NCERT Solutions for Class 12 Biology Chapter 2 Human Reproduction

NCERT
SOLUTIONS
CLASS - 12th

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Class : 12th
Subject : Biology
Chapter : 3
Chapter Name : Human Reproduction

Q1 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 the ovum from a mature follicle is called _________.
(g) Ovulation is induced by a hormone called the ____________ .
(h) The fusion of the male and the female gametes is called ___________ .
(i) Fertilization takes place in the ____________ .
(j) The zygote divides to form __________, which is implanted in uterus.
(k) The structure which provides vascular connection between the fetus 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 the ovum from a mature follicle is called ovulation .
(g) Ovulation is induced by a hormone called the luteinizing hormone .
(h) The fusion of the male and the female gametes is called Fertilization
(i) Fertilization takes place in the fallopian tube
(j) The zygote divides to form blastocyst which is implanted in uterus.
(k) The structure which provides vascular connection between the fetus and uterus is called
placenta .

Page : 55 , Block Name : Exercise

Q2 Draw a labeled diagram of male reproductive system.

Answer.

Q3 Draw a labeled diagram of female reproductive system.

Answer.

Q4 Write two major functions each of testis and ovary.

Answer. Functions of the Testis:
(a) They produce male gametes called spermatozoa by the process of spermatogenesis.
(b) The leydig cells of the seminiferous tubules secrete the male sex hormone called testosterone.
Testosterone aids the development of secondary sex characteristics in males.

Functions of the ovary:
(a) They produce female gametes called ova by the process of oogenesis.
(b) The growing Graa an follicles secrete the female sex hormone called estrogen. Estrogen aids
the development of secondary sex characteristics in females.

Q5 Describe the structure of a seminiferous tubule.

Answer. The production of sperms in the testes takes place in a highly coiled structure called the
seminiferous tubules. These tubules are located in the testicular lobules. Each seminiferous tubule
is lined by germinal epithelium. It is lined on its inner side by two types of cells namely
spermatogonia and sertoli cells respectively. Spermatogonia are male germ cells which produce
primary spermatocytes by meiotic divisions. Primary spermatocytes undergo further meiotic
division to form secondary spermatocytes and nally, spermatids. Spermatids later
metamorphoses into male gametes called spermatozoa. Sertoli cells are known as nurse cells of
the testes as they provide nourishment to the germ cells. There are large polygonal cells known as
interstitial cells or leydig cells just adjacent to seminiferous tubules. These cells secrete the male
hormone called testosterone.

Q6 What is spermatogenesis? Brie y describe the process of spermatogenesis.

Answer. Spermatogenesis is the process of the production of sperms from the immature germ cells
in males. It takes place in seminiferous tubules present inside the testes. During spermatogenesis,
a diploid spermatogonium (male germ cell) increases its size to form a diploid primary
spermatocyte. This diploid primary spermatocyte undergoes rst meiotic division (meiosis l),
which is a reductional division to form two equal haploid secondary spermatocytes. Each
secondary spermatocyte then undergoes second meiotic division (meiosis II) to form two equal
haploid spermatids. Hence, a diploid spermatogonium produces four haploid spermatids. These
spermatids are transformed into spermatozoa (sperm) by the process called spermiogenesis.

Q7 Name the hormones involved in regulation of spermatogenesis.

Answer. Follicle-stimulating hormones (FSH) and luteinizing hormones (LH) are secreted by
gonadotropin releasing hormones from the hypothalamus . These hormones are involved in the
regulation of the process of spermatogenesis. FSH acts on sertoli cells, whereas LH acts on leydig
cells of the testis and stimulates the process of spermatogenesis.

Q8 De ne spermiogenesis and spermiation.

Answer. Spermiogenesis: It is the process of transforming spermatids into mature spermatozoa or
sperms.
Spermiation: It is the process when mature spermatozoa are released from the sertoli cells into the
lumen of seminiferous tubules.

Q9 Draw a labeled diagram of sperm.

Answer.

Q10 What are the major components of seminal plasma?

Answer. Semen (produced in males) is composed of sperms and seminal plasma. The major
components of the seminal plasma in the male reproductive system are mucus, spermatozoa, and
various secretions of accessory glands. The seminal plasma is rich in fructose, calcium, ascorbic
acid, and certain enzymes. It provides nourishment and protection to sperms.

Q11 What are the major functions of male accessory ducts and glands?

Answer. The male accessory ducts are vasa efferentia, epididymis, vas deferens, and rete testis.
They play an important role in the transport and temporary storage of sperms. On the contrary,
male accessory glands are seminal vesicles, prostate glands, and bulbourethral glands. These
glands secrete uids that lubricate the reproductive system and sperms. The sperms get dispersed
in the uid which makes their
transportation into the female body easier. The uid is rich in fructose, ascorbic acid, and certain
enzymes. They also provide nutrients and activate the sperm.

Q12 What is oogenesis? Give a brief account of oogenesis.

Answer. Oogenesis is the process of the formation of a mature ovum from the oogonia in females.
It takes place in the ovaries. During oogenesis, a diploid oogonium or egg mother cell increases in
size and gets transformed into a diploid primary oocyte. This diploid primary oocyte undergoes
rst meiotic division i.e., meiosis I or reductional division to form two unequal haploid cells. The

smaller cell is known as the rst polar body, while the larger cell is known as the secondary oocyte.
This secondary oocyte undergoes second meiotic division i.e., meiosis II or equational division and
gives rise to a second polar body and an ovum. Hence, in the process of oogenesis, a diploid
oogonium produces a single haploid ovum while two or three polar bodies are produced.

Q13 Draw a labeled diagram of a section through ovary.

Answer.

Q14 Draw a labeled diagram of a Graa an Follicle?

Answer.

Q15 Name the functions of the following.
(a) Corpus luteum
(b) Endometrium
(c) Acrosome
(d) Sperm tail
(e) Fimbriae

Answer. (a) Corpus luteum - Corpus luteum is formed from the ruptured Graa an follicle. It
secretes progesterone hormone during the luteal phase of the menstrual cycle. A high level of
progesterone inhibits the secretions of FSH and LH, thereby preventing ovulation. It also allows
the endometrium of the uterus to proliferate and to prepare itself for implantation.
(b) Endometrium — It is the innermost lining of the uterus. It is rich in glands and undergoes
cyclic changes during various phases of the menstrual cycle to prepare itself for the implantation
of the embryo.
(c) Acrosome — It is a cap-like structure present in the anterior part of the head of the sperm. It
contains hyaluronidase enzyme, which hydrolyses the outer membrane of the egg, thereby helping
the sperm to penetrate the egg at the time of fertilization.
(d) Sperm tail — It is the longest region of the sperm that facilitates the movement of the sperm
inside the female reproductive tract.
(e) Fimbriae — They are nger-like projections at the ovarian end of the fallopian tube. They help
in the collection of the ovum (after ovulation), which is facilitated by the beating of the cilia.

Q16 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) Androgens are produced by Sertoli cells. (False) Androgens are produced by Leydig
cells found in seminiferous tubules of the testis.
(b) Spermatozoa get nutrition from Sertoli cells. (True)
(c) Leydig cells are found in ovary. (False) Leydig cells are found in the seminiferous tubules of the
testis.
(d) Leydig cells synthesise androgens. (True)
(e) Oogenesis takes place in corpus luteum. (False) Oogenesis takes place in the ovary.
(f) Menstrual cycle ceases during pregnancy. (True)
(g) Presence or absence of the hymen is not a reliable indicator of virginity or sexual experience.
(True)

Q17 What is menstrual cycle? Which hormones regulate menstrual cycle?

Answer. The menstrual cycle is a series of cyclic physiologic changes that take place inside the
female reproductive tract in primates. The whole cycle takes around 28 days to complete. The end
of the cycle is accompanied by the breakdown of uterine endothelium, which gets released in the
form of blood and mucous through the vagina. This is known as menses. The follicle stimulating
hormone (FSH), luteinizing hormone (LH), estrogen, and progesterone are the various hormones
that regulate the menstrual cycle. The level of FSH and LH secreted from the anterior pituitary
gland increases during the follicular phase. FSH secreted under the in uence of RH (releasing
hormone) from the hypothalamus stimulates the conversion of a primary follicle into a graa an
follicle. The level of LH increases gradually leading to the growth of follicle and secretion of
estrogen. Estrogen inhibits the secretion of FSH and stimulates the secretion of luteinizing
hormone. It also causes the thickening of the uterine endometrium. The increased level of LH
causes the rupturing of the graa an follicle and release the ovum into the fallopian tube. The
ruptured graa an follicle changes to corpus luteum and starts secreting progesterone hormone
during the luteal phase. Progesterone hormone helps in the maintenance and preparation of
endometrium for the implantation of the embryo. High levels of progesterone hormone in the
blood decrease the COD sh

Q18 What is parturition? Which hormones are involved in induction of parturition?

Answer. Parturition is the process of giving birth to a baby as the development of the foetus gets

completed in the mother's womb. The hormones involved in this process are oxytocin and relaxin.
Oxytocin leads to the contraction of smooth muscles of myometrium of the uterus, which directs
the full term foetus towards the birth canal. On the other hand, relaxin hormone causes relaxation
of the pelvic ligaments and prepares the uterus for childbirth.

Q19 In our society the women are often blamed for giving birth to daughters. Can you explain why
this is not correct?

Answer. All human beings have 23 pairs of chromosomes. Human males have 22 pairs of
autosomes and contain one or two types of sex chromosome. They are either X or Y. On the
contrary, human females have 22 pairs of autosomes and contain only the X sex chromosome. The
sex of an individual is determined by the type of the male gamete (X or Y), which fuses with the X
chromosome of the female. If the fertilizing sperm is X, then the baby will be a girl and if it is Y,
then the baby will be a boy. Hence, it is incorrect to blame a woman for the gender of the child.

Q20 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. An ovary releases an egg every month. When two babies are produced in succession, they
are called Wins. Generally, Wins are produced from a single egg by the separation of early
blastomeres resulting from the rst zygotic cleavage. As a result, the young ones formed will have
the same genetic make- up and are thus, called identical twins. If the twins born are fraternal,
then they would have developed from two separate eggs. This happens when two eggs (one from
each ovary) are released at the same time and get fertilized by two separate sperms. Hence, the
young ones developed will have separate genes and are therefore, called non-identical or fraternal
twins.

Q21 How many eggs do you think were released by the ovary of a female dog which gave birth to 6
puppies?

Answer. Dogs and rodents are polyovulatory species. In these species, more than one ovum is
released from the ovary at the time of ovulation. Hence, six eggs were released by the ovary of a
female dog to produce six puppies.