RowQ
The Vault
RowQ
The Vault
CBSE Class 12 Biology · 11 questions · 27 marks
This chapter follows the human reproductive process as a sequence of well-timed biological events, from the formation of gametes deep inside the gonads to the birth of a fully formed infant. You will study the anatomy of the male and female reproductive systems, the hormonal control of the menstrual cycle, fertilisation and implantation, and the role of the placenta. Learning the hormone names alongside the events they trigger is the fastest way to master this chapter.
Which cells of the seminiferous tubules provide nutrition and structural support to the developing male gametes?
Answer
Sertoli cells are correct — they lie within the seminiferous tubules and nourish and anchor the developing germ cells throughout spermatogenesis. Leydig cells lie outside the tubules in the interstitial spaces and secrete testosterone, while spermatogonia and primary spermatocytes are the germ cells themselves, not supporting cells.
In a normal 28-day menstrual cycle, ovulation is directly triggered by:
Answer
A surge of luteinising hormone is correct — rising oestrogen from the mature Graafian follicle triggers a sharp LH peak around the middle of the cycle, and this surge causes the follicle to rupture and release the secondary oocyte. Progesterone rises only after ovulation from the corpus luteum, FSH does not fall to zero, and hCG appears only if implantation occurs.
In the human female, fertilisation of the ovum normally takes place in the:
Answer
The ampullary-isthmic junction of the fallopian tube is correct — the ovum released at ovulation is swept into the tube, and sperm swimming up meet it at this junction, so fertilisation happens there and not in the uterus. The zygote only reaches the uterine cavity days later, as a blastocyst ready to implant.
How many functional ova are ultimately produced from one primary oocyte that completes both meiotic divisions?
Answer
One is correct — oogenesis is unequal: meiosis I gives one large secondary oocyte and a tiny first polar body, and meiosis II gives one ovum and a second polar body. The polar bodies receive almost no cytoplasm and degenerate, so a single primary oocyte yields only one functional ovum, unlike spermatogenesis, where one primary spermatocyte yields four sperm.
Assertion (A): In humans the testes are located outside the abdominal cavity in the scrotum. Reason (R): Normal spermatogenesis requires a temperature slightly lower than the core body temperature.
Answer
Both A and R are true and R is the correct explanation of A — the scrotum acts as a thermoregulatory sac that keeps testicular temperature roughly 2 to 2.5 °C below core body temperature, which is the condition required for normal sperm production. This is also why undescended testes are associated with failure of spermatogenesis.
State the function of the acrosome of a human sperm and explain what the cortical reaction achieves immediately after a sperm makes contact with the ovum.
Answer
The acrosome is a cap-like structure over the sperm head filled with hydrolytic enzymes. On contact with the zona pellucida of the ovum, these enzymes are released in the acrosomal reaction and digest a path through the egg's outer coverings, allowing the sperm to reach and fuse with the plasma membrane of the ovum. The cortical reaction occurs immediately afterwards: granules just beneath the egg's plasma membrane release their contents, which alter the zona pellucida so that no further sperm can bind or penetrate. This blocks polyspermy and ensures the zygote receives exactly one set of paternal chromosomes, keeping the chromosome number diploid.
Describe the structure of the human placenta and list any three of its functions.
Answer
After implantation, finger-like projections called chorionic villi grow out from the trophoblast of the embryo and interdigitate with the tissue of the uterine endometrium. This intimate combination of foetal and maternal tissue forms the placenta, a disc-shaped organ connected to the foetus by the umbilical cord. Its functions include: supplying oxygen and nutrients such as glucose and amino acids from the mother's blood to the developing foetus; removing carbon dioxide and nitrogenous waste from foetal blood into the maternal circulation for excretion; and acting as an endocrine gland, secreting hormones such as human chorionic gonadotropin, human placental lactogen, oestrogens, and progestogens that maintain the pregnancy. It also acts as a partial barrier, preventing direct mixing of maternal and foetal blood.
Explain what happens to the corpus luteum and the endometrium in the second half of a cycle in which fertilisation does not occur.
Answer
After ovulation, the remains of the ruptured Graafian follicle become the corpus luteum, which secretes large amounts of progesterone. Progesterone maintains and further thickens the endometrium, keeping it richly supplied with blood vessels and glands so that it is ready to receive an implanting embryo. If fertilisation and implantation do not occur, no hCG is produced to sustain it, so the corpus luteum degenerates into a scar called the corpus albicans. Progesterone and oestrogen levels then fall sharply. Without hormonal support the thickened endometrial lining cannot be maintained; its blood vessels constrict and the lining breaks down and is shed along with blood as menstrual flow, marking day 1 of the next cycle.
Compare spermatogenesis and oogenesis in humans under the headings site, timing, number of gametes formed, cytoplasmic distribution, and completion of meiosis.
Answer
Site: Spermatogenesis takes place in the seminiferous tubules of the testes, with Sertoli cells providing nourishment. Oogenesis takes place in the ovaries, within follicles whose granulosa cells nourish the oocyte. Timing: Spermatogenesis begins only at puberty under the influence of GnRH, FSH, and LH, and then continues without interruption throughout adult life. Oogenesis begins in the foetal ovary, where all primary oocytes are formed before birth and then arrested in prophase I; the process resumes only from puberty onwards, one follicle at a time per cycle, and ceases at menopause. Number of gametes: One primary spermatocyte produces four functional spermatozoa. One primary oocyte produces only one functional ovum, along with polar bodies that degenerate. Cytoplasmic distribution: The meiotic divisions in spermatogenesis are equal, so all four products receive similar cytoplasm and the mature sperm is small and highly motile. In oogenesis the divisions are markedly unequal, so nearly all the cytoplasm and stored nutrients are retained by the ovum, which is large and non-motile, while the polar bodies get almost none. Completion of meiosis: In the male, both meiotic divisions are completed continuously within the tubule before spermiogenesis. In the female, meiosis I is completed only just before ovulation, and meiosis II is completed only after a sperm penetrates the secondary oocyte, so an unfertilised secondary oocyte never finishes meiosis.
Trace the sequence of events in a human female from ovulation to implantation, naming the structures formed at each stage.
Answer
Ovulation: The LH surge causes the mature Graafian follicle to rupture and release a secondary oocyte, arrested in metaphase II, into the abdominal cavity, from where the fimbriae sweep it into the fallopian tube. The remaining follicle becomes the corpus luteum and starts secreting progesterone. Fertilisation: Sperm deposited in the female tract swim up and reach the oocyte at the ampullary-isthmic junction. The acrosomal reaction allows one sperm to penetrate the zona pellucida and fuse with the oocyte membrane; the cortical reaction then blocks any further sperm. Entry of the sperm stimulates the oocyte to complete meiosis II, releasing the second polar body and leaving a haploid ovum whose nucleus fuses with the sperm nucleus to form the diploid zygote. Cleavage: As it is moved along the tube by ciliary action, the zygote undergoes repeated mitotic divisions without growth, forming 2, 4, 8, and 16 celled stages; the solid 8 to 16 celled ball is the morula. Blastocyst formation: The morula continues dividing and develops a fluid-filled cavity, becoming a blastocyst. Its cells are now arranged as an outer layer, the trophoblast, and an inner cell mass attached to one side, which will form the embryo proper. Implantation: The blastocyst reaches the uterus, sheds the zona pellucida, and its trophoblast attaches to the progesterone-primed endometrium. The uterine tissue grows over and encloses the blastocyst, embedding it in the uterine wall — this is implantation, and it marks the establishment of pregnancy. The trophoblast then forms chorionic villi that develop into the placenta.
A 29-year-old woman with regular 28-day cycles visits a clinic. Her last period began on 1 September. On 26 September she takes a home test that detects human chorionic gonadotropin in urine, and the result is positive. An ultrasound two weeks later confirms an implanted embryo in the uterine wall. (a) On roughly which date would she have ovulated, and from which structure was the oocyte released? (b) Which structure secretes the hCG detected by the test? (c) Explain why her period did not begin around 29 September. (d) Name the stage of the embryo that implants in the uterine wall and the layer of it that makes contact with the endometrium.
Answer
(a) In a regular 28-day cycle, ovulation occurs around the middle of the cycle, roughly 14 days after the start of the last period — so about 15 September. The secondary oocyte is released from a mature Graafian follicle in the ovary. (b) hCG is secreted by the trophoblast cells of the implanted embryo, which go on to form the chorion and placenta. Its presence in urine is therefore a reliable early indicator of pregnancy. (c) Menstruation happens when the corpus luteum degenerates and progesterone falls, so the endometrium can no longer be maintained. Here, hCG from the embryo signals the corpus luteum to persist and keep secreting progesterone. The endometrium is therefore maintained rather than shed, and the expected period does not occur — this missed period is a first sign of pregnancy. (d) The stage that implants is the blastocyst. Its outer layer, the trophoblast, makes contact with and attaches to the uterine endometrium, while the inner cell mass gives rise to the embryo itself.
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