Human Reproduction Class 12 NCERT Solutions - Download PDF

Concept used. Each blank tests one core fact about human sexual reproduction. Sexual reproduction means two parents make haploid gametes that fuse. A viviparous animal gives birth to live young that developed inside the mother's body. Internal fertilisation means the sperm meets the egg inside the female body. Haploid (n) means one set of chromosomes; diploid (2n) means two sets. We answer each blank from these definitions.

1. (a) sexually. Humans have two sexes. A male and a female each contribute a gamete, so reproduction is sexual, not asexual.
2. (b) viviparous. The young one develops inside the mother's uterus and is born alive, so humans are viviparous (not egg-laying like birds).
3. (c) internal. Sperm are deposited inside the female tract and the egg is fertilised inside the body, so fertilisation is internal.
4. (d) haploid. Gametes are formed by meiosis. A human sperm and a human ovum each carry 23 chromosomes, so both gametes are haploid (n).
5. (e) diploid. A haploid sperm (n = 23) fuses with a haploid ovum (n = 23). The zygote therefore has 23 + 23 = 46 chromosomes, so it is diploid (2n).
6. (f) ovulation. The release of the secondary oocyte (ovum) from a mature Graafian follicle is called ovulation.
7. (g) Luteinising Hormone (LH). A sharp rise in LH (the LH surge) from the anterior pituitary triggers ovulation.
8. (h) fertilisation (syngamy). The fusion of the male gamete (sperm) and the female gamete (ovum) is fertilisation, also called syngamy.
9. (i) the ampulla of the fallopian tube. Fertilisation occurs at the ampullary–isthmic junction of the oviduct, that is, in the ampulla region of the fallopian tube.
10. (j) blastocyst. The zygote divides (cleavage) into a morula and then forms a hollow blastocyst, which implants in the uterine wall.
11. (k) placenta. The placenta is the structure that links the foetal blood supply with the maternal blood supply (a vascular connection between foetus and uterus).

(a) sexually    (b) viviparous    (c) internal    (d) haploid    (e) diploid    (f) ovulation    (g) Luteinising Hormone (LH)    (h) fertilisation (syngamy)    (i) ampulla of the fallopian tube    (j) blastocyst    (k) placenta

Concept used. The male reproductive system is made of a pair of testes (where sperms and the male hormone are made), a set of accessory ducts (rete testis, vasa efferentia, epididymis, vas deferens, ejaculatory duct, urethra) that carry sperm out, accessory glands (seminal vesicles, prostate, bulbourethral glands) that add fluid, and the external genitalia (penis). We reproduce the two standard NCERT views: a sectional view of the male pelvis and a front view that opens up the testis to show inner ducts.

1. The testes sit outside the body in a pouch called the scrotum. The scrotum keeps the testes about 2--2.5^∘C below body temperature, which sperm formation needs.
2. Sperms made in the testis pass through the rete testis and vasa efferentia into the epididymis, where they mature and are stored.
3. From the epididymis the vas deferens carries sperm upward, loops over the urinary bladder, and joins the duct of the seminal vesicle to form the ejaculatory duct.
4. The ejaculatory duct opens into the urethra, the common passage for urine and semen, which runs through the penis and opens at the urethral meatus.
5. Accessory glands add secretions: the paired seminal vesicles, the single prostate, and the paired bulbourethral glands. Their fluids plus the sperms make up semen.

The labelled male reproductive system is shown above in Fig. 2.1(a) and Fig. 2.1(b): testis, scrotum, epididymis, vas deferens, seminal vesicle, prostate, bulbourethral gland, ejaculatory duct, urethra and penis.

Concept used. The female reproductive system has a pair of ovaries (make ova and female hormones), a pair of oviducts (fallopian tubes), the uterus (womb), the cervix, the vagina, and the external genitalia. The mammary glands are functionally part of the female reproductive system. We give the two standard NCERT views: a sectional view of the female pelvis and a front view of the system.

1. Each ovary lies in the lower abdomen, is about 2--4 cm long, and produces ova and the hormones oestrogen and progesterone.
2. The funnel-shaped infundibulum of the oviduct has finger-like fimbriae that collect the released ovum. The infundibulum leads to the wider ampulla, then the narrow isthmus, which opens into the uterus.
3. The uterus is a muscular, pear-shaped organ. Its wall has three layers: outer perimetrium, thick muscular myometrium, and inner glandular endometrium (it is the endometrium that changes during the menstrual cycle).
4. The uterus opens into the vagina through a narrow cervix; the cervical cavity is the cervical canal.
5. The external genitalia include the mons pubis, labia majora, labia minora, hymen and clitoris.

The labelled female reproductive system is shown above in Fig. 2.3(a) and Fig. 2.3(b): ovary, fallopian tube (infundibulum, ampulla, isthmus, fimbriae), uterus (endometrium, myometrium, perimetrium), cervix, cervical canal and vagina.

Concept used. A gonad has two jobs: a gametogenic job (making gametes) and an endocrine job (making sex hormones). The testis is the male gonad and the ovary is the female gonad. We state the two jobs for each.

1. Testis, function 1 (gametogenic). The seminiferous tubules of the testis produce male gametes, spermatozoa, by the process of spermatogenesis.
2. Testis, function 2 (endocrine). The Leydig (interstitial) cells of the testis secrete male sex hormones called androgens, mainly testosterone.
3. Ovary, function 1 (gametogenic). The ovary produces the female gamete, the ovum, by the process of oogenesis.
4. Ovary, function 2 (endocrine). The ovary secretes the female sex hormones oestrogen and progesterone, which control the menstrual cycle and support pregnancy.

Testis: (1) produces sperms (spermatogenesis); (2) secretes androgens (testosterone). Ovary: (1) produces ova (oogenesis); (2) secretes oestrogen and progesterone.

Concept used. A seminiferous tubule is the coiled tube inside each testicular lobule where sperms are made. To describe it we name its lining cells (germ cells and Sertoli cells) and the tissue around it (interstitial Leydig cells), and say what each does.

1. Each testicular lobule has 1--3 highly coiled seminiferous tubules. The inner lining of each tubule is made of two kinds of cells: male germ cells (spermatogonia) and Sertoli cells.
2. The spermatogonia lie near the wall of the tubule. They divide and mature into primary spermatocytes, secondary spermatocytes, spermatids and finally spermatozoa, moving toward the centre (lumen) of the tubule.
3. The Sertoli cells are tall cells that extend from the wall to the lumen. They provide nourishment and support to the developing germ cells (so they are also called nurse cells).
4. The regions outside the seminiferous tubules, between them, contain small blood vessels and Leydig cells (interstitial cells). The Leydig cells synthesise and secrete the testicular androgens.
5. A few other immunologically competent cells are also present in the interstitial spaces.

A seminiferous tubule is lined by spermatogonia (which form sperms) and supporting Sertoli (nurse) cells; the spaces around the tubules contain Leydig cells that secrete androgens.

Concept used. Spermatogenesis is the process by which immature male germ cells (spermatogonia) become mature spermatozoa in the testis. It involves mitosis (to multiply cells), meiosis (to halve the chromosome number from 2n to n) and a final change of shape called spermiogenesis.

1. Multiplication. At puberty, spermatogonia on the inside wall of the seminiferous tubules increase in number by mitosis. Each spermatogonium is diploid and has 46 chromosomes.
2. Growth into primary spermatocyte. Some spermatogonia, called primary spermatocytes, periodically undergo growth. A primary spermatocyte is still diploid (2n = 46).
3. First meiotic division. A primary spermatocyte completes the first meiotic (reduction) division to form two equal haploid secondary spermatocytes, each with 23 chromosomes (n).
4. Second meiotic division. Each secondary spermatocyte undergoes the second meiotic division to give four equal haploid spermatids (each n = 23).
5. Spermiogenesis. The spermatids are transformed into spermatozoa (sperms) by spermiogenesis. After this, sperm heads become embedded in Sertoli cells and are finally released into the lumen by spermiation.

Spermatogenesis is the formation of haploid spermatozoa from diploid spermatogonia: spermatogonium → primary spermatocyte → (meiosis I) → secondary spermatocytes → (meiosis II) → spermatids → (spermiogenesis) → spermatozoa.

Concept used. Spermatogenesis starts at puberty and is under the control of the hypothalamus–pituitary–testis axis. We name the hormones in the order they act along this axis.

1. GnRH. At puberty the hypothalamus releases increased amounts of Gonadotropin Releasing Hormone (GnRH).
2. LH and FSH. GnRH acts on the anterior pituitary and stimulates secretion of two gonadotropins: Luteinising Hormone (LH) and Follicle Stimulating Hormone (FSH).
3. Action of LH. LH acts on the Leydig cells and stimulates them to secrete androgens (mainly testosterone). Androgens in turn stimulate the process of spermatogenesis.
4. Action of FSH. FSH acts on the Sertoli cells and stimulates secretion of factors that help the process of spermiogenesis.

The hormones are GnRH (from the hypothalamus), LH and FSH (from the anterior pituitary), and androgens / testosterone (from the Leydig cells).

Concept used. These are two successive late events of spermatogenesis. Spermiogenesis is a change of shape; spermiation is a release. We define each separately.

1. Spermiogenesis. It is the process by which spermatids (round, non-motile cells) are transformed into spermatozoa (sperms) that have a head, neck, middle piece and tail. No cell division occurs here; only the shape and parts change.
2. Spermiation. After spermiogenesis the sperm heads remain embedded in the Sertoli cells for a while. Spermiation is the process by which these mature sperms are finally released from the Sertoli cells into the lumen of the seminiferous tubule.

Spermiogenesis: transformation of spermatids into spermatozoa (shape change). Spermiation: release of the mature spermatozoa from the Sertoli cells into the seminiferous tubule lumen.

Concept used. A sperm is a microscopic single cell made of a head, a neck, a middle piece and a tail, all wrapped by a plasma membrane. We draw the NCERT figure and label every part with its job.

1. The head contains an elongated haploid nucleus. Its front part is covered by a cap-like acrosome filled with enzymes that help the sperm penetrate the ovum at fertilisation.
2. A plasma membrane envelops the whole body of the sperm.
3. The middle piece has many mitochondria that produce the energy needed for the movement of the tail.
4. The tail is the longest part; its lashing movement gives the sperm its motility, which is essential for fertilisation.

The labelled sperm (Fig. 2.6) shows plasma membrane, acrosome, nucleus, head, neck, middle piece (with mitochondria) and tail.

Concept used. Semen = sperms + seminal plasma. The seminal plasma is the fluid part of semen, contributed by the male accessory glands (seminal vesicles, prostate, bulbourethral glands). We list what it is rich in and why.

1. Seminal plasma is rich in fructose, a sugar that provides energy to the sperms for their motility.
2. It contains calcium and certain enzymes (secreted mainly by the prostate and seminal vesicles).
3. It also contains other ions and secretions that nourish the sperms and keep the medium suitable for their movement and survival.

Seminal plasma is rich in fructose, calcium and certain enzymes (it is the secretion of the seminal vesicles, prostate and bulbourethral glands that nourishes and activates the sperms).

Concept used. The male system has accessory ducts (rete testis, vasa efferentia, epididymis, vas deferens) and accessory glands (paired seminal vesicles, a prostate, paired bulbourethral glands). We state the function of each group.

1. Function of accessory ducts. The accessory ducts store the sperms and transport them from the testis to the outside through the urethra. In the epididymis the sperms also mature and gain motility.
2. Function of accessory glands (energy). The seminal vesicles and prostate secrete a fluid rich in fructose, which nourishes the sperms.
3. Function of accessory glands (medium and motility). Their secretions also provide calcium and enzymes, activate the sperms and make a fluid medium that is essential for the easy transport and motility of sperms.

Accessory ducts store, mature and transport sperms; accessory glands secrete a fructose-, calcium- and enzyme-rich fluid that nourishes, activates and provides a transport medium for the sperms.

Concept used. Oogenesis is the process of formation of a mature female gamete (ovum) from the female germ cells in the ovary. Like spermatogenesis it uses mitosis and meiosis, but it begins in the embryo, pauses for years, and produces only one ovum per cycle.

1. Starts in foetal life. Oogenesis is initiated during the embryonic (foetal) stage. Gamete mother cells, the oogonia, are formed in the foetal ovary; no more oogonia are formed after birth.
2. Primary oocyte, arrested. Oogonia enter prophase I of the first meiotic division and get arrested at that stage as primary oocytes. Each primary oocyte gets surrounded by a layer of granulosa cells, forming the primary follicle.
3. Follicle development. The primary follicle becomes a secondary follicle and then a tertiary follicle, which has a fluid-filled cavity called the antrum. The primary oocyte in the tertiary follicle completes meiosis I.
4. Unequal division. Meiosis I gives a large haploid secondary oocyte and a tiny first polar body; the secondary oocyte keeps the bulk of the nutrient-rich cytoplasm.
5. Graafian follicle and ovulation. The tertiary follicle becomes the mature Graafian follicle, which ruptures to release the secondary oocyte (ovulation). Meiosis II of the secondary oocyte completes only if a sperm enters.

Oogenesis is the formation of a haploid ovum from oogonia in the ovary: oogonia → primary oocyte (arrested in meiosis I) → secondary oocyte + first polar body → ovum (meiosis II completed at fertilisation).

Concept used. A section through the ovary shows follicles at different stages of growth, a corpus luteum, blood vessels and the secondary oocyte being released. We embed and label the NCERT sectional view.

1. The primary follicle is an early follicle, a primary oocyte surrounded by a layer of granulosa cells.
2. The tertiary follicle has a fluid-filled cavity, the antrum; it matures into the Graafian follicle.
3. At ovulation the Graafian follicle ruptures and releases the secondary oocyte.
4. The empty ruptured follicle becomes the corpus luteum, which secretes large amounts of progesterone.
5. Blood vessels supply the ovary and support follicle growth and hormone secretion.

The labelled ovary section (Fig. 2.7) shows blood vessels, primary follicle, tertiary follicle (with antrum), Graafian follicle, secondary oocyte and corpus luteum.

Concept used. The Graafian follicle is the mature ovarian follicle just before ovulation. It contains the secondary oocyte surrounded by follicular layers and a fluid-filled antrum. The NCERT chapter shows it inside the sectional view of the ovary (Fig. 2.7); we use that and label its parts.

1. At the centre lies the secondary oocyte (the cell that will be released as the ovum).
2. The oocyte is surrounded by a new membrane called the zona pellucida, around which lie the granulosa (follicular) cells.
3. A large fluid-filled cavity, the antrum, develops within the follicle.
4. The whole follicle is enclosed by layers of theca cells. At maturity this Graafian follicle bursts to release the secondary oocyte, the event called ovulation.

The Graafian follicle (mature follicle in Fig. 2.7) shows the secondary oocyte, zona pellucida, granulosa cells, the fluid-filled antrum and the theca layers; it ruptures at ovulation.

Concept used. Each of these is a named reproductive structure; the question asks only its function. We give one clear function per structure, naming the hormone or process involved.

1. (a) Corpus luteum. The corpus luteum secretes large amounts of progesterone, which is essential for maintaining the endometrium and supporting pregnancy.
2. (b) Endometrium. The endometrium is the inner lining of the uterus; it undergoes cyclic changes and is the site of implantation of the blastocyst and is essential for pregnancy.
3. (c) Acrosome. The acrosome is the cap over the sperm head; it is filled with enzymes that help the sperm penetrate and fertilise the ovum.
4. (d) Sperm tail. The tail provides motility: its lashing movement propels the sperm so it can reach the ovum (this motility is essential for fertilisation).
5. (e) Fimbriae. The fimbriae are finger-like projections of the infundibulum; they help in collection of the ovum after ovulation.

(a) secretes progesterone (maintains endometrium/pregnancy); (b) site of implantation and supports pregnancy; (c) enzymes for sperm to penetrate the ovum; (d) motility to reach the ovum; (e) collect the released ovum.

Concept used. We test each statement against a single known fact and, if it is false, replace only the wrong word so the statement becomes true. Key facts: Leydig cells (not Sertoli cells) make androgens; Sertoli cells nourish germ cells; Leydig cells are in the testis; oogenesis occurs in the ovary; ovarian cycling stops during pregnancy; the hymen is not a virginity indicator.

1. (a) False. Androgens are produced by the Leydig cells, not Sertoli cells. Corrected: ``Androgens are produced by Leydig cells.''
2. (b) True. Sertoli cells (nurse cells) provide nutrition to the developing spermatozoa.
3. (c) False. Leydig cells are found in the testis, not the ovary. Corrected: ``Leydig cells are found in the testis.''
4. (d) True. Leydig cells synthesise and secrete the testicular androgens.
5. (e) False. Oogenesis takes place in the ovary (in the germ cells/follicles), not in the corpus luteum. Corrected: ``Oogenesis takes place in the ovary.''
6. (f) True. During pregnancy the menstrual cycle ceases, because high progesterone prevents new cycles.
7. (g) True. The presence or absence of the hymen is not a reliable indicator of virginity or sexual experience.

(a) False (Leydig cells); (b) True; (c) False (testis); (d) True; (e) False (ovary); (f) True; (g) True.

Concept used. The menstrual cycle is the cyclic change in the reproductive tract of primate females, repeating about every 28 days. It is controlled by the same hypothalamus–pituitary–ovary axis: pituitary hormones (FSH, LH) and ovarian hormones (oestrogen, progesterone).

1. Definition. The menstrual cycle is the monthly cycle of changes (in the ovary and uterine endometrium) in human females from puberty (menarche) up to menopause; one cycle is the period from one menstruation to the next, about 28 days.
2. Phases. It has the menstrual phase (breakdown of the endometrium, bleeding for 3--5 days), the follicular (proliferative) phase, ovulation (around day 14), and the luteal (secretory) phase.
3. Pituitary hormones. FSH stimulates follicle growth; a mid-cycle surge of LH (the LH surge) induces ovulation.
4. Ovarian hormones. The growing follicle secretes oestrogen; after ovulation the corpus luteum secretes progesterone, which maintains the endometrium.
5. Falling oestrogen and progesterone (if no pregnancy) cause the endometrium to break down, starting the next cycle.

The menstrual cycle is the ∼ 28-day cyclic change in the female reproductive tract; it is regulated by the pituitary hormones FSH and LH and the ovarian hormones oestrogen and progesterone (under hypothalamic GnRH).

Concept used. Parturition is the process of delivery of the foetus, that is, childbirth, at the end of pregnancy. It is induced by a complex neuroendocrine mechanism involving signals from the fully developed foetus and the placenta.

1. Definition. Parturition is the expulsion of the fully developed foetus from the mother's uterus at the end of gestation, brought about by vigorous contractions of the uterus.
2. Trigger. Signals from the fully developed foetus and the placenta produce mild uterine contractions called the foetal ejection reflex.
3. Oxytocin. This reflex triggers release of oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscle and causes stronger uterine contractions.
4. Positive feedback. Stronger contractions stimulate more oxytocin secretion, and the cycle of contraction and oxytocin release continues (positive feedback) until the baby is delivered, followed by expulsion of the placenta. Relaxin also assists by relaxing the pelvic ligaments.

Parturition is childbirth, the expulsion of the foetus from the uterus at the end of pregnancy; it is mainly induced by oxytocin (with relaxin assisting), driven by a foetus-and-placenta signal in a positive-feedback loop.

Concept used. The sex of a child is decided by the sex chromosomes. Human females are XX and males are XY. The mother can give only an X; the father gives either X or Y. So it is the father's sperm, not the mother, that determines the child's sex.

1. All eggs (ova) made by the mother carry one X chromosome, because the mother is XX. The mother can never pass a Y.
2. The father is XY, so he makes two kinds of sperm: half carry an X chromosome and half carry a Y chromosome.
3. If an X-bearing sperm fertilises the egg, the zygote is XX, a girl. If a Y-bearing sperm fertilises the egg, the zygote is XY, a boy. X_ovum + X_sperm → XX (girl); X_ovum + Y_sperm → XY (boy).
4. Therefore the sex of the baby depends entirely on which sperm (X or Y) of the father fertilises the egg. The mother has no role in deciding it, so blaming the woman for the child's sex is scientifically wrong and unjust.

The mother contributes only an X chromosome; the father's sperm (X or Y) decides the child's sex. So the sex of the child is determined by the father, not the mother, and blaming women for the birth of daughters is biologically incorrect.

Concept used. A human female is normally uniparous: one secondary oocyte (egg) is released per menstrual cycle (per month). Identical (monozygotic) twins come from one fertilised egg that splits; fraternal (dizygotic) twins come from two eggs fertilised by two sperms.

1. Eggs per month. In humans, generally only one ovum is released by one of the two ovaries in one menstrual cycle, so one egg is released per month.
2. Identical twins. Identical twins develop from a single fertilised egg (zygote) that divides into two embryos. So even for identical twins, only one egg was released.
3. Fraternal twins. Fraternal twins develop from two different eggs, each fertilised by a different sperm. So if the twins were fraternal, two eggs must have been released. Hence the answer does change: one egg for identical, two eggs for fraternal.

One egg is released per month. For identical twins, still only one egg (it splits after fertilisation). For fraternal twins the answer changes to two eggs (two eggs fertilised by two sperms).

Concept used. A dog is a polyovulatory (multiparous) animal: unlike humans, its ovaries release many eggs in one cycle, so it can produce a litter. Each puppy normally develops from its own fertilised egg.

1. In multiparous animals such as the dog, more than one ovum is released from the ovary at the time of ovulation.
2. Each puppy in the litter develops from a separate egg fertilised by a separate sperm.
3. Since the female dog gave birth to 6 puppies, at least 6 eggs must have been released by its ovary (assuming one puppy per fertilised egg).

At least 6 eggs were released, because the dog is a polyovulatory animal and each of the 6 puppies develops from a separate fertilised egg.