NCERT Solutions for Class 12 Biology Chapter 3 Human Reproduction

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NCERT Solutions for Class 12 Biology Chapter 3 Human Reproduction

Question 1.
Fill in the blanks.

1. Humans reproduce___(asexually/ sexually).
2. Humans are ___(oviparous/ viviparous/ ovoviviparous).
3. Fertilisation is___in humans (external/internal).
4. Male and female gametes are___ (diploid/haploid).
5. Zygote is___(diploid/haploid).
6. The process of release of ovum from a mature follicle is called___
7. Ovulation is induced by a hormone called ___
8. The fusion of male and female gametes is called___
9. Fertilisation takes place in ___
10. Zygote divides to form___which is implanted in uterus.
11. The structure which provides vascular connection between foetus and uterus is called ___

Solution:

1. sexually;
2. viviparous;
3. internal;
4. haploid;
5. diploid;
6. ovulation;
7. LH;
8. fertilisation;
9. ampullary-isthmic junction of Fallopian tube;
10. blastocyst;
11. placenta

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

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

Question 4.
Write two major functions each of testis and ovary.
Solution:
Testes – Testes are the primary sexual organs in male. Two major functions of testes are –
(i) Testosterone production from leydig cells.
(ii) Production of sperm cells in seminiferous tubules.

Ovary – Ovaries are the primary female sex organs. Two main functions of ovaries are
(i) Productions of female germ cells called eggs or oocytes.
(ii)Production of hormones – estrogens & progesterone which affect many of the female secondary sexual characters & reproductive functions.

Question 5.
Describe the structure of a seminiferous tubule
Solution:
Seminiferous tubules are located in the testicles, and are the specific location of meiosis, and the subsequent creation of gametes, namely spermatozoa.

The lining of seminiferous tubules called germinal epithelium contains two types of cells – primary germ ceils which undergo spermatogenesis to form spermatozoa and columnar indifferent cells (derived from coelomic epithelium) which enlarge to form Sertoli cells. Sertoli cells function as nurse cells for providing nourishment to the developing spermatozoa, phagocvtise defective sperm and secrete protein hormone inhibin (which inhibits FSH secretion).

The seminiferous tubules are situated in testicular lobules. Both ends of the tubule are connected to the central region of the testis and form a network of small ductules called the rete testis.

Question 6.
What is spermatogenesis ? Briefly describe the process of spermatogenesis.
Solution:
Spermatogenesis is the process by which male spermatogonia develop into mature male gamete, spermatozoa. It starts at puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age. The process of spermatogenesis includes the formation of spermatogonia from germinal epithelium (primordial germ cell) through mitosis (multiplication phase). Finally they stop undergoing mitosis, grow and become primary spermatocytes (growth phase). Each spermatocyte undergoes meiosis (maturation phase). First maturation division is reductional, and produces two secondary spermatocytes. The latter divides by equational division (second maturation division) to form four haploid spermatids. Spermatids receive nourishment from the Sertoli cells to form sperms. This step is called spermiogenesis.

During this process one spermatogonium produces four sperms having half number of chromosomes.

Question 7.
Name the hormones involved in regulation of spermatogenesis.
Solution:
After sexual maturity, spermatogenesis starts due to the secretion of gonadotropin releasing hormone (GnRH) from the hypothalamus of brain. GnRH acts on pituitary gland and stimulates secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH). LH induce the Leydig’s cells of the testis to produce male sex hormones called androgens. High level of androgens stimulate the process of spermatogenesis. FSH acts on the Sertoli cells and stimulates secretion of some factors which help in the process of spermiogenesis.

Question 8.
Define spermiogenesis and spermiation.
Solution:
Spermiogenesis is the final stage of spermatogenesis in which there is maturation of spermatids into mature, motile spermatozoa. Spermiation is the release of mature spermatozoa from the surface of the Sertoli cell into the lumen of the seminiferous tubules

Question 9.
Draw a labelled diagram of sperm
Solution:

Question 10.
What are the major components of seminal plasma?
Solution:
Seminal plasma consists of secretions of seminal vesicles, prostate gland and bulbourethral glands. It contains fructose, prostaglandins, citrate, inositol, dotting proteins, (secretions of seminal vesicles), calcium, phosphate, bicarbonate, enzymes, prostaglandins (secretions of prostate gland) and mucus (secretion of bulbourethral glands).

Question 11.
What are the major functions of male accessory ducts and glands?
Solution:
Major functions of male-accessory ducts are
•Aid in sperm transport.
•Temporary storage of spermatozoa.
Male accessory glands secretions constitute the seminal plasma. These secretions are rich in fructose, ascorbic acid, citrate, calcium, certain enzymes and prostaglandins. These secretions nourish and activate the spermatozoa to swim.

Question 12.
What is oogenesis? Give a brief account of oogenesis.
Solution:
Oogenesis is the production and growth of the ova (egg cell) in the ovary. It starts only after the female has attained puberty. The process is induced by FSH from the anterior pituitary. It leads to the growth of a single Graafian follicle in one of the two ovaries every month. The developing ovary is colonised by primordial germ cells prior to birth which differentiate into oogonia. These enlarge within the follicle under the influence of mitotic division to form primary oocyte containing diploid number of chromosomes. These undergo reductional division (1st meiotic division) to form a secondary oocyte and first polar body. The secondary oocyte proceeds with meiosis II but the division gets arrested until fertilisation occurs. The ‘egg’ is released at secondary oocyte stage under the effect of LH. A second polar body is extruded. The first polar body may also divide to form two polar bodies of equal sizes which do not take part in reproduction and ultimately degenerates. During oogenesis one oogonium produces one ovum and three polar bodies. Polar bodies containing small amount of cytoplasm helps to retain sufficient amount of cytoplasm in the ovum which is essential for the development of early embryo. Formation of polar bodies maintains the half number of chromosomes in the ovum.

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

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

Question 15.
Give the functions of the following.

1. Corpus luteum
2. Endometrium
3. Acrosome
4. Sperm tail
5. Fimbriae

Solution:

1. Corpus luteum: It secretes the hormone progesterone, which prepares the uterus for implantation. If implantation fails the corpus luteum becomes inactive and degenerates. If an embryo gets implanted, the corpus luteum continues to secrete progesterone until the fourth month of pregnancy, after that the placenta takes over this function.
2. Endometrium : It is the mucous membrane lining the uterus, which becomes progressively thicker and more glandular and has an increased blood supply in the latter part of the menstrual cycle. This prepares the endometrium for implantation of the embryo, but if this does not occur much of the endometrium breaks down and is lost in menstruation. If pregnancy is established the endometrium becomes the decidua, which is shed after birth.
3. Acrosome : A membranous sac at or near the front of a sperm that assists in penetration of the egg. The acrosome contains enzymes, which are released when the sperm contacts the egg prior to fertilisation. The enzymes break down the outer layers of the egg to permit entry of
   the sperm.
4. Sperm tail : Tail is a fine vibrating posterior portion of the sperm which helps in swimming. This ability to swim (called motility) is essential for male fertility as the sperm has to swim up the vaginal canal, cervix and cervical canal to reach to the ovtim.
5. Fimbriae: They help in collection of ovum after ovulation.

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

1. Androgens are produced by Sertoli cells.
2. Spermatozoa get nutrition from Sertoli cells.
3. Leydig cells are found in ovary.
4. Leydig cells synthesise androgens.
5. Oogenesis takes place in corpus luteum.
6. Menstrual cycle ceases during pregnancy.
7. Presence or absence of hymen is not a reliable indicator of virginity Or sexual experience.

Solution:

1. False : Androgens are produced by interstitial cells or Leydig cells.
2. True
3. False: Leydig’s cells are found in the testes (in between the seminiferous tubules).
4. True
5. False : Oogenesis takes place in the ovary.
6. True
7. True

Question 17.
What is menstrual cycle ? Which hormones regulate menstrual cycle ?
Solution:
The recurring cycle of physiological changes in the uterus, ovaries and other sexual structures that occur from the beginning of one menstrual period through the beginning of the next is called menstrual cycle. The beginning of menstruation is called menarche. Hormones involved in the regulation of menstrual cycle are pituitary or ovarian hormones. These are LH, FSH, estrogen and progesterone.

Question 18.
What is parturition? Which hormones are involved in induction of parturition?
Solution:
The average duration of human pregnancy is about 9 months which is called 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 is called parturition.It is induced by hormone oxytocin which acts on the uterine muscle and causes stronger uterine contractions.

Question 19.
In our society the women are often blamed for giving birth to daughters. Can you explain why this is not correct?
Solution:
This is not correct that women in our society are often blamed for giving birth to daughters because the sex of the baby is determined by the father, not by the mother. As we know that the chromosome pattern in the human female is XX and that in the male is XY.

Therefore, all the haploid gametes produced by the female (ova) have the sex chromosome X whereas in the male gametes (sperms) the sex chromosome could be either X or Y, hence, 50 per cent of sperms carry the X chromosome while the other 50 per cent carry the Y. After fusion of the male and female gametes the zygote would carry either XX or XY depending on whether the sperm carrying X or Y fertilised the ovum. The zygote carrying XX would develop into a female baby and XY would form a male.

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 ?
Solution:
Each ovary develops a number of immature eggs associated with groups of other cells called follicles. Normally, in humans, only one egg is released at one time; occasionally, two or more erupt during the menstrual cycle. The egg erupts from the ovary on the 14th to 16th day of the approximately 28 day menstrual cycle. Identical twins occur when a single egg is fertilised to form one zygote (monozygotic) which then divides into two separate embryos. And if the twins were born fraternal two eggs are released. Fraternal twins (commonly known as “non-identical twins”) usually occur when two fertilised eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as “binovular” twins.

Question 21.
How many eggs do you think were released by the ovary of a female dog which gave birth to 6 puppies?
Solution:
One oogonium produces one ovum and three polar bodies. The ovum is the actual female gamete. The polar bodies take no part in reproduction and hence, soon degenerate. In human beings, ovum is released from the ovary in the secondary oocyte stage. So, six ova (eggs), were released by ovary of a female dog which gave birth to 6 puppies.

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