Reproduction — Set 3

Correct Answer: B. Urethra

• **Urethra** = The urethra is the final common pathway in males that carries both urine (from the bladder) and semen (from the ejaculatory ducts); a sphincter mechanism prevents simultaneous passage of both fluids. • **Key fact** — The male urethra is about 20 cm long and has three sections — prostatic, membranous, and penile (spongy); the female urethra (4 cm) carries only urine. • 💡 Option A (Vas deferens) is wrong because the vas deferens carries only sperm from the epididymis to the ejaculatory duct, not urine; Option C (Epididymis) is wrong because the epididymis is a coiled tube attached to each testis where sperm mature and are stored; Option D (Ureter) is wrong because the ureter carries urine from the kidney to the bladder and is separate from the reproductive tract.

Correct Answer: D. Morula

• **Morula** = After fertilization, the zygote undergoes cleavage (repeated mitotic divisions without growth), forming a solid, mulberry-like ball of 16–32 cells called the morula, which occurs approximately 3–4 days after fertilization. • **Key fact** — The morula then develops into the blastula (blastocyst) as a fluid-filled cavity forms inside; it is at the blastocyst stage that implantation into the uterine wall occurs. • 💡 Option A (Fetus) is wrong because the fetus is the later developmental stage from about 8 weeks after fertilization; Option B (Gastrula) is wrong because the gastrula is a later stage characterised by the formation of three germ layers; Option C (Blastula) is wrong because the blastula (blastocyst) forms after the morula when a fluid cavity (blastocoel) develops inside the solid ball.

Correct Answer: C. Epididymis

• **Epididymis** = The epididymis is a highly coiled tube (about 6 metres long when uncoiled) attached to the back of each testis where immature sperm from the seminiferous tubules spend 2–3 weeks acquiring motility and fertilising ability. • **Key fact** — Sperm stored in the epididymis can remain viable for several weeks; during ejaculation, muscular contractions propel them through the vas deferens. • 💡 Option A (Scrotum) is wrong because the scrotum is the external pouch of skin that houses the testes and regulates their temperature but does not store or mature sperm; Option B (Testes) is wrong because the testes produce sperm in the seminiferous tubules but do not store mature sperm; Option D (Prostate) is wrong because the prostate adds secretions to semen but is not a sperm storage organ.

Correct Answer: C. Fertilization

• **Fertilization** = Fertilization is the process in which a haploid sperm cell fuses with a haploid egg cell to form a diploid zygote; in humans this occurs in the ampulla of the Fallopian tube and restores the full chromosome number of 46. • **Key fact** — Out of millions of sperm released, typically only one penetrates the egg; the acrosome reaction and zona pellucida binding are essential steps that precede actual membrane fusion. • 💡 Option A (Regeneration) is wrong because regeneration is the regrowth of lost or damaged body parts in certain organisms; Option B (Pollination) is wrong because pollination is the transfer of pollen between flowers in plants — not gamete fusion; Option D (Budding) is wrong because budding is asexual reproduction where a new organism grows from an outgrowth of the parent, as seen in yeast and Hydra.

Correct Answer: A. Prolactin

• **Prolactin** = Prolactin, secreted by the anterior pituitary gland, is the primary hormone that stimulates the mammary glands to synthesise and secrete milk (lactogenesis) after childbirth. • **Key fact** — During pregnancy, high estrogen and progesterone suppress prolactin's milk-producing effect; after delivery, their sharp drop allows prolactin to trigger milk production; each act of breastfeeding further boosts prolactin via a suckling reflex. • 💡 Option B (Estrogen) is wrong because estrogen promotes ductal growth in the breast during puberty and pregnancy but suppresses milk production during pregnancy; Option C (Oxytocin) is wrong because oxytocin causes milk ejection (let-down) from already-formed milk but does not produce the milk itself; Option D (Thyroxine) is wrong because thyroxine regulates the basal metabolic rate and has no direct role in milk synthesis.

Correct Answer: C. Maintaining temperature

• **Maintaining temperature** = The scrotum is the external pouch of skin that keeps the testes approximately 2–3°C below core body temperature, a prerequisite for normal spermatogenesis because sperm production is inhibited at normal body temperature (37°C). • **Key fact** — The cremaster muscle and dartos muscle in the scrotal wall contract or relax to move the testes closer to or farther from the body in response to temperature changes, acting as a thermoregulatory mechanism. • 💡 Option A (Producing testosterone) is wrong because testosterone is produced by the Leydig cells inside the testes, not by the scrotum; Option B (Protecting the bladder) is wrong because the bladder is protected within the pelvic cavity; the scrotum is located far below and is unrelated; Option D (Storing urine) is wrong because urine is stored in the bladder, which is a completely separate organ from the scrotum.

Correct Answer: B. Corpus luteum

• **Corpus luteum** = After a Graafian follicle ruptures during ovulation, the remaining follicular cells transform into the corpus luteum ('yellow body'), a temporary endocrine gland that secretes progesterone and estrogen to maintain the uterine lining. • **Key fact** — If fertilization occurs, hCG from the embryo keeps the corpus luteum active until the placenta takes over hormone production around weeks 8–10; if no fertilization occurs, the corpus luteum degenerates into the corpus albicans within ~14 days. • 💡 Option A (Placenta) is wrong because the placenta develops from embryonic and uterine tissues after implantation, not from the follicle itself; Option C (Zygote) is wrong because the zygote is formed in the Fallopian tube by sperm-egg fusion, not inside the ovary; Option D (Primary follicle) is wrong because a primary follicle is an early-stage follicle that develops before ovulation — it does not form after rupture.

Correct Answer: D. Parturition

• **Parturition** = Parturition is the scientific term for childbirth — the process of delivering the fully developed fetus from the uterus through uterine contractions, triggered by a surge of oxytocin and prostaglandins. • **Key fact** — Parturition occurs in three stages: dilation of the cervix, expulsion of the baby, and delivery of the placenta (afterbirth); the entire process is regulated by a hormonal positive feedback loop. • 💡 Option A (Lactation) is wrong because lactation refers to milk production and secretion by the mammary glands after birth; Option B (Insemination) is wrong because insemination is the introduction of semen into the female reproductive tract, the very first step, not the last; Option C (Gestation) is wrong because gestation is the entire period of pregnancy from conception to birth, not the act of delivery itself.

Correct Answer: B. One egg and one sperm

• **One egg and one sperm** = Identical (monozygotic) twins arise when a single fertilized egg (one egg + one sperm) splits into two separate embryos early in development, resulting in two genetically identical individuals with the same sex and DNA. • **Key fact** — Identical twins share ~100% of their genetic material, unlike fraternal (dizygotic) twins which form from two separate eggs fertilized by two separate sperm and share only ~50% of genes like any siblings. • 💡 Option A (Two different sperm) is wrong because two different sperm fertilizing one egg is biologically blocked by the zona reaction after first fertilization; Option C (One egg and two sperm) is wrong because polyspermy is prevented in normal fertilization; Option D (Two different eggs) is wrong because two different eggs fertilized by two sperm produce fraternal (non-identical) twins, not identical ones.

Correct Answer: A. 46

• **46** = A human zygote contains 46 chromosomes (23 pairs) — 23 contributed by the haploid sperm and 23 by the haploid egg — restoring the full diploid genome of the species after fertilization. • **Key fact** — Of the 46 chromosomes, 44 are autosomes (22 pairs) and 2 are sex chromosomes (XX in females, XY in males); the sex of the offspring is determined by whether the sperm carries an X or a Y chromosome. • 💡 Option B (23) is wrong because 23 is the haploid number found in gametes (sperm and egg), not the zygote; Option C (44) is wrong because 44 counts only the autosomes, omitting the 2 sex chromosomes; Option D (92) is wrong because 92 would represent a tetraploid state, which is abnormal and not found in a healthy human zygote.