Class 12 Biology: Reproduction - Important Exam Questions

Very Short Answer Type Questions (1 Mark Each)

Q1: What is the primary function of the seminal vesicles? A1: To secrete seminal plasma, which is rich in fructose, prostaglandins, and clotting factors.

Q2: Name the cells that provide nutrition to the developing spermatozoa. A2: Sertoli cells (or nurse cells).

Q3: What is the ploidy level of a zygote? A3: Diploid (2n).

Q4: Give an example of a dioecious plant. A4: Carica papaya (Papaya) or Phoenix dactylifera (Date palm).

Q5: What is parthenogenesis? A5: The development of an embryo directly from an unfertilized egg.

Q6: Where does fertilization typically occur in humans? A6: Ampulla of the fallopian tube (oviduct).

Q7: Name the hormone responsible for the maturation of Graafian follicles. A7: Follicle-Stimulating Hormone (FSH).

Q8: What is the function of the acrosome in a sperm? A8: It contains hydrolytic enzymes that help the sperm penetrate the egg during fertilization.

Q9: Define implantation. A9: The embedding of the blastocyst into the endometrium of the uterus.

Q10: Which part of the flower develops into a fruit after fertilization? A10: Ovary.

Short Answer Type Questions (2-3 Marks Each)

Q1: Differentiate between asexual and sexual reproduction.

A1:

Asexual Reproduction:

Involves a single parent, no gamete fusion, offspring are genetically identical to the parent (clones), faster process, e.g., budding in

Hydra

.

Sexual Reproduction:

Involves two parents (usually), involves gamete fusion, offspring show genetic variation, slower process, e.g., reproduction in humans.

Q2: Explain the role of the placenta in human development.

A2: The placenta is a temporary organ that connects the mother and the developing fetus. Its roles include: * Nutrient and Oxygen Supply: Facilitates the transport of nutrients and oxygen from maternal blood to fetal blood. * Waste Removal: Removes metabolic wastes (e.g., urea, carbon dioxide) from fetal blood to maternal blood. * Hormone Production: Produces hormones like hCG, hPL, estrogen, and progesterone essential for maintaining pregnancy.

Q3: What is meant by external fertilization? Give two examples.

A3: External fertilization is a type of fertilization where the fusion of male and female gametes occurs outside the body of the female parent, usually in an aquatic medium. * Examples: Most aquatic organisms like fishes and amphibians (e.g., frogs).

Q4: List the major events of the menstrual cycle.

A4: The menstrual cycle comprises four main phases: 1. Menstrual Phase (Day 1-5): Breakdown of the endometrial lining, leading to bleeding. 2. Follicular/Proliferative Phase (Day 5-13): Development of primary follicles into Graafian follicles, endometrium regenerates. 3. Ovulatory Phase (Day 14): Rupture of the Graafian follicle and release of the ovum (ovulation) due to LH surge. 4. Luteal/Secretory Phase (Day 15-28): Formation of corpus luteum from the ruptured follicle, endometrium thickens further and becomes secretory.

Q5: Describe the structure of a mature embryo sac.

A5: A mature embryo sac (female gametophyte) is typically 7-celled and 8-nucleate. It contains: * Egg Apparatus: Consists of one egg cell and two synergids at the micropylar end. The synergids have filiform apparatus. * Central Cell: A large cell in the center with two polar nuclei (which fuse to form a diploid secondary nucleus). * Antipodal Cells: Three cells located at the chalazal end.

Long Answer Type Questions (5 Marks Each)

Q1: With the help of a well-labelled diagram, describe the structure of a human sperm. Explain its significance in reproduction.

A1: (Diagram of a human sperm with labels: Head, Neck, Middle Piece, Tail, Acrosome, Nucleus, Mitochondria, Axil Filament)

Structure of Human Sperm: A human sperm is a microscopic structure composed of four main parts:

Head: Contains a haploid nucleus, which carries the genetic material (chromosomes). Anterior to the nucleus is the acrosome, a cap-like structure derived from the Golgi apparatus. The acrosome contains hydrolytic enzymes (hyaluronidase, acrosin) crucial for penetrating the egg's protective layers.

Neck: A very short region connecting the head and the middle piece. It contains proximal and distal centrioles, which play a role in the first cleavage of the zygote.

Middle Piece: Contains numerous mitochondria spirally arranged around an axial filament. These mitochondria provide the energy (ATP) for the movement of the sperm tail.

Tail (Flagellum): The longest part, a slender, tapering structure containing the axial filament. Its whip-like movements propel the sperm through the female reproductive tract towards the egg.

Significance in Reproduction:

Genetic Contribution:

The sperm carries the paternal genetic material (haploid set of chromosomes) to the egg, ensuring the formation of a diploid zygote and genetic diversity.

Motility: The tail provides motility, enabling the sperm to reach and fertilize the immotile egg.

Penetration of Egg: The acrosome's enzymes are essential for breaking down the egg's zona pellucida and corona radiata, allowing the sperm to fuse with the egg membrane.

Initiation of Development: Upon successful fertilization, the sperm's entry triggers changes in the egg that prevent polyspermy and initiate embryonic development.

Q2: Describe the process of megasporogenesis and the development of the female gametophyte in angiosperms.

A2: Megasporogenesis: Megasporogenesis is the process of formation of megaspores from the megaspore mother cell (MMC).

In the ovule, a single cell in the nucellus differentiates into a large megaspore mother cell (MMC). The MMC is diploid (2n).

 

The MMC undergoes meiosis I, forming two haploid (n) cells.

 

These two cells then undergo meiosis II, resulting in a linear tetrad of four haploid megaspores.

 

In most angiosperms (monosporic development, e.g., Polygonum type), three of these four megaspores degenerate, and only one functional megaspore remains, usually the one at the chalazal end.

Development of Female Gametophyte (Embryo Sac): The single functional megaspore enlarges and develops into the female gametophyte, also known as the embryo sac. This development involves three successive mitotic divisions of the megaspore nucleus:

The nucleus of the functional megaspore undergoes its first mitotic division to form two nuclei, which move to opposite poles, forming a 2-nucleate embryo sac.

These two nuclei undergo a second mitotic division, resulting in four nuclei, two at each pole.

A third mitotic division occurs, forming an 8-nucleate stage, with four nuclei at each pole.

Subsequently, three of these nuclei at the micropylar end form the egg apparatus (one large egg cell and two synergids). The synergids have special cellular thickenings called the filiform apparatus, which guides the pollen tube.

Three nuclei at the chalazal end organize into three antipodal cells.

The remaining two nuclei, one from each pole, move to the center of the embryo sac and are called polar nuclei. They fuse to form a large diploid central cell (or secondary nucleus).

Thus, a typical mature angiosperm embryo sac is 7-celled (3 antipodal cells + 2 synergids + 1 egg cell + 1 central cell) and 8-nucleate.

Q3: Explain the hormonal control of the male reproductive system in humans.

A3: The male reproductive system is regulated by hormones secreted by the hypothalamus, pituitary gland, and testes.

Hypothalamus: Secretes Gonadotropin-Releasing Hormone (GnRH). GnRH acts on the anterior pituitary gland.

Anterior Pituitary Gland: In response to GnRH, the anterior pituitary secretes two gonadotropins:

Luteinizing Hormone (LH) / Interstitial Cell Stimulating Hormone (ICSH): LH acts on the Leydig cells (interstitial cells) present in the interstitial spaces of the seminiferous tubules. It stimulates Leydig cells to synthesize and secrete androgens, primarily testosterone.

Follicle-Stimulating Hormone (FSH): FSH acts on the Sertoli cells (sustentacular cells) of the seminiferous tubules. It stimulates Sertoli cells to secrete factors (e.g., Androgen Binding Protein - ABP) that help concentrate testosterone within the seminiferous tubules, and also aids in spermiogenesis (the transformation of spermatids into spermatozoa). Sertoli cells also secrete inhibin, which selectively inhibits FSH secretion from the anterior pituitary.

Testes (Leydig Cells and Sertoli Cells):

Testosterone (Androgen): Secreted by Leydig cells under LH stimulation. Testosterone is crucial for:

Initiation and maintenance of spermatogenesis.

Development and maintenance of male secondary sexual characteristics (e.g., facial hair, deepened voice, muscle mass).

Development and maturation of accessory sex organs (epididymis, vas deferens, seminal vesicles, prostate gland).

Exerts negative feedback on the hypothalamus (inhibiting GnRH release) and anterior pituitary (inhibiting LH release).

Inhibin: Secreted by Sertoli cells. It primarily inhibits the secretion of FSH from the anterior pituitary, providing a negative feedback loop to regulate spermatogenesis.

 

This intricate hormonal interplay ensures the proper functioning of the male reproductive system, leading to continuous sperm production and maintenance of male sexual characteristics.