormone (LH), Oestrogens and Progesterone.

18. What is parturition? Which hormones are involved in induction of parturition?

Answer –

• Vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the foetus. This process of delivery of the foetus (childbirth) is called parturition.
• This process is induced by a complex neuroendocrine mechanism.
• Hormones involved in induction of parturition are:
  • Oxytocin- It causes the contraction of smooth muscles of the myometrium wall of the uterus while childbirth.
  • Relaxin- It relaxed the pelvic ligaments.

19. In our society the women are often blamed for giving birth to daughters. Can you explain why this is not correct?

Answer –

• Out of 23 pairs of human chromosomes, 22 are autosomes and the last pair is sex chromosomes, which vary in both males and females.
• While males are heterogametic, females are homogametic, i.e. two types of gametes are produced in males 50% of sperms carry the X chromosome and 50% carry the Y chromosome, while females produce one type of gamete or ova containing X chromosome only.
• When the egg of a female containing the X chromosome is fertilised with the sperm of a male containing the Y chromosome, the zygote of XY is formed. This zygote develops into a male baby. And if the sperm of a male containing the X chromosome fertilizes, the zygote of XX is formed which would develop into a female baby.
• In both cases, the sex of the baby is attributed to the sperm of the male fertilizing the ovum of the female.
• Therefore, the sex of the child is determined by the sperm of the father.
• This is why it is incorrect to blame a woman for the gender of the baby.

20. How many eggs are released by a human ovary in a month? How many eggs do you think would have been released if the mother gave birth to identical twins? Would your answer change if the twins born were fraternal?

Answer –

• A human ovary releases one egg in a month. Very rarely two are released sometimes which leads to fraternal twins if fertilised.
• If the mother gave birth to identical twins, one egg is released which splits after fertilisation into two identical parts. Even though twins share the same genetic constitution, they can look different due to environmental factors.
• When two eggs are fertilised, fraternal twins or dizygotic twins are formed. They share a different set of genes, thus making their attributes different.

21. How many eggs do you think were released by the ovary of a female dog which gave birth to 6 puppies?

Answer – Dogs give multiple births, therefore, they release several eggs at the same time. If the eggs get fertilised, they implant themselves on the uterine wall. The gestation period for dogs is about 9 weeks. An average pregnancy consists of about six puppies. For a female dog to give birth to six puppies, six eggs were released.

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