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NCERT Solutions for CBSE 12th Biology - Chapter 3, Human Reproduction

 

CBSE Class 12 Biology - NCERT Solutions (Answers)

HUMAN REPRODUCTION

Question 1: Fill in the blanks-
a)       Humans reproduce _______. (asexually / sexually)
b)       Humans are _______. (oviparous, viviparous, ovoviviparous)
c)        Fertilization 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)         Fertilization 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 ______.
Solution: a) sexually, b) viviparous, c) internal, d) haploid, e) diploid, f) ovulation, g) estrogens, h) fertilization, i) ampullary-isthmic junction, j) blastomers, k) placenta.

Question 2: Draw a labelled diagram of male reproductive system.
Solution:
www.cbsencertsolution.com - diagram of male reproductive system

Question 3: Draw a labelled diagram of female reproductive system.
Solution
www.cbsencertsolution.com - diagram of female reproductive system

Question 4: Write two major functions each of testis and ovary.
Solution: Major functions of testis-
a) Produce sperms.
b) Provide nutrition to germ cells and secrete testicular hormones called androgens.
   Major functions of ovary-
a) Produce female gamete, i.e., ovum.
b) Produce several steroid hormones (ovarian hormones).

Question 5: Describe the structure of seminiferous tubule.
Solution: Sperms are produced in the seminiferous tubules. Each seminiferous tubule is lined on its inside by two types of cells called male germ cells (spermatogonia) and sertoli cells. The male germ cells undergo meiotic divisions finally leading to sperm formation, while sertoli cells provide nutrition to the germ cells. The regions outside the seminiferous tubules called interstitial spaces contain small blood vessels and interstitial cells or Leydig cells. These cells synthesize and secrete testicular hormones called androgens.

Question 6: What is spermatogenesis? Briefly describe the process of spermatogenesis.
Solution: Spermatogenesis: It is the process of formation of spermatozoa in the seminiferous tubules of the testis.
www.cbsencertsolution.com - fig of Spermatogenesis
Fig: Spermatogenesis
As sperm mother cell or spermatogonia which are diploid (2n = 46) differentiates by mitosis into primary spermatocytes (2n). The primary spermatocytes undergoes meiosis I (reduction division) to produce secondary spermatocytes (n = 23). Secondary spermatocytes undergo meiosis II to form haploid spermatids that differentiates into spermatozoa (sperm). This whole process is referred as spermiogenesis or spermatogenesis. 

Question 7: Name the hormones involved in regulation of spermatogenesis.
Solution: At puberty, the hypothalamus produces GTH (Gonado Tropin Hormone). It acts on pituitary to release LH – (Lutenising Hormone) and FSH – (Follicle Stimulating Hormone). LH acts on sertoli cell and Leydig cell helping in spermatogenesis and androgen hormone secretion (Testosterone). It is also supported by ICSH from pituitary that acts on Leydig cells.
FSH acts on testicular follicles helping in spermatogenesis and on enlargement of secondary sex organs.

Question 8: Define spermiogenesis and spermiation.
Solution: Spermiogenesis – It is the process of transformation of spermatids into spermatozoa. Spermiation – Spermiation is the process of release of the spermatozoa from the seminiferous tubules.

Question 9: Draw a labelled diagram of sperm.
Solution:
www.cbsencertsolution.com image of Structure of a Sperm
Fig: Structure of a Sperm

Question 10: What are the major components of seminal plasma?
Solution: The major components of seminal plasma are as follows:
1. It is a mixture of secretions from seminal vesicles, prostate and bulbourethral glands. 
2. Seminal plasma is rich in fructose, calcium and certain enzymes.

Question 11: What are the major functions of male accessory ducts and glands?
Solution: The male accessory ducts store and transport the sperms from the testis to the outside through urethra. The secretions of accessory glands constitute the seminal plasma, which is rich in fructose, calcium and certain enzymes. The secretions of bulbourethral glands help in the lubrication of the penis.

Question 12: What is oogenesis? Give a brief account of oogenesis.
Solution: Oogenesis: It is the process of formation of ova in the ovary.
www.cbsencertsolution.com - fig of Oogenesis
Fig: Oogenesis
It is a non-motile female gamete, which is an oval or ovoid in shape. These are covered by plasma membrane called oolema having an anterior cone of reception through which the sperm enters into the ovum. During ovum production, the secondary oocytes undergo equal karyokinesis but unequal cytokinesis. Thus, the second polar body and ovum are morphologically different though haploid in nature. Unequal cytokinesis helps in getting major share of food-laded cytoplasm, which is helpful in nourishment for an alecithal egg (no yolk). The oolema is surrounded by outer zona pellucida and outer-most corona-radiata. The oolema encloses ooplasm, which has cytoplasm rich in ribosomes, mitochondria, Golgi bodies etc. It has a central vesicular nucleus with nucleolus. The ovum is polar with anterior animal pole and posterior vegetal pole. 

Question 13: Draw a labelled diagram of a section through ovary.
Solution: (Give figure from NCERT textbook page no 49).

Question 14: Draw a labelled diagram of a Graafian follicle.
Solution:  
www.cbsencertsolution.com - fig of Graffian follicle
Fig: Graffian follicle

Question 15: Name the functions of the following:
a)     Corpus luteum.
b)     Endometrium.
c)     Acrosome.
d)     Sperm tail.
e)     Fimbriae.
Solution
a) Corpus luteum: It secretes large amounts of progesterone, which is essential for maintenance of the endometrium. 
b) Endometrium: It helps in the implantation of the fertilized ovum and other events of pregnancy. 
c) Acrosome: A cap-like structure found in sperm that help in fertilization of the ovum. 
d) Sperm tail: It facilitates sperm motility essential for fertilization. 
e) Fimbriae: It helps in the collection of ovum after ovulation.

Question 16: Identify True / False statements: 
a) Androgens are produced by Sertoli cells. 
b) Spermatozoa get nutrition from Sertoli cells. 
c) Leydig cells are found in ovary. 
d) Leydig cells synthesize androgens. 
e) Oogenesis takes place in corpus luteum. 
f) Menstrual cycle ceases during pregnancy. 
g) Presence or absence of hymen is not a reliable indicator of virginity or sexual experience.
Solution: (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?
Solution: It is a periodic process occurring every 28-29 days. It is time-period when the primary follicle undergoes development, ovulates, and transforms into corpus albicans. The cycle is seen in primate’s like- humans, monkeys, and apes. Menstrual cycle does not occur during pregnancy. In other animals, estrous cycle is seen during which bleeding does not occur. It is seen in the breeding season and the animal is on the treat (when the female allows the male for coitus). In primates, breeding period is not definite. Menstrual cycle consists of 3 phases:
1.     Follicular Phase - 10 days.
2.     Luteal Phase - 14 days.
3.     Menstrual Phase - 3-5 days.
Follicular Phase: It is also called proliferative phase averaging about 14 days. During this time-period, the primary follicles develop into secondary and tertiary follicles. These start producing GTH releasing hormone from the hypothalamus. It causes release of FSH from pituitary. FSH act on follicle cells helping in maturation. The follicle cells produce estradiol which when more act on pituitary inhibiting FSH secretion and cause LH secretions. LH act on Graafian follicle and pushes the secondary oocytes out. This is called ovulation. At the 14th day when ovulation occurs, the level of LH is maximum. This is called LH surge. Ovulation marks the end of follicular phase.
Luteal Phase: It is also called as secretory phase ranging about 10 days. During this time-period after ovulation, the Graafian follicle becomes lighter and floats on the surface of ovary. The cell mass becomes dense, yellow-coloured corpus luteum. It starts producing high-level of progesterone and little estradiol. Increased progesterone decreased LH level. Progesterone act on uterine wall causing structural changes in myometrium and long coiled projections in endometrium. The uterus is ready for pregnancy. The uterine body secrets nutritious fluid into the uterine cavity. This is helpful for the developing embryo.
Menstrual Phase: It is called menstruation, which is the distortion of uterine wall and flowing of the uterine fluid and blood through vagina. T occurs due to degeneration of corpus luteum and transforming into white scar called corpus albicans. The process occurs due to absence of fertilization. Atrophy of corpus luteum decreases progesterone and estradiol level. Lack of progesterone, results in degeneration of uterus. Decreased estradiol stimulates pituitary to produce FSH again. This marks the beginning of next cycle.

Question 18: What is parturition? Which hormones are involved in induction of parturition?
Solution: Parturition refers to the process of delievery of the fully developed foetus, i.e., childbirth.
Relaxin and oxytocin are the hormones involved in the process.

Question 19: In our society, the women are often blamed for giving birth to daughters. Can you explain why this is not correct?
Solution: The chromosomes pattern in the human female is XX and that in male is XY. Thus, all the haploid gametes produced by the female have the sex chromosome X whereas that in the male gametes could be either X or Y chromosome. Hence, 50% carry the X chromosome while the other 50% carry the Y, after fusion of the gametes the zygote would carry either XX or XY depending on whether the sperm carrying X or Y fertilized the ovum. The zygote carrying XX would be a female baby and XY could be a male. Thus, from the above reason it is not correct to say the woman are responsible for giving birth to a girl child.

Question 20: How many eggs are released by a human ovary in a month? How many eggs do you think have been released if the mother gave birth to identical twins? Would your answer change if the twins born were fraternal?
Solution: (1) One egg. (2) One egg. Twins are born from one egg by the separation of early blastomere resulting from the first zygotic cleavage. (3) Yes, because if twins born are non-identical (fraternal) they would have developed from two separate eggs.  

Question 21: How many eggs do you think were released by the ovary of a female dog, which gave birth to 6 puppies?
Solution: Six eggs. The reason is dogs and rodents being polyovulatory species, can release more eggs. Since the dog gave birth to 6 puppies, so the ovary of the female dog would have released 6 eggs. 
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