NCERT Class 12 Biology Chapter 3 Notes Human Reproduction

NCERT Class 12 Biology Chapter 2 Human Reproduction: Download PDF

The Human Reproduction chapter covers the structure and function of male and female reproductive systems, gamete formation, and embryonic development. The NCERT Class 12 Biology Chapter 2 Notes present these topics in a simple and organised format for easy revision. Students can download the PDF and use it offline, making it convenient for anytime study. Regular study of the NCERT Notes for Class 12 Biology helps students strengthen their understanding and perform better in exams.

Download PDF

Also Read:

• NCERT Solutions for Class 12 Biology Chapter 2 Human Reproduction

Class 12 Biology Chapter 2 Human Reproduction Notes

Introduction to Reproduction

We know that humans are sexually reproducing and viviparous. The reproductive events in humans include the formation of gametes called gametogenesis, i.e., when the sperm in males transfer into the female genital tract, and fusion with the female gametes, which is called fertilisation, leads to the formation of a zygote. This is followed by the formation and development of the blastocyst and its attachment to the uterine wall, which is called implantation; the gestation period, in which embryonic development takes place; and finally, the delivery of the baby, which is parturition.

Male Reproductive System

The male reproductive system consists of the following organs:

Testes:

• Situated outside the abdominal cavity in a pouch called the scrotum, which helps in maintaining the low temperature of the testes necessary for the process called spermatogenesis.

• Each testis has around 250 testicular lobules, and each of them contains highly coiled seminiferous tubules in which sperm are produced. Each seminiferous tubule is lined by two types of cells, spermatogonia (male germ cells) and Sertoli cells.

Leydig cells:

It is also called the Interstitial cells. It is present around the seminiferous tubules that synthesize and secrete the hormone called androgen.

Ejaculatory duct:

Its work is to store and transport the sperm from the testes to the outside through the urethra. The urethra originates from the urinary bladder and extends through the penis to open at the tip.

Penis:

It is the male external genitalia. The enlarged end of the penis is called the glans penis, is covered by a loose fold of skin called the foreskin.

Male accessory glands

The male accessory ducts include:

• paired seminal vesicles

• prostate

• paired bulbourethral glands

Secretion of these glands forms the seminal plasma, which contains fructose, calcium, and enzymes, and the secretion of bulbourethral glands also helps in the lubrication of the penis.

Female Reproductive System

Given below are the organs that together form the female reproductive system.

Ovaries:

They are the primary female sex organ that produces female gametes, eggs, and several other steroid hormones. Each ovary is covered by a thin epithelium that encloses the ovarian stroma, which is divided into two parts: a peripheral cortex and an inner medulla.

Fallopian tube:

The fallopian tube extends from the periphery of the ovary to the uterus. The part closer to the ovary is a funnel-shaped structure called the infundibulum, and it also has a finger-like projection called fimbriae.

The infundibulum leads to the ampulla and joins with the uterus through the isthmus. The uterus is pear-shaped in structure and is also called the womb.

Uterus:

It opens the vagina through a narrow cervix. The cavity of the cervix (cervical canal), along with the vagina, forms the birth canal.

The wall of the uterus has three layers of tissue:

• Perimetrium: The outermost thin layer that covers the external surface of the uterus.

• Myometrium: The middle thick layer of smooth muscle that shows strong contractions during childbirth.

• Endometrium: The innermost lining of the uterus that undergoes cyclic changes during the menstrual cycle to prepare for possible implantation.

Female external genitalia

Female external genitalia includes:

1. Mons pubis- It is a cushion of fatty tissue covered by skin and pubic hair.

2. Labia majora- It is a fleshy fold that surrounds the vaginal opening.

3. Labia minora- It is a paired fold of tissue under the labia majora.

The opening of the vagina is often partially covered by a membrane called the hymen. The tiny finger-like projection present at the upper junction of the two labia minora above the urethral opening is called the clitoris.

Mammary glands

They are paired structures that contain glandular tissues and variable fats. Each glandular tissue contains 15-20 mammary lobes containing alveoli that secrete milk. Mammary ducts join to form the mammary ampulla.

Gametogenesis

It is the process of formation of male and female gametes in the testes and the ovary, respectively, is called gametogenesis.

Gametogenesis is of two types:

1. Spermatogenesis- In males

2. Oogenesis- In females

Spermatogenesis

It is the process through which sperm formation in males takes place. When testes are immature, male germ cells (spermatogonia) produce sperm that begin at the stage of puberty.

The spermatogonia present at the inner side of the seminiferous tubules multiply by mitotic division and increase in number. Each spermatogonium contains a total of 46 chromosomes, and then spermatogonia form a primary spermatocyte that undergoes meiosis to produce the secondary spermatocytes having 23 chromosomes.

The spermatids are transformed into spermatozoa by the process called spermiogenesis. The sperm heads remain embedded in Sertoli cells and are released from seminiferous tubules by the process of spermiation.

Hormonal control of spermatogenesis

1. Spermatogenesis is initiated due to an increase in the secretion of Gonadotropin-Releasing Hormone (GnRH) by the hypothalamus.

2. An increase in GnRH acts on the anterior pituitary and stimulates the secretion of two gonadotropins, LH and FSH.

3. LH acts on Leydig cells and stimulates them to secrete androgens.

4. FSH acts on Sertoli cells, stimulating the secretion of some factors that help in spermiogenesis.

Structure of Sperm

Sperm is a microscopic structure composed of a head, neck, middle piece, and tail. The sperm head contains an elongated haploid nucleus, the anterior portion of which is covered by a cap-like structure acrosome.

• A human male ejaculates about 200-300 million sperm during coitus. The seminal plasma, along with the sperm, constitutes the semen. The function of the male sex secondary ducts and glands is maintained by androgen hormones.

Oogenesis

It is the process of formation of mature female gametes is called oogenesis. It started during the embryonic development stage when millions of oogonia (gamete mother cells) are formed in each fetal ovary.

• The gamete mother cells start division and enter prophase I of meiotic division, and are temporarily arrested at that stage, called primary oocytes.

• Each primary oocyte is surrounded by granulosa cells, forming the primary follicle. At puberty, about 60,000- 80,000 primary follicles are left in each ovary.

• The primary follicle gets surrounded by more layers of granulosa cells called secondary follicles that transform into tertiary follicles that contain fluid-filled cavities called the antrum.

• The tertiary follicles further change into the mature follicle called the Graafian follicle, which ruptures to release secondary oocytes (ovum) from the ovary by the process of ovulation.

Menstrual cycle

The reproductive cycle in female primates is called the menstrual cycle. It starts at puberty and is called menarche.

There are Four Phases of the Menstrual Cycle (MC):

Menstrual Phase:

• In a 28-day menstrual cycle, the menses take place on a cycle of 3-5 days.

• The production of LH from the anterior lobe of the pituitary gland is reduced.

• The withdrawal of this hormone causes degeneration of the corpus luteum, and, therefore, progesterone production is reduced.

• Production of oestrogen is also reduced in this phase.

• The endometrium of the uterus breaks down & menstruation begins.

• The cells of endometrium secretions, blood & unfertilised ovum constitute the menstrual flow.

Follicular Phase:

• This phase usually includes cycle days 6-13 or 14 in a 28-day cycle.

• The follicle-stimulating hormone (FSH) secreted by the anterior lobe of the pituitary gland stimulates the ovarian follicle to secrete oestrogens.

• Oestrogen stimulates the proliferation of the endometrium of the uterine wall.

• The endometrium becomes thicker by rapid cell multiplication, and this is accompanied by an increase in uterine glands & blood vessels.

Ovulatory Phase:

• Both LH & FSH attain a peak level in the middle of the cycle (about the 14th day).

• Oestrogen concentration in the blood increases.

• Rapid secretion of LH induces the rupture of the Graafian follicle and thereby the release of the ovum.

• In fact, LH causes ovulation.

Luteal Phase:

• Includes cycle days 15 to 28.

• Corpus luteum secretes progesterone.

• Endometrium thickens.

• Uterine glands become secretory.

Hormonal Control of Menstrual Cycle:

(i) FSH stimulates the ovarian follicles to produce oestrogens.

(ii) LH stimulates the corpus luteum to secrete progesterone.

(iii) The menstrual phase begins due to the fall in progesterone and estrogen levels, following degeneration of the corpus luteum.

(iv) LH causes ovulation

(v) The Proliferative phase is caused by the increased production of oestrogens.

(vi) The secretory phase is caused by increased production of progesterone.

Fertilisation and Implantation

The process of fertilisation and implantation is explained below:

The process of fusion of sperm with an ovum is called fertilisation.

During coitus (copulation), semen is released into the vagina. The motile sperm swim rapidly to reach the junction of the isthmus and ampulla of the fallopian tube. The ovum also reaches there, and fusion of gametes takes place at the ampullary-isthmic junction.

The acrosome of sperm undergoes an acrosomal reaction and releases certain sperm lysins, which dissolve the egg envelopes locally and make a path for the penetration of sperm.

These sperm lysins contain a lysing enzyme. hyaluronidase, which dissolves the hyaluronic acid polymers in the intercellular spaces that hold the granulosa cells of corona radiata together; corona penetrating enzyme that dissolves the corona radiata, and acrosin, which dissolves the zona pellucida. Then it dissolves the zona pellucida.

Cortical reaction

(a) Immediately after the entry of sperm into the egg, the latter shows a cortical reaction to check the entry of more sperm.

(b) In this reaction, the cortical granules present beneath the egg’s plasma membrane release chemical substances between the ooplasm and the plasma membrane vitelline membrane.

(c) These substances raise the vitelline membrane above the egg surface. The elevated vitelline membrane is called the fertilization membrane.

(d) The increased space between the ooplasm and the fertilization membrane and the chemicals present in it effectively check the entry of other sperm.

(e) If polyspermy occurs, that is, more than one sperm enters the secondary oocyte, the resulting cell has too much genetic material to develop normally

The haploid gametes fuse together to form a diploid zygote. As the zygote moves towards the uterus, the mitotic division starts and forms cleavage to change into 2, 4,8,16 celled blastomeres.

• The blastomeres with 8 to 16 cells are called the morula. Morula divides to change into blastocysts.

• The blastomeres in the blastocyst are arranged into an outer layer called the trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass.

• The outer layer of the blastocyst is called the trophoblast, which attaches to the endometrium of the uterus, called implantation, which leads to pregnancy.

Pregnancy and embryonic development:

The finger-like projections on trophoblasts after implantation are called chorionic villi that, along with the uterine wall, form a functional unit between the developing embryo and maternal body called the placenta.

The placenta is attached to the fetus with an umbilical cord that transports food and oxygen to the embryo.

• Hormones hCG (human chorionic gonadotropin)

• hPL (human placental lactogen)

• Relaxin

They are produced in women only during pregnancy by the placenta.

After implantation, the inner cell mass embryo differentiates into an outer layer called ectoderm and an inner layer called endoderm. A mesoderm soon appears between the ectoderm and the endoderm.

These three layers give rise to all tissues; organs in adults. It is important to note that the inner cell mass contains certain cells called stem cells, which have the potency to give rise to all the tissues and organs.

In humans, after one month of pregnancy, the embryo’s heart is formed. By the end of, 2nd month, limbs and digits are formed. By the end of the third month, major organs and external genital organs are well developed.

The first movement of the foetus is observed in 5 months. By the end of 24 weeks, the body is covered with fine hair, and eyelids and eyelashes are formed. At the end of 9 months fetus is fully developed.

Parturition And Lactation

• Parturition is the process of delivery of a fully developed foetus is called parturition.
• Signals for parturition originate from the fully developed fetus and placenta, inducing mild uterine contractions called the foetal ejection reflex.
• It triggers the release of oxytocin from the maternal pituitary.

• The mammary glands of females start producing milk at the end of pregnancy through the process of lactation.
• The milk produced during the initial few days of lactation is called colostrum. Colostrum is rich in IgA antibodies that provide passive immunity to the newborn.

Also, Read

• NCERT Notes for Class 12 Physics
• NCERT Notes for Class 12 Chemistry
• NCERT Notes for Class 12 Mathematics

Chapter 2 Human Reproduction: Previous Year Question and Answers

Some of the questions that have come in past years from the chapter are given below. Students will know how reproduction takes place in humans by referring to the Human Reproduction Class 12 Notes.

Question 1. Which of the following is not a component of the Fallopian tube?

Option 1. Uterine fundus

Option 2. Isthmus

Option 3. Infundibulum

Option 4. Ampulla

Answer :

The correct answer is option (1) as the uterine fundus is the upper, dome-shaped part of the uterus, above the opening of the fallopian tubes.
• Option (2) is incorrect as the isthmus is the last and narrow part of the oviduct that links to the uterus.
• Option (3) is incorrect as the infundibulum is the part of the oviduct that is closer to the ovary.
• Option (4) is incorrect as the ampulla is the wider part of the oviduct.

Hence, the correct answer is option (1). Uterine Fundus

Question 2. The Fallopian Tube provides an ideal environment for fertilization due to its:

Option 1. High acidity

Option 2. Thick mucus secretion

Option 3. Presence of spermicidal substances

Option 4. Optimal pH and supportive secretions

Answer :

The fallopian tube maintains a slightly alkaline pH, which helps sperm survive and move effectively. Its secretions contain nutrients and factors that support both sperm viability and egg fertilization. This favorable environment ensures successful fusion of gametes.

Hence, the correct answer is option (4), Optimal pH and supportive secretions

Question 3. In terms of their layered structures, which layers are shared between the allantois and the yolk sac?

Option 1. Outer Mesoderm and Inner Endoderm

Option 2. Outer Endoderm and Inner Mesoderm

Option 3. Outer Endoderm and Inner Ectoderm

Option 4. Outer Endoderm and Inner Endoderm

Answer :

Both the allantois and yolk sac are extraembryonic membranes that share a similar structure, consisting of an outer mesodermal layer and an inner endodermal lining. This arrangement helps in their role in nutrition, waste removal, and exchange during early embryonic development.

Hence, the correct answer is option (1), Outer Mesoderm and Inner Endoderm

Also Read:

• NCERT Solutions
• NCERT Solutions for Class 12
• NCERT Solutions for Class 12 Biology

How to Use NCERT Class 12 Biology Chapter 2 Notes Effectively?

Understanding Human Reproduction is a bit difficult sometimes, but using notes in a smart and structured way makes the process much easier. A planned approach helps you grasp key concepts quickly and retain them for exams.

• Start with the male and female reproductive systems and their functions step by step from the Class 12 Biology Chapter 2 Human Reproduction Notes.

• Revise the process of spermatogenesis and oogenesis using flowcharts, as they make the stages easy to understand.

• Go through the hormonal control of reproduction, especially the menstrual cycle, with the help of diagrams showing FSH, LH, estrogen, and progesterone.

• Highlight topics like fertilisation, pregnancy, and parturition, as questions are frequently asked about these concepts in the exam.

• Use diagrams and flowcharts in the Class 12 Biology Chapter 2 Human Reproduction Notes for quick last-minute revision.

Chapter-Wise NCERT Class 12 Notes Biology

The following notes cover all chapters, including concepts, important definitions, and diagrams. They are useful for quick revision before exams and for strengthening understanding of the subject.