NCERT Solutions for Class 12 Biology Chapter 2: Human Reproduction

Conclusion: the answers follow from the basic facts that humans reproduce sexually, are viviparous with internal fertilisation, and form haploid gametes whose fusion restores the diploid number in the zygote.

The human male reproductive system lies in the pelvic region. It consists of a pair of testes (primary sex organs), accessory ducts, accessory glands and the external genitalia (penis).

Parts shown in the diagram and their role:

• Testes (in scrotum): produce sperms and the male hormone testosterone. The scrotum keeps them 2–2.5 °C below body temperature, which is needed for spermatogenesis. Each testis has about 250 testicular lobules, each with 1–3 highly coiled seminiferous tubules.
• Accessory ducts: rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra. They store and carry sperms from the testis to the outside.
• Accessory glands: paired seminal vesicles, a single prostate and paired bulbourethral (Cowper’s) glands; their secretions form the seminal plasma.
• Penis: the copulatory organ made of special erectile tissue; its enlarged tip, the glans penis, is covered by a loose fold of skin, the foreskin. The urethra runs through it and opens outside at the urethral meatus.

Conclusion: the male reproductive system is built around the testes (sperm + testosterone), a series of ducts that carry sperm, and glands that make the seminal plasma, all opening through the penis.

The female reproductive system consists of a pair of ovaries, a pair of oviducts (fallopian tubes), the uterus, cervix, vagina and the external genitalia, located in the pelvic region. Together with the mammary glands, they support ovulation, fertilisation, pregnancy, birth and child care.

Parts shown in the diagram and their role:

• Ovaries (2–4 cm each): primary female sex organs; produce the ovum and the hormones oestrogen and progesterone. Each is attached to the pelvic wall and uterus by ligaments.
• Fallopian tube (10–12 cm): has three parts — infundibulum (funnel near the ovary with finger-like fimbriae that collect the ovum), ampulla (wider middle part, site of fertilisation) and isthmus (narrow part that joins the uterus).
• Uterus (womb): single, inverted pear-shaped organ. Its wall has three layers: outer thin perimetrium, middle thick muscular myometrium (strong contractions at childbirth) and inner glandular endometrium (changes during the menstrual cycle; site of implantation).
• Cervix and cervical canal: the narrow lower part of the uterus that opens into the vagina; cervical canal + vagina form the birth canal.
• Vagina: receives sperms during coitus and acts as the birth canal.

Conclusion: the female system provides for gamete formation (ovaries), fertilisation (ampulla), development of the embryo (uterus) and delivery (birth canal), supported by the mammary glands.

The testes and ovaries are the primary sex organs (gonads). Each has a gametogenic (gamete-forming) and an endocrine (hormone-forming) function.

Conclusion: both gonads perform a gametogenic function (sperm/ovum) and an endocrine function (testosterone / oestrogen and progesterone).

Seminiferous tubules are the highly coiled tubules inside each testicular lobule (1–3 per lobule) in which sperms are produced. A transverse section shows the following structure:

1. Outer covering: each tubule is bounded by a thin basement membrane.
2. Lining of two types of cells:
   • Male germ cells (spermatogonia): lie next to the basement membrane. They divide and pass through the stages primary spermatocytes → secondary spermatocytes → spermatids → spermatozoa, arranged in layers from the periphery towards the lumen.
   • Sertoli cells: tall cells extending from the basement membrane to the lumen. They provide nourishment to the developing germ cells; the spermatids remain embedded in them until they mature.
3. Lumen: the central cavity into which mature sperms are released (spermiation), with their tails pointing inwards.
4. Interstitial spaces (outside the tubules): contain small blood vessels and interstitial or Leydig cells, which secrete androgens. Other immunologically competent cells are also present.

Conclusion: the seminiferous tubule combines germ cells at all stages, Sertoli cells for nourishment, and, just outside it, Leydig cells for androgen secretion — everything needed for sperm production.

Spermatogenesis is the process by which haploid sperms (spermatozoa) are formed from diploid male germ cells (spermatogonia) in the seminiferous tubules of the testis. It begins at puberty.

Stages of the process:

1. Multiplication phase: the spermatogonia (sperm mother cells) on the inner wall of the seminiferous tubules multiply by mitosis and increase in number. Each spermatogonium is diploid and has 46 chromosomes.
2. Growth phase: some spermatogonia grow and become primary spermatocytes (2n = 46), which periodically undergo meiosis.
3. Maturation phase (meiosis):
   • A primary spermatocyte completes the first meiotic division (reduction division) and forms two equal, haploid secondary spermatocytes, each with 23 chromosomes.
   • Each secondary spermatocyte undergoes the second meiotic division to produce two equal, haploid spermatids. Thus one primary spermatocyte gives four spermatids.
4. Spermiogenesis: the non-motile, round spermatids are transformed into motile spermatozoa (sperms) — the head, neck, middle piece and tail are formed.
5. Spermiation: the sperm heads, which were embedded in the Sertoli cells, are finally released into the lumen of the seminiferous tubule.

Hormonal control: it starts at puberty due to the rise in GnRH, which increases LH and FSH (details in Q7).

Conclusion: spermatogenesis converts one diploid spermatogonium into four haploid sperms through mitosis, meiosis, spermiogenesis and spermiation.

Spermatogenesis is controlled by a chain of hormones from the hypothalamus → anterior pituitary → testis:

1. Gonadotropin releasing hormone (GnRH): a hypothalamic hormone. Its secretion increases significantly at puberty, which starts spermatogenesis. It acts on the anterior pituitary.
2. Luteinising hormone (LH): a gonadotropin from the anterior pituitary. It acts on the Leydig (interstitial) cells and stimulates the synthesis and secretion of androgens.
3. Androgens (mainly testosterone): secreted by Leydig cells; they directly stimulate the process of spermatogenesis.
4. Follicle stimulating hormone (FSH): another gonadotropin; it acts on the Sertoli cells and stimulates the secretion of some factors which help in spermiogenesis.

Conclusion: the hypothalamus–pituitary–testis axis (GnRH → LH/FSH → Leydig and Sertoli cells) controls spermatogenesis.

• Spermiogenesis: the process in which the spermatids (round, non-motile, haploid cells) are transformed into spermatozoa (motile sperms). No cell division takes place; only the shape changes — the nucleus condenses to form the head, the Golgi body forms the acrosome, centrioles form the neck and the axial filament, and mitochondria gather in the middle piece; most of the cytoplasm is lost.
• Spermiation: the release of sperms from the seminiferous tubules. After spermiogenesis, the sperm heads remain embedded in the Sertoli cells; they are finally set free into the lumen of the tubule — this release is called spermiation.

Conclusion: spermiogenesis (spermatid → sperm) precedes spermiation (release of the sperm), so the two are consecutive final steps of sperm formation.

A human sperm is a microscopic, motile cell made of a head, neck, middle piece and tail, all enclosed by a plasma membrane.

• Head: contains an elongated haploid nucleus; its anterior part is covered by a cap-like acrosome filled with enzymes that help the sperm penetrate the ovum during fertilisation.
• Neck: a short region containing the centrioles.
• Middle piece: contains numerous mitochondria, which produce the energy (ATP) for the movement of the tail — hence it is the “powerhouse” of the sperm.
• Tail: long and whip-like; its beating gives the sperm the motility essential for fertilisation.

Conclusion: the sperm is a compact motile cell — acrosome to penetrate, nucleus to carry genes, mitochondria for energy and tail for movement.

Seminal plasma is the fluid part of semen. It is formed by the secretions of the male accessory glands — the paired seminal vesicles, the prostate and the paired bulbourethral glands. Seminal plasma + sperms = semen.

Major components:

1. Fructose — a sugar that is the main energy source for the sperms.
2. Calcium — needed for sperm activity and motility.
3. Certain enzymes — help in maturation and motility of sperms and in keeping semen fluid.
4. Mucus from the bulbourethral glands — helps in the lubrication of the penis.

Conclusion: seminal plasma nourishes and activates the sperms; its NCERT keywords are fructose, calcium and enzymes.

A. Male accessory ducts — rete testis, vasa efferentia, epididymis and vas deferens:

• They store and transport sperms from the testis to the outside through the urethra (seminiferous tubules → rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra).
• The epididymis stores sperms temporarily and its secretion helps in their maturation.
• The vas deferens carries sperms upward, loops over the urinary bladder and, with the duct of the seminal vesicle, forms the ejaculatory duct which opens into the urethra.
• The urethra carries both urine and semen to the outside through the urethral meatus.

B. Male accessory glands — seminal vesicles, prostate and bulbourethral glands:

• Their secretions form the seminal plasma, which is rich in fructose, calcium and certain enzymes.
• Seminal plasma provides a medium and nourishment for sperms and is essential for their maturation and motility.
• Secretion of the bulbourethral glands helps in the lubrication of the penis.

Note: the functions of these ducts and glands are maintained by the testicular hormones, the androgens.

Conclusion: the ducts store and transport sperm, while the glands supply the seminal plasma; both are maintained by androgens.

Oogenesis is the process of formation of a mature female gamete (ovum) in the ovary. It is markedly different from spermatogenesis because it starts before birth, is interrupted for years, and gives only one functional ovum from each mother cell.

1. Formation of oogonia (before birth): during embryonic development, a couple of million oogonia (gamete mother cells) are formed in each fetal ovary. No more oogonia are formed after birth.
2. Primary oocytes: the oogonia enter prophase I of meiosis and get temporarily arrested at that stage. These are called primary oocytes.
3. Primary follicle: each primary oocyte gets surrounded by a layer of granulosa cells. Many follicles degenerate from birth to puberty, so only 60,000–80,000 primary follicles are left in each ovary at puberty.
4. Secondary follicle: the primary follicle gets more layers of granulosa cells and a new theca.
5. Tertiary follicle: has a fluid-filled cavity, the antrum; the theca forms an inner theca interna and an outer theca externa. At this stage the primary oocyte completes meiosis I — an unequal division giving a large haploid secondary oocyte (which keeps most of the nutrient-rich cytoplasm) and a tiny first polar body.
6. Graafian follicle: the tertiary follicle becomes the mature Graafian follicle; the secondary oocyte forms a new membrane, the zona pellucida.
7. Ovulation: the Graafian follicle ruptures and releases the secondary oocyte (ovum) from the ovary.
8. Completion of meiosis II: happens only when a sperm enters the secondary oocyte; it forms the haploid ovum (ootid) and a second polar body.

Conclusion: oogenesis begins before birth, is arrested in prophase I, and gives only one functional ovum per mother cell through unequal divisions.

A section of the ovary shows an outer thin germinal epithelium enclosing the ovarian stroma, which has a peripheral cortex and an inner medulla. The cortex shows follicles at different stages of development.

Structures seen (in order of development):

1. Primary follicle — primary oocyte surrounded by a single layer of granulosa cells.
2. Secondary follicle — more layers of granulosa cells and a new theca.
3. Tertiary follicle — has a fluid-filled antrum; theca interna and theca externa.
4. Graafian follicle — the mature follicle containing the secondary oocyte.
5. Ovulation — rupture of the Graafian follicle releasing the ovum.
6. Corpus luteum — formed from the remains of the ruptured follicle; secretes progesterone. If there is no fertilisation it degenerates into the corpus albicans.

Conclusion: the ovary shows a developmental sequence of follicles, ovulation, and the corpus luteum, all embedded in the cortex.

The Graafian follicle is the mature ovarian follicle formed from the tertiary follicle. It ruptures at ovulation to release the secondary oocyte.

• Theca externa — outer fibrous layer of the follicle.
• Theca interna — inner vascular layer (secretes hormones).
• Granulosa cells (membrana granulosa) — many layers of follicle cells lining the antrum.
• Antrum — large cavity filled with follicular fluid.
• Cumulus oophorus — a mound of granulosa cells that holds the oocyte on one side of the antrum.
• Secondary oocyte — haploid cell with the first polar body, covered by the non-cellular zona pellucida and the cellular corona radiata.

Conclusion: the Graafian follicle is the mature follicle whose rupture releases the secondary oocyte at ovulation.

Conclusion: each structure has a specific role in reproduction, from hormone secretion (corpus luteum) to fertilisation (acrosome) and ovum pickup (fimbriae).

Conclusion: correcting the false statements reinforces that androgens come from Leydig cells (testis) and oogenesis occurs in the ovary, not the corpus luteum.

The menstrual cycle is the reproductive cycle of female primates (monkeys, apes and humans). In human females it is repeated at an average interval of about 28/29 days; the cycle of events from one menstruation to the next is called the menstrual cycle. The first menstruation at puberty is menarche, and the cycle stops at around 50 years of age (menopause). One ovum is released in the middle of each cycle.

Hormones regulating the cycle:

1. GnRH from the hypothalamus — stimulates the anterior pituitary.
2. FSH (follicle stimulating hormone) — pituitary; stimulates follicular development.
3. LH (luteinising hormone) — pituitary; its mid-cycle surge induces ovulation and formation of the corpus luteum.
4. Oestrogen — ovarian (from growing follicles); causes proliferation of the endometrium.
5. Progesterone — ovarian (from the corpus luteum); maintains the endometrium.

Conclusion: the menstrual cycle is driven by pituitary hormones (FSH, LH) and ovarian hormones (oestrogen, progesterone), with the LH surge causing ovulation at mid-cycle.

Parturition is the process of delivery of the foetus (childbirth) by vigorous contractions of the uterus at the end of pregnancy. In humans the average duration of pregnancy (gestation) is about 9 months.

Mechanism (neuroendocrine):

1. Parturition is induced by a complex neuroendocrine mechanism. The signals originate from the fully developed foetus and the placenta.
2. These signals cause mild uterine contractions called the foetal ejection reflex.
3. This reflex triggers the release of oxytocin from the maternal pituitary.
4. Oxytocin acts on the uterine muscle (myometrium) and causes stronger contractions, which in turn stimulate further secretion of oxytocin (a positive feedback loop). The contractions become stronger and stronger, leading to expulsion of the baby through the birth canal.
5. Soon after the infant is delivered, the placenta is also expelled out of the uterus.

Hormones involved: Oxytocin (the main hormone of parturition); relaxin, secreted during pregnancy, helps by relaxing the pelvic ligaments and softening the cervix; signals (hormones) from the foetus and placenta start the process.

Conclusion: parturition is triggered by foetal/placental signals and driven by oxytocin through a positive feedback loop.

Blaming women for the birth of daughters is scientifically wrong, because the sex of the baby is decided by the father’s sperm, not by the mother. The reason lies in the sex chromosomes:

1. A human female has XX sex chromosomes and a male has XY.
2. All the ova formed by a female carry only the X chromosome (22 + X).
3. A male produces two types of sperms in equal proportion: 50% carry X (22 + X) and 50% carry Y (22 + Y).
4. If an X-bearing sperm fertilises the ovum, the zygote is XX → girl; if a Y-bearing sperm fertilises it, the zygote is XY → boy.

Conclusion: the sex of the child is determined at fertilisation by the type of sperm that fuses with the ovum; in every pregnancy there is a 50% chance of a boy or a girl. The mother has no role in deciding it, so she should not be blamed.

Conclusion: the sex of the child depends on whether an X- or Y-bearing sperm fertilises the ovum, so it is determined by the father, never the mother.

1. Eggs per month: normally one egg (ovum) is released in each menstrual cycle, i.e. once a month — usually alternately from the left and right ovary.
2. Identical (monozygotic) twins: only one egg was released. It was fertilised by one sperm to form one zygote, which later split into two embryos at an early stage. Such twins have the same genes and are always of the same sex.
3. Fraternal (dizygotic) twins: yes, the answer changes — two eggs were released and each was fertilised by a separate sperm, forming two zygotes. Such twins are genetically different and may be of the same or different sex.

Conclusion: one egg gives identical twins (splitting of one zygote); two eggs give fraternal twins, so the number of eggs changes with the type of twins.

• Dogs are polyovulatory animals — several eggs are released in one breeding cycle, and each puppy normally develops from a separate fertilised egg.
• Therefore, for 6 puppies, at least 6 eggs must have been released by the ovaries and fertilised by 6 different sperms.
• (Only if some of the puppies were identical, formed by splitting of one zygote, could the number be less — but this is rare, so the expected answer is 6 eggs.)

Conclusion: dogs are polyovulatory, so at least six eggs must have been released and fertilised to produce six puppies.