NCERT Solution Class 12th Biology Chapter – 3 Human Reproduction
| Textbook | NCERT |
| class | Class – 12th |
| Subject | Biology |
| Chapter | Chapter -3 |
| Chapter Name | Human Reproduction |
| Category | Class 12th Biology Question & Answer |
| Medium | English |
| Source | last doubt |
NCERT Solution Class 12th Biology Chapter – 3 Human Reproduction
?Chapter – 3?
✍Human Reproduction✍
?Question & Answer?
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) Fertilisation 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) Fertilisation takes place in ampulla of oviduct
(j) Zygote divides to form blastocyst which is implanted in uterus.
(k) The structure which provides vascular connection between foetus and uterus is called placenta
2. Draw a labelled diagram of male reproductive system.
Answer: The diagram of male reproductive system is as follows:
3. Draw a labelled diagram of female reproductive system.
Answer: The diagram of female reproductive system is as follows:
4. Write two major functions each of testis and ovary.
Answer: Two major functions of each are as follows:
Testis:
- The process of spermatogenesis produces sperms through the seminiferous tubules
- Testosterone, the male sex hormone is secreted by the Leydig cells
Ovary:
- In the process of oogenesis, ovaries produces ova
- Progesterone and oestrogen, the female sex hormones are secreted
5. Describe the structure of a seminiferous tubule.
Answer: Structure of seminiferous tubules:
- Seminiferous tubules are found in the testicular lobules and are highly coiled structures. It is here that the production of sperms in the testes occurs
- Each of the seminiferous tubule has a lining of germinal epithelium
- On the inner side, it is lined by two types of cells – sertoli cells and spermatogonia
- Spermatogonia – they are the male germ cells that produce the primary spermatocytes through the process of meiotic divisions. The primary spermatocytes furthermore undergo meiotic divisions for the formation of secondary spermatocytes and ultimately spermatids. Later on, spermatids metamorphosize into the male gametes termed as spermatozoa
- Sertoli cells are referred to as nurse cells of the testes. It is because they nourish the germ cells.
- Just adjacent to the seminiferous tubules, there are large polygonal cells referred to as leydig cells or interstitial cells which secrete testosterone – the male hormone.
6. What is spermatogenesis? Briefly describe the process of spermatogenesis.
Answer: The phenomena of sperm production from the immature germ cell in males is termed as spermatogenesis. The process occurs in the seminiferous tubules located inside the testes. In this process, a diploid male germ cell or spermatogonium enlarges (in size) for the formation of a diploid primary spermatocyte which inturn goes through the first meiotic division or meiosis I. This division is a reductional division for the formation of two equal haploid secondary spermatocytes, each of which further undergoes second meiotic division or meiosis II for the formation of two equal haploid spermatids.
Subsequently, four haploid spermatids are formed from a diploid spermatogonium. The spermatids hence produced alter to form spermatozoa(sperm) through the process of spermiogenesis.
7. Name the hormones involved in regulation of spermatogenesis.
Answer: Some hormones involved in the regulation of spermatogenesis are as listed below:
- Lutenising hormone (LH) – serves as Leydig cells triggering synthesis & secretion of androgens
- Gonadotrophin-releasing hormone (GnRH) – it is a hypothalamic hormone that is secreted at the age of puberty, acting at the anterior pituitary gland and stimulating the secretion of LH & FSH
- Follicle stimulating hormone (FSH) – acts on sertoli cells, stimulating the secretion of factors which aid in spermiogenesis process
- Androgens – triggers inhibin production regulating the spermatogenesis process
- Inhibin
8. Define spermiogenesis and spermiation.
Answer: Spermiogenesis – It is the phenomena of transformation of non-motile spermatids to mature, motile spermatozoa.
Spermiation – It is the phenomena where mature spermatozoa are released from the Serotoli cells into the lumen of the seminiferous tubules of the testes.
9. Draw a labelled diagram of sperm.
Answer: The diagram of sperm is as below:
10. What are the major components of seminal plasma?
Answer: The major components of seminal plasma are:
- Secretions of the accessory sex glands of males – prostate gland, seminal vesicles, bulbourethral glands.
- Mainly composed of – calcium, fructose, and other enzymes.
11. What are the major functions of male accessory ducts and glands?
Answer: The major functions of the male accessory ducts and glands are as follows:
| Male accessory ducts | Vasa efferentia | Conducts sperms from the rete testis to the epididymis |
| Rete testis | Stores sperms that is produced by seminiferous tubules | |
| Vasa deferentia | Conducts sperms from epididymis to the urethra | |
| Epididymis | Physiological maturation of sperms, storage & nourishment | |
| Male accessory gland | Seminal vesciles | Activates and provides energy to facilitate sperm motility after ejaculation |
| Prostate gland | Nourishes and activates sperms, enhances sperm motility, provides alkalinity to the ejaculate, neutralizes urine acidity | |
| Cowper’s gland | Enhances mobility & survival potentiality of sperms in the genital tract of female reproductive system, neutralizes the activity of acidic female vaginal secretions |
12. What is oogenesis? Give a brief account of oogenesis.
Answer: Ooogenesis is the phenomena of formation of haploid female gametes known as ova from diploid oogonia in the ovary, Graffian follicles, to be precise. This process is discontinuous which is initiated during the period of foetal development that is terminated only after puberty sets in.
The process of Oogenesis takes place in three phases:
Multiplicative phase –
- Follicle cells are differentiated from the germinal epithelium of the ovary due to repeated mitosis division. Few follicle cells enlarge and are termed as egg mother cells and undergo mitosis to multiply which are referred to as oogonia.
- Growth phase – one of the oogonium of the egg nest differentiates while the rest change into surrounding nutritive follicular epithelium. There is an increase in the size of the differentiated isolated oogonium as it gets nourished from the girdling follicle cells thereby transforming into a diploid primary oocyte.
- Maturation phase – The diploid primary oocyte in this phase passes through two maturation divisions. Meiosis I – the first meiotic division splits the diploid primary oocyte into two haploid cells wherein the larger one is the secondary oocyte and the minor one is the polar body (polocyte). In meiosis II or the second meiotic division, the secondary oocyte splits to form one large ootid and a tiny second polar body. Furthermore, the first polar body splits through mitosis to form two polar bodies. The ootid matures into a functional haploid ovum. Therefore, one primary oocyte produces one large ovum and three polar bodies which inturn degenerate. They degenerate as they do no participate in reproduction, thus leaving behind one functional ovum.
13. Draw a labelled diagram of a section through ovary.
Answer: The diagram of a section of an ovary is as follows:
14. Draw a labelled diagram of a Graafian follicle.
Answer: The diagram of a Graafian follicle is as follows:
15. Name the functions of the following:
(a) Corpus luteum (b) Endometrium
(c) Acrosome (d) Sperm tail
(e) Fimbriae
Answer: The functions are as follows:
(a) Corpus luteum – It is formed when the Graafian follicle ruptures. The corpus luteum secretes the hormone progesterone during the luteal phase of the menstrual cycle. When progesterone is secreted in high levels, the secretion of LH and FSH is inhibited which further prevents ovulation. The corpus luteum facilitates the endometrium of the uterus to proliferate and prepare for the process of implantation.
(b) Endometrium – as the name suggests, the endometrium is the innermost lining of the uterus comprising glands that undergoes cyclic changes during different stages of the menstrual cycle in order to prepare itself for the embryo-implantation process.
(c) Acrosome – The acrosome is located in the anterior section of the head of the sperm, resembling a cap-like structure. It consists of the hyaluronidase enzyme that hydrolyses the outer membrane of the egg which facilitates the sperm to perforate through the egg during fertilization.
(d) Sperm tail – the sperm tail makes up for the longest part of the sperm, enabling the movement of the sperm, once it has entered the female reproductive tract.
(e) Fimbriae – Towards the ovarian end of the fallopian tube, finger-like projections emerge. These are the Fimbriae which assist in gathering the ovum after the ovulation process. This is facilitated by the beating of the cilia.
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 the 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:
- False.
Androgens are produced by the Leydig cells that are present in the seminiferous tubules - True
- False
Leydig cells are present in the seminiferous tubules of the testis in males. - True
- False
Oogenesis occurs in the ovary. - True
- True
17. What is menstrual cycle? Which hormones regulate menstrual cycle?
Answer:
- It is a cycle observed to be taking place in females, lasting for around 28 days on an average to complete
- It is a series of cyclic physiological changes occurring in the female reproductive tract in primates, the end of which is combined with the collapsing of the uterine endothelium that is liberated in the form of blood and mucus through the vaginal opening, which is termed as menses.
- The different hormones that regulate the menstrual cycle are LH – leutinizing hormone, FSH – follicle-stimulating hormone, progesterone, estrogen.
- During the follicular phase, the levels of LH and FSH that is secreted from the anterior pituitary gland increases. The FSH that is secreted under the effect of the releasing hormone (RH) from the hypothalamus, triggers the primary follicle to convert into a Graafian follicle.
- There is a gradual increase in the level of LH which causes the follicle to grow, also causing the secretion of estrogen.
- Estrogen obstructs the FSH secretion, triggering the secretion of the LH, which also results in the thickening of the uterine endometrium.
- The amplified secretion of LH also results in the Graffian follicle to rupture, thereby causing the release of the ovum into the fallopian tube.
- This ruptured graafian follicle transforms into corpus luteum which secretes the hormone – progesterone during the luteal phase.
- The hormone progesterone assists in maintaining and preparing the endometrium for the process of embryo-implantation
- When the progesterone level in the blood is high, the FSH and LH secretion decreases which furthermore hinders the process of ovulation.
18. What is parturition? Which hormones are involved in induction of parturition?
Answer: Parturution is the process wherein a fully developed foetus, from the mother’s womb, is expelled out after the completion of the gestation period. There are two critical hormones that are involved in the induction of parturition, they are:
- Oxytocin – It directs the full-term foetus towards the birth canal, as it causes the contraction of the smooth muscles of the myometrium of the uterus, leading the baby to be expelled out
- Relaxin – It relaxes the pelvic ligaments, widening the pelvis to assist in an easier childbirth
19. In our society the women are often blamed for giving birth to daughters. Can you explain why this is not correct?
Answer: We know that human beings have 23 pairs of chromosomes, out of these, we contain 22 pairs of autosomes, and the last pair varies in males and females. Males are heterogametic – they produce two types of male gametes or sperms, where 50% of the sperms carry the ‘X’ chromosome, while the rest 50% carry the ‘Y’ chromosome. On the other hand, females are homogametic – they produce only one type of gamete, the ova, each of which carries the ‘X’ chromosome only.
Once the male and female gametes have fused to form the zygote, it would carry either XX chromosome or XY chromosome, depending on whether sperm carrying X or Y fertilized the ovum. Hence, if the sperm carrying ‘X’ fertilizes the ovum (zygote XX), then it would develop into a female baby and if the sperm carrying ‘Y’ fertilizes the ovum (zygote XY), then it would develop into a male baby. Both of these cases are attributed to the sperm that carries the chromosome fertilizing the ovum. Hence, we say that the sex of the child is determined by the father and not the mother. This is why it is incorrect to blame women for the gender of the child.
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: Typically in a month, human ovaries release only one egg, rarely two.
Incase of identical twins or monozygotic twins, one egg is released by the ovary which splits into two post-fertilization. This is why identical twins exhibit the same genetic features. On the other hand, in fraternal twins or dizygotic twins, two eggs are released which are fertilized by two different sperms causing the fraternal twins to exhibit different genetic characters.
21. How many eggs do you think were released by the ovary of a female dog which gave birth to 6 puppies?
Answer: In order to have given birth to six puppies, the ovary of the female dog released six eggs. Hence, six zygotes were formed for each to develop into a puppy.