Chapter 2: Human Reproduction

Class 12 Biology Chapter 2 Human Reproduction Exercises Questions and Answers :

1. Fill in the blanks:

(a) Humans reproduce _____________ (asexually/sexually)

(b) Humans are _____________ (oviparous, viviparous, ovoviviparous)

(c) Fertilisation is _____________ in humans (external/internal)

(d) Male and female gametes are _____________ (diploid/haploid)

(e) Zygote is _____________ (diploid/haploid)

(f) The process of release of ovum from a mature follicle is called _____________ .

(g) Ovulation is induced by a hormone called _____________ .

(h) The fusion of male and female gametes is called _____________ .

(i) Fertilisation takes place in _____________ .

(j) Zygote divides to form _____________which is implanted in uterus.

(k) The structure which provides vascular connection between foetus and uterus is called _____________ .

Answer :

(a) Humans reproduce sexually.

(b) Humans are viviparous.

 (c) Fertilization is internal in humans.

(d) Male and female gametes are haploid.

(e) Zygote is diploid.

(f) The process of release of ovum from a mature follicle is called ovulation.

(g) Ovulation is induced by a hormone called luteinizing hormone (LH).

(h) The fusion of male and female gametes is called fertilization.

(i) Fertilization takes place in the ampulla of the oviduct.

 (j) Zygote divides to form a blastocyst , which is implanted in the uterus.

(k) The structure which provides vascular connection between the fetus and uterus is called the placenta.

2. Draw a labelled diagram of male reproductive system.

Answer : The diagram of male reproductive system :

3. Draw a labelled diagram of female reproductive system.

Answer : The diagram of female reproductive system :

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

Answer :  The two major functions of both the testis and the ovary are :

For Testis:

(i) The primary function of the testes is to produce sperm through a process called spermatogenesis. Sperm are male reproductive cells that are essential for fertilization during sexual reproduction.

(ii) The testes also produce the male sex hormone testosterone. Testosterone is responsible for the development and maintenance of male secondary sexual characteristics, such as facial hair, deepening of the voice, muscle development, and the growth of the male reproductive organs.

For Ovary:

(i) The ovaries are responsible for the production of eggs (ova) through a process known as oogenesis. These eggs are crucial for fertilization, and one is typically released each month during the menstrual cycle.

(ii) The ovaries produce female sex hormones, primarily estrogen and progesterone. These hormones play a key role in regulating the female reproductive system, including the development of secondary sexual characteristics, the menstrual cycle, and pregnancy-related changes in the body.

5. Describe the structure of a seminiferous tubule.

Answer :  A seminiferous tubule is a small, coiled tube found in the testes (testicles), and it is the place where sperm production occurs.

The structure of a seminiferous tubule :

(i)  Seminiferous tubules are long, narrow, and tubular in shape, resembling tiny tubes.

(ii) The walls of these tubules are lined with different types of cells that play crucial roles in sperm production.

(a) These are large, supportive cells found within the tubule. Sertoli cells provide physical support and nourishment to developing sperm cells.

(b) Germ cells are the sperm-forming cells. They start as immature germ cells and undergo a series of divisions and transformations as they move through the seminiferous tubules, ultimately developing into mature sperm cells.

(iii) Outside the seminiferous tubules, in the spaces between them, you find interstitial cells, also known as Leydig cells. These cells produce the male sex hormone testosterone, which is essential for sperm production and the development of male sexual characteristics.

Seminiferous tubules are the sites of sperm production in the testes. They consist of layers of supportive Sertoli cells and developing germ cells. These tubules, along with the interstitial cells, work together to ensure the production of sperm and the maintenance of male reproductive functions.

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

Answer : Spermatogenesis is the process of sperm cell production that takes place in the testes of males. It involves the transformation of germ cells (spermatogonia) into fully mature sperm cells (spermatozoa). Spermatogenesis starts at the age of puberty due to significant increase in the secretion of gonadotropin releasing hormone .

The process of spermatogenesis are :

(i) Spermatogonia, which are diploid (containing two sets of chromosomes), undergo mitotic division to produce more spermatogonia. Some of these spermatogonia remain as stem cells, ensuring a continuous supply of germ cells.

(ii) Spermatogonia that are committed to becoming sperm cells enter meiosis. Meiosis is a specialized cell division process that reduces the chromosome number by half. It consists of two stages: primary spermatocytes (diploid) undergo the first meiotic division to form secondary spermatocytes (haploid), and then, the secondary spermatocytes undergo the second meiotic division to produce spermatids (haploid).

(iii) Spermatids are immature and morphologically different from mature sperm. During spermiogenesis, spermatids undergo a series of structural changes that transform them into fully mature spermatozoa. This process involves the development of a tail (flagellum) for mobility and the formation of a head with a nucleus containing a condensed set of chromosomes.

(iv) Mature spermatozoa are released from the seminiferous tubules into the epididymis, where they undergo further maturation and become capable of swimming and fertilizing an egg.

Spermatogenesis is a continuous and highly regulated process that ensures a constant supply of sperm throughout a male's reproductive life. It is essential for sexual reproduction and the transmission of genetic material to the next generation .

7. Name the hormones involved in regulation of spermatogenesis .

Answer : The name the hormones involved in regulation of spermatogenesis are :  luteinising hormone (LH) , Androgens , GnRH and follicle stimulating hormone (FSH).

 Spermatogenesis starts at the age of puberty due to significant increase in the secretion of gonadotropin releasing hormone (GnRH). The increased levels of GnRH then acts at the anterior pituitary gland and stimulates secretion of two gonadotropins – luteinising hormone (LH) and follicle

stimulating hormone (FSH). LH acts at the Leydig cells and stimulates synthesis and secretion of androgens. Androgens, in turn, stimulate the process of spermatogenesis. FSH acts on the Sertoli cells and stimulates secretion of some factors which help in the process of spermiogenesis.

8. Define spermiogenesis and spermiation.

Answer : The primary sex organs – the testis in the males and the ovaries in the females – produce gametes, i.e, sperms and ovum, respectively, by the process called gametogenesis. In testis, the immature male germ cells (spermatogonia) produce sperms by spermatogenesis that begins at puberty. The spermatogonia (sing. spermatogonium) present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers.

The spermatids are transformed into spermatozoa (sperms) by the process called s permiogenesis. After spermiogenesis, sperm heads become embedded in the Sertoli cells, and are finally released from the seminiferous tubules by the process called spermiation.

9. Draw a labelled diagram of sperm.

Answer : The diagram of sperm :

10. What are the major components of seminal plasma?

Answer : The major components of seminal plasma are :

(i) Spermatozoa (sperm cells)

(ii) Fluid from the seminal vesicles

(iii)  Fluid from the prostate gland

(iv) Fluid from the bulbourethral glands

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

Answer :  The major functions of male accessory ducts and glands are:

Accessory ducts (epididymis, vas deferens) transport sperm from the testes to the urethra for ejaculation.

Glands (seminal vesicles, prostate, bulbourethral glands) produce fluids that mix with sperm to create semen, providing nutrients, mobility, and protection for sperm.

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 the female counterpart to spermatogenesis in males and is crucial for sexual reproduction.

The brief account of oogenesis:

(i)  Oogenesis begins before birth when a female fetus contains all the primary oogonia (precursor cells) she will ever have. These oogonia multiply through mitosis.

(ii) Some of the oogonia develop into primary oocytes during fetal development but remain arrested in prophase I of meiosis.

(iii) At puberty, a small number of primary oocytes (about one each menstrual cycle) are stimulated to resume meiosis. Each month, one primary oocyte is released from the ovary, entering the first meiotic division.

(iv) As the primary oocyte undergoes the first meiotic division, it divides unequally, producing one large secondary oocyte and a smaller polar body. The secondary oocyte contains most of the cytoplasm and organelles, making it the functional egg cell.

(v) The secondary oocyte remains arrested in metaphase II of meiosis until fertilization occurs.

(vi) Upon ovulation, the secondary oocyte is released from the ovary into the fallopian tube.

(vi) If fertilization occurs, the secondary oocyte completes meiosis II, producing a mature ovum (egg) and another polar body. The mature ovum is now ready to fuse with a sperm cell to form a zygote.

Oogenesis results in the production of one mature egg cell (ovum) from each primary oocyte that successfully completes meiosis. This process ensures the availability of eggs for fertilization during a female's reproductive years.

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

Answer : The diagram of a section through ovary :

14. Draw a labelled diagram of a Graafian follicle .

Answer : The diagram of a Graafian follicle :

        

15. Name the functions of the following : (a) Corpus luteum (b) Endometrium  (c) Acrosome (d) Sperm tail (e) Fimbriae

Answer : (a) Function of Corpus Luteum : The corpus luteum is a temporary endocrine structure formed in the ovary after ovulation. Its main functions are to produce the hormone progesterone and a small amount of estrogen. These hormones are crucial for maintaining the uterine lining (endometrium) and supporting early pregnancy if fertilization occurs.

(b) Function of Endometrium : The endometrium is the inner lining of the uterus. Its main function is to provide a suitable environment for the implantation of a fertilized egg (zygote) and, if pregnancy occurs, to support the developing embryo and fetus by supplying nutrients and blood. If pregnancy doesn't occur, the endometrium is shed during menstruation.

(c) Function of Acrosome :  The acrosome is a structure at the tip of a sperm cell's head. It contains enzymes that are essential for the sperm to penetrate and break through the outer protective layers of the egg (oocyte) during fertilization. The acrosome reaction releases these enzymes, facilitating the fusion of sperm and egg.

(d) Function of Sperm Tail : The sperm tail, also called the flagellum, is responsible for the motility (movement) of sperm. It propels the sperm forward, allowing it to swim through the female reproductive tract and reach the egg for fertilization. The tail's whip-like movement is critical for the sperm's ability to reach the egg.

(e) Function of Fimbriae:  Fimbriae are finger-like projections at the end of the fallopian tube closest to the ovary. Their primary function is to capture the egg (oocyte) when it is released from the ovary during ovulation. Fimbriae create a current that guides the egg into the fallopian tube, where it can meet a sperm for fertilization.

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 synthesise 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 primarily produced by Leydig cells in the testes.

(b) True.

(c) False. Leydig cells are found in the testes, not the ovary.

(d) True.

(e) False. Oogenesis takes place in the ovaries, specifically within ovarian follicles.

(f) True. The menstrual cycle typically ceases during pregnancy because ovulation and menstruation do not occur when a woman is pregnant.

(g) True. The presence or absence of the hymen is not a reliable indicator of virginity or sexual experience because it can be stretched or torn for various reasons other than sexual activity

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

Answer : The menstrual cycle is a recurring, natural process that occurs in the female reproductive system. It involves a series of hormonal and physiological changes that prepare the body for potential pregnancy. The menstrual cycle typically lasts about 28/29 days, although it can vary from person to person.

Hormones that regulate the menstrual cycle are :

(i) Produced by the hypothalamus in the brain, GnRH stimulates the anterior pituitary gland to release gonadotropins, which are crucial for regulating the cycle.

(ii) Secreted by the anterior pituitary gland, FSH initiates the development of follicles (small sacs within the ovaries) that contain eggs.

(iii) Also produced by the anterior pituitary gland, LH plays a central role in triggering ovulation (the release of a mature egg from the ovary) and the formation of the corpus luteum.

(iv) Produced by developing ovarian follicles, estrogen levels rise during the follicular phase and play a key role in stimulating the thickening of the uterine lining (endometrium) and the maturation of the egg.

(v) Produced primarily by the corpus luteum (a structure formed after ovulation), progesterone levels increase during the luteal phase. It helps prepare the uterine lining for possible embryo implantation and helps maintain pregnancy if fertilization occurs.

 (vi) If fertilization occurs and an embryo implants in the uterus, hCG (Human Chorionic Gonadotropin) is produced by the developing placenta. It signals the corpus luteum to continue producing progesterone during early pregnancy.

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

Answer : The average duration of human pregnancy is about 9 months which is called the gestation period. 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.

The initiation of parturition is a complex process that involves hormonal changes and signals from both the mother and the fetus.

Two main hormones are primarily involved in the induction of parturition:

(i) Oxytocin is a hormone produced by the hypothalamus and released by the posterior pituitary gland. It plays a central role in stimulating uterine contractions during labor. As the baby's head presses against the cervix and the cervix stretches, this triggers the release of oxytocin. Oxytocin then causes the uterine muscles to contract, which pushes the baby down the birth canal.

(ii) Prostaglandins are a group of hormone-like substances produced by various tissues, including the uterine lining. They help soften and ripen the cervix, making it more receptive to contractions. Prostaglandins also enhance uterine contractions and play a role in the initiation of labor.

Together, oxytocin and prostaglandins coordinate the rhythmic contractions of the uterine muscles and the dilation of the cervix, which are necessary for the successful delivery of the baby. Additionally, other hormones, such as estrogen and cortisol, also play supporting roles in the process of parturition.

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

Answer : Blaming women for giving birth to daughters is not only incorrect but also unfair and unjust. It is essential to understand that the sex of a child is determined by the combination of genetic factors contributed by both the mother and the father, specifically the sex chromosomes.

In humans, women typically carry two X chromosomes (XX), while men carry one X and one Y chromosome (XY). When fertilization occurs, the father's sperm can carry either an X or a Y chromosome, determining the sex of the offspring. If an X-carrying sperm fertilizes the egg, the result is a female (XX), and if a Y-carrying sperm fertilizes the egg, the result is a male (XY).

This is not correct because of :

(i) Both parents contribute equally to the genetic makeup of their child, including the sex chromosomes. It is the father's sperm that determines whether the child will be a boy or a girl.

(ii) The sex of a child is determined by the random chance of which sperm fertilizes the egg. It is not within the control of the mother.

(iii) Blaming or assigning responsibility to a woman for the sex of her child perpetuates harmful gender biases and stereotypes. It is essential to promote gender equality and challenge discriminatory attitudes.

(iv) Each child is a unique individual, regardless of their gender. It is crucial to celebrate and value each child for their unique qualities and potential, rather than attaching societal expectations based on gender.

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 typically releases only one egg (ovum) during each menstrual cycle in a process called ovulation. This egg may potentially be fertilized by a sperm cell, leading to the development of an embryo and pregnancy.

If a mother gives birth to identical twins, it means that a single fertilized egg (zygote) split into two embryos during early development. In this case, only one egg was initially released from the mother's ovary, and the subsequent splitting of that egg resulted in identical twins.

If the twins born are fraternal (non-identical), it means that two separate eggs were fertilized by two separate sperm cells during the same menstrual cycle. In this scenario, two eggs were released from the mother's ovaries

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

Answer : Female dogs typically release multiple eggs during a single estrus (heat) cycle, and these eggs can be fertilized by sperm from a male dog, resulting in multiple embryos and puppies. The number of eggs released can vary among individual dogs and breeds, but it's not uncommon for a dog to release multiple eggs during each cycle.

In the case of a female dog giving birth to 6 puppies, it suggests that multiple eggs were released, fertilized by multiple sperm cells, and subsequently developed into embryos. The exact number of eggs released by the dog's ovaries would likely have been at least 6 or more to account for each puppy born.