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