Is it correct to say: Saying oogenesis produces four ova from one primary oocyte, just like spermatogenesis?

One primary oocyte gives only one ovum. Both meiotic divisions are unequal, and the other nuclei end up in polar bodies.

Is it correct to say: Writing that secondary spermatocytes are diploid?

Meiosis I is the reduction division. Secondary spermatocytes are already haploid, with 23 chromosomes each.

Is it correct to say: Swapping the target cells of LH and FSH in the male?

LH acts on Leydig cells to make androgens. FSH acts on Sertoli cells, which secrete factors that help spermiogenesis.

Is it correct to say: Saying the ovary releases a mature ovum that has finished meiosis?

Ovulation releases the secondary oocyte. Meiosis II is completed only when a sperm enters it.

Is it correct to say: Mixing up spermiogenesis and spermiation?

Spermiogenesis turns spermatids into sperms. Spermiation is the release of sperms from the seminiferous tubules.

Is it correct to say: Saying menstruation is caused by the LH surge?

The LH surge causes ovulation. Menstruation follows when the corpus luteum degenerates without fertilisation, so progesterone falls and the endometrium breaks down.

📄 Formula sheet (PDF)

Human Reproduction

Male and female reproductive systems, spermatogenesis and oogenesis, the menstrual cycle and its hormones, fertilisation, implantation, pregnancy and parturition — NCERT Class 12 Biology Ch 2

📐 18 formulas✏️ 4 examples🎯 6 practice⚖️ Unit VI Reproduction: 16 marks (with Ch 1 and 3)🏫 CBSE📚 Class 12✓ 2025–26 syllabus

💡

Board Exam Tips

→Draw spermatogenesis and oogenesis as side-by-side flow charts and write the chromosome number (2n = 46 or n = 23) at every stage. Comparing the two is a standard long-answer question.
→For the menstrual cycle, tie each phase to its days and its hormone: menstrual (3–5 days), follicular (FSH, LH and estrogen rising), ovulation (LH surge, about day 14), luteal (progesterone from the corpus luteum).
→Know where meiosis pauses in the female. The primary oocyte is arrested in prophase I from before birth, and meiosis II finishes only when a sperm enters the secondary oocyte.
→LH acts on Leydig cells (androgens); FSH acts on Sertoli cells (factors for spermiogenesis). Swapping the two is a frequent slip.
→Practise the labelled diagrams NCERT gives: sectional view of a seminiferous tubule, structure of a sperm, sectional view of the ovary, and the blastocyst.

📐 Formulas(18)

Position of Testes

T_{testes} \approx T_{body} - (2 to 2.5)^{\circ} C

Symbol	Meaning
$T_{testes}$	Temperature of the testes (°C)
$T_{body}$	Normal internal body temperature (°C)

Organisation of a Testis

1 testis \approx 250 testicular lobules; 1 lobule = 1 to 3 seminiferous tubules

Spermatogenesis Sequence★ Board fav

Primary spermatocyte (2 n = 46) meiosis I 2 secondary spermatocytes (n = 23) meiosis II 4 spermatids (n = 23)

Sperm Count from Spermatocytes

N_{sperms} = 4 \times N_{primary} = 2 \times N_{secondary}

Symbol	Meaning
$N_{sperms}$	Number of sperms (equal to number of spermatids)
$N_{primary}$	Number of primary spermatocytes
$N_{secondary}$	Number of secondary spermatocytes

Hormonal Control of Spermatogenesis★ Board fav

GnRH \to LH \to Leydig cells (androgens); GnRH \to FSH \to Sertoli cells

Semen Quality for Normal Fertility

Sperms per ejaculate \approx (200 to 300) \times 1 0^{6}; \geq 60% normal shape and size; \geq 40% vigorous motility

Oogenesis Sequence★ Board fav

Primary oocyte (2 n) meiosis I secondary oocyte (n) + first polar body meiosis II, on sperm entry ovum (n) + second polar body

Ovum Count from Oocytes

N_{ova} = N_{primary oocytes} = N_{secondary oocytes}

Symbol	Meaning
$N_{ova}$	Number of ova formed
$N_{primary oocytes}$	Number of primary oocytes completing oogenesis

Follicle Development

Primary \to Secondary \to Tertiary (antrum) \to Graafian follicle

Menstrual Cycle Length

Average interval \approx 28 to 29 days; ovulation \approx day 14

Phases of the Menstrual Cycle★ Board fav

Menstrual (3 to 5 days) \to Follicular \to Ovulatory (day 14) \to Luteal (to day 28)

Hormones Across the Cycle★ Board fav

Follicular: LH, FSH, estrogen rise; Mid-cycle: LH surge \Rightarrow ovulation; Luteal: progesterone

Sex of the Child

Ovum (X) + Sperm (X) \to XX (female); Ovum (X) + Sperm (Y) \to XY (male)

Cleavage and Morula

Zygote cleavage 2, 4, 8, 16 blastomeres; 8 to 16 blastomeres = morula

Blastocyst and Implantation

Morula \to Blastocyst = trophoblast + inner cell mass

Hormones of Pregnancy

Placenta: hCG, hPL, estrogens, progestogens; Ovary (late pregnancy): relaxin

Milestones of Gestation

Heart: 1 month; limbs and digits: 2 months; major organs: 12 weeks; delivery: about 9 months

Parturition Reflex

Foetal ejection reflex \to oxytocin (maternal pituitary) \to stronger uterine contractions

✏️ Solved Examples

1Solved Exampleeasy2 steps

How many sperms are formed from 25 primary spermatocytes, and how many ova are formed from 25 primary oocytes?

Each primary spermatocyte completes two meiotic divisions and gives four spermatids, each of which becomes a sperm.

N_{sperms} = 4 \times 25 = 100

2Solved Exampleboard2 steps

A semen sample contains 250 million sperms. For normal fertility, what is the minimum number of sperms that must (a) have normal shape and size and (b) show vigorous motility?

For normal fertility at least 60% of sperms must have normal shape and size.

0.60 \times 250 \times 1 0^{6} = 150 \times 1 0^{6}

3Solved Exampleboard3 steps

A woman with a regular 28-day cycle starts menstruating on 1 March (day 1). Around which date does ovulation occur, which hormone triggers it, and when is the next menstruation expected if the ovum is not fertilised? (3 marks)

In a 28-day cycle, ovulation happens about mid-cycle, on day 14. Day 1 is 1 March, so day 14 is 13 days later.

Day 14 = 1 March + 13 days = 14 March

4Solved ExampleHOTS3 steps

To produce 400 sperms and 400 ova, how many (a) primary spermatocytes, (b) secondary spermatocytes, (c) primary oocytes and (d) secondary oocytes are needed? Explain why the numbers differ.

One primary spermatocyte gives 4 sperms; one secondary spermatocyte gives 2.

N_{primary} = \frac{400}{4} = 100; N_{secondary} = \frac{400}{2} = 200

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Saying oogenesis produces four ova from one primary oocyte, just like spermatogenesis
✓One primary oocyte gives only one ovum. Both meiotic divisions are unequal, and the other nuclei end up in polar bodies.
2
Writing that secondary spermatocytes are diploid
✓Meiosis I is the reduction division. Secondary spermatocytes are already haploid, with 23 chromosomes each.
3
Swapping the target cells of LH and FSH in the male
✓LH acts on Leydig cells to make androgens. FSH acts on Sertoli cells, which secrete factors that help spermiogenesis.
4
Saying the ovary releases a mature ovum that has finished meiosis
✓Ovulation releases the secondary oocyte. Meiosis II is completed only when a sperm enters it.
5
Mixing up spermiogenesis and spermiation
✓Spermiogenesis turns spermatids into sperms. Spermiation is the release of sperms from the seminiferous tubules.
6
Saying menstruation is caused by the LH surge
✓The LH surge causes ovulation. Menstruation follows when the corpus luteum degenerates without fertilisation, so progesterone falls and the endometrium breaks down.

🎯 Practice Yourself

🎯Practice Yourself6 questions

Q1
How many spermatids are formed from 50 secondary spermatocytes?
Q2
Give the chromosome number in a human spermatogonium, a primary oocyte, a secondary oocyte and an ovum.
Q3
A semen sample contains 300 million sperms. What is the minimum number that must show vigorous motility for normal fertility?
Q4
Name the hormone whose mid-cycle surge causes ovulation, and the day on which it happens in a 28-day cycle.
Q5
What is an embryo with 8 to 16 blastomeres called, and what does it develop into?
Q6
Which hormone of pregnancy is secreted by the ovary rather than the placenta?

📝 Notes

Human Reproduction

Humans reproduce sexually and are viviparous. The chapter follows a sequence: gametogenesis, insemination, fertilisation, implantation, gestation and parturition. Typical board questions test either a gametogenesis comparison or the hormone timeline of the menstrual cycle.

Gametogenesis: same meiosis, different outcome

Both pathways start with a diploid cell (2n = 46) and end with haploid gametes (n = 23). The difference is how the cytoplasm is divided:

Spermatogenesis (seminiferous tubules, from puberty): divisions are equal, so 1 primary spermatocyte gives 4 sperms.
Oogenesis (ovary, starts before birth): divisions are unequal, so 1 primary oocyte gives 1 ovum plus polar bodies.

When asked "how many gametes from N cells", multiply by 4 for the male and by 1 for the female. Always say where meiosis pauses in the female: prophase I from fetal life until the tertiary follicle stage (after puberty), and meiosis II until sperm entry.

The menstrual cycle is a hormone story

Read the cycle as cause and effect. Rising FSH and LH grow the follicle, and the follicle makes estrogen, which rebuilds the endometrium. The LH surge at about day 14 causes ovulation. The empty follicle becomes the corpus luteum, whose progesterone keeps the endometrium ready for implantation. If no fertilisation happens, the corpus luteum degenerates and menstruation starts the next cycle. During pregnancy the cycle stops.

From zygote to birth

Fertilisation happens in the ampulla. Cleavage gives the morula (8–16 blastomeres), then the blastocyst, which implants in the endometrium. The placenta exchanges materials and also acts as an endocrine tissue (hCG, hPL, estrogens, progestogens). At term, the foetal ejection reflex releases oxytocin, which drives labour. The mammary glands secrete colostrum first, which contains antibodies for the newborn.

Approach to long answers

Draw labelled flow charts with chromosome numbers, name the hormone at each step, and finish with a one-line comparison (number of gametes, timing, equal or unequal division). A neat diagram with correct labels often earns marks that text alone does not.