CBSE Class 12 Biology Notes Chapter 3 Human Reproduction

Spermatogenesis : In males, spermatogenesis occurs in the testes, where immature male germ cells called spermatogonia develop into mature sperm cells.

Oogenesis : In females, oogenesis takes place in the ovaries, where immature oogonia produce mature ovum.

Spermatogenesis Process : Spermatogenesis begins at puberty and proceeds as follows:

• Spermatogonia multiply through mitosis within the seminiferous tubules, increasing in number.
• Primary spermatocytes occasionally undergo meiosis to form two haploid secondary spermatocytes, each containing 23 chromosomes.
• Secondary spermatocytes undergo a second meiotic division, resulting in four haploid spermatids, each with 23 chromosomes.
• Spermatids undergo spermiogenesis to mature into spermatozoa (sperms), with their heads embedded in Sertoli cells.
• Spermatozoa are released from the seminiferous tubules through spermiation.

Key Terms :

• Spermatogonia : Immature male germ cells undergoing meiotic divisions to form sperms, containing 46 chromosomes.
• Primary Spermatocytes : Spermatogonia undergoing meiosis to produce two haploid secondary spermatocytes, each with 46 chromosomes.
• Secondary Spermatocytes : Resulting haploid cells from meiosis I, containing 23 chromosomes.
• Spermatids : Haploid cells formed from secondary spermatocytes after meiosis II, containing 23 chromosomes.
• Spermiogenesis : Process where spermatids mature into spermatozoa.
• Spermiation : Release of sperms from seminiferous tubules.

Menstrual Cycle

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Menstruation : Menstruation is the shedding of blood and mucosal tissue from the inner lining of the uterus, occurring in a periodic manner approximately once a month. This process, known as a period, is triggered by the breakdown of the uterine lining and typically lasts for 3-5 days.

Menarche : Menarche marks the onset of menstruation in human females, typically occurring at puberty. The age of menarche varies from person to person and signifies the beginning of the reproductive age in females.

Menopause : Menopause signifies the permanent cessation of the menstrual cycle in females. It occurs due to the depletion of oocytes and the decline in estrogen production by the ovaries as a result of aging. The average age of menopause ranges between 45-50 years, but it varies among individuals.

Phases of the Menstrual Cycle : The menstrual cycle comprises four phases:

Menstrual Phase : Characterized by menstrual flow lasting 3-5 days due to the shedding of the endometrial lining when the released ovum remains unfertilized.

Follicular Phase : Involves the growth and maturation of Graafian follicles in the ovary, stimulated by hormones LH and FSH. The endometrium regenerates in preparation for potential implantation.

Ovulation/Ovulatory Phase : Peak levels of LH trigger the release of an ovum from a mature Graafian follicle, known as ovulation, typically occurring around the 14th day of the cycle.

Luteal Phase : Formation of the corpus luteum, which secretes progesterone to maintain the endometrium for potential implantation. If fertilization doesn't occur, the corpus luteum degenerates, leading to menstruation and the start of a new cycle.