12 Biology

Reproduction: Human

NCERT Solution

Question 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 fetus and uterus is called _____________

Answer: (a) sexually, (b) viviparous, (c) internal, (d) haploid, (e) diploid, (f) ovulation, (g) Luteinizing hormone, (h) fertilization, (i) fallopian tube, (j) blastocyst, (k) chorionic villi




Question 2: Draw a labelled diagram of male reproductive system.

Answer:

male reproductive system

Question 3: Draw a labelled diagram of female reproductive system.

Answer:

female reproductive system

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

Answer: Functions of testis: Production of sperms, and production of male sex hormones.
Functions of ovaries: Production of ovum, and production of female sex hormones.

Question 5: Describe the structure of a seminiferous tubule.

Answer: Each lobule contains one to three highly coiled seminiferous tubules. Sperms are produced in the seminiferous tubules. A seminiferous tubule is lined on the inside by two types of cells; called the male germ cells (spermatogonia) and Setoli cells. The male germ cells undergo meiosis; resulting in sperm formation. Sertoli cells provide nutrition to the germ cells.

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

Answer: In testis, the immature male germ cells (spermatogonia) produce sperms by spermatogenesis. Spermatogenesis begins at puberty. The sprematogonia are present on the inside wall of seminiferous tubules. These cells multiply by mitosis 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 completes the first meiotic division; resulting in formation of two equal haploid cells; called secondary spermatocytes. A secondary spermatocyte has 23 chromosomes. The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids.

Spermiogenesis: The process leading to transformation of spermids into spermatozoa (sperms) is called spermiogenesis. After spermiogenesis, sperm heads get embedded in the Sertoli cells, and are finally released from the seminiferous tubules. This process is called spermiation.

Question 7: Name the hormones involved in regulation of spermatogenesis.

Answer: Gonadotropin Releasing Hormone (GnRH), Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH)

Question 8: Define spermiogenesis and spermiation.

Answer: The process leading to transformation of spermids into spermatozoa (sperms) is called spermiogenesis. After spermiogenesis, sperm heads get embedded in the Sertoli cells, and are finally released from the seminiferous tubules. This process is called spermiation.

9. Draw a labelled diagram of sperm.

Answer:

structure of sperm

Question 10: What are the major components of seminal plasma?

Answer: The seminal plasma is rich in fructose, calcium and certain enzymes.



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

Answer: The accessory ducts transport the sperm from testis to urethra. The accessory glands secrete substances which constitute the seminal plasma.

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

Answer: The process of formation of a mature female gamete is called oogenesis. Oogenesis is initiated during the embryonic development stage, i.e. when the female child is still in the womb. At this stage, a couple of million gamete mother cells (oogonia) are formed within each fetal ovary. These cells start division and enter into prophase-I of meiosis, and get temporarily arrested at that stage. These cells are called primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells, and then it is called the primary follicle.

Secondary Follicle: A large number of primary follicles degenerate during the phase from birth to puberty. Thus, at puberty only 60,000 to 80,000 primary follicles are left in each ovary. The primary follicle gets surrounded by more layers of granulose and a new theca, and is now called secondary follicle.

Tertiary Follicle: The secondary follicle soon changes into tertiary follicle. The tertiary follicle is characterized by a fluid-filled cavity called antrum. The theca layer is organized into an inner theca interna and an outer theca externa. At this stage, the primary oocyte (within tertiary follicle) grows in size and completes its first meiotic division. This is an unequal division which results in the formation of a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte retains bulk of the nutrient rich cytoplasm of the primary oocyte. There is not enough knowledge on what happens to the first polar body. The tertiary follicle further changes into the mature follicle or Graafian follicle.

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

Answer:

sectional view of ovary

Question 14. Draw a labelled diagram of a Graafian follicle?

Answer: Look the figure given in previous answer

Question 15: Name the functions of the following:

(a) Corpus luteum

Answer: Produces estrogen and progesterone

(b) Endometrium

Answer: Is the site of implantation for embryo

(c) Acrosome

Answer: Acrosome releases enzyme which helps the sperm to enter the ovum

(d) Sperm tail

Answer: Helps the sperm in swimming

(e) Fimbriae

Answer: Catches the ovum during ovulation and transfers it to fallopian tube



Question 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, (b) True, (c) False, (d) True, (e) False, (f) True, (g) True

Question 17: What is menstrual cycle? Which hormones regulate menstrual cycle?

Answer:Answer: The cycle of events; starting from one menstruation till the next menstruation is called menstrual cycle. LH and FSH regulate menstrual cycle.

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

Answer:Answer: The process of delivery of the foetus (childbirth) is called parturition. Oxytocin is involved in induction of parturition.

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

Answer:Answer: The gender of the baby is decided at the stage of fertilization itself. You may recall, that the chromosome pattern in the human female is XX and that in the male is XY. Hence, all the female gametes have sex chromosome X. On the other hand, the male gametes can have X or Y as sex chromosome. If a sperm with X chromosome fertilizes the ovum, the resulting zygote will lead to the birth of a girl child. If a sperm with Y chromosome fertilizes the ovum, the resulting zygote will lead to the birth of a male child. Hence, there is 50:50 chance of the birth of either a boy or a girl. Neither the father nor the mother has control on the gender of the child. Hence, it is wrong to blame the woman for giving birth to daughters.

Question 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:Answer: Generally, one egg is released by a human ovary in a month. In case of identical twins, only one egg is released in a menstrual cycle. Two eggs are released in case of fraternal twins.

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

Answer:Answer: 6 eggs