NCERT Exemplar for Class 12 Biology Chapter 3

NCERT Exemplar for Class 12 Biology will aid in deepening the knowledge of students related to the topic of Human Reproduction. This provided exemplar by SimplyAcad allows learners to cover up all the sections presented in the chapter 3 of the biology textbook. There are a total of 20 questions containing detailed diagrams wherever required to ensure students understand the concepts better. It will benefit the students as they can prepare their own study and revision notes from them. Students can easily access this NCERT exemplar for class 12 biology in this article below to perform incredibly well in their upcoming 12th board examinations. Apart from these there are several NCERT exemplar for class 12 science of all the chapters provided in a detailed manner.

Access the NCERT Exemplar Class 12 Biology Chapter 3 Human Reproduction

Question:1 What role do pituitary gonadotropins play during follicular and ovulatory phases of the menstrual cycle? Explain the shifts in steroidal secretions.

Answers (1)

Pituitary gonadotropins (FSH and LH) play a very vital role during the follicular and ovulatory phases of the menstrual cycle.

• FSH stimulates the release of ovum from the Graafian follicle. It also stimulates the development and subsequent rupture of the Graafian follicle. Thus, FSH plays a major role during the follicular and ovulatory phases.
• The above figure shows varying levels of LH. FSH and steroid hormones during follicular and ovulation phases of the menstrual cycle.
• At this stage, the level of estradiol (a steroid hormone) increases. This suppresses the level of Luteinizing hormone (LH). Once the level of estradiol reaches a threshold level, the effect is reversed.
• After that, the level of estrogen (another steroid hormone) increases. This stimulates a surge in levels of LH. High level of L.H stimulates the development of corpus luteum and proliferation of the endometrium, thus, marking the beginning of the luteal phases.

Ques 2.Meiotic division during oogenesis is different from that in spermatogenesis. Explain how and why?

Answer 2.

It is true that the meiotic division during oogenesis is different from that in spermatogenesis. Following are the details about significant differences between them:

• In the case of spermatogenesis, meiotic division begins at the time of puberty. This is the time when spermatogenesis begins in a male. On the other hand, oogenesis begins in the female when the female is still in the womb. Thus, it can be said that meiotic division during oogenesis begins when the girl child is still in the womb.
• Formation of primary oocytes stops by the 20th week of gestation of the female child. On the contrary, the production of spermatocytes continues throughout the reproductive phase of a male.
• All the stages of meiosis take place quickly after one another in case of spermatogenesis and there is no time lag between them. In case of oogenesis, meiosis is deferred at the Prophase I state.

Meiosis resumes only once puberty begins.

Following are some possible reasons for this difference in patterns of oogenesis and spermatogenesis:

• Male gametes need to be formed in very large numbers to ensure Fertilisation.
• Female gametes are not exposed to external hazards, and hence a less number of them would be enough. Limiting the production of female gametes helps in conservation of resources from the female body. The resources can then be properly utilised during the development of the foetus.

Ques 3. The zygote passes through several developmental stages till implantation, Describe each stage briefly with suitable diagrams.

Answers (3)

The following figure shows various stages of zygote development till implantation:

(a) The zygote divides into two-cell stage called cleavage. This development takes place when the zygote is still in the isthmus.

(b) The cleavage undergoes further rounds of mitosis to form 2, 4, 8 and finally 16-celled stage.
These daughter cells are called blastomere.

(c) The 16-celled stage is known as morula.

(d) The morula changes into blastocyst; with more rounds of meiosis. At this stage, the blastomeres are arranged in an outer layer of cells; called trophoblast and an inner mass of cells.

(e) Implantation to uterine wall happens at the blastocyst stage.

Ques 4. Draw a neat diagram of the female reproductive system and label the parts associated with the following

(a) production of gamete,

(b) site of fertilisation

(c) site of implantation and,

(d) birth canal.

Answers (4)

Ques 5. With a suitable diagram, describe the organization of the mammary gland.

Answers (5)

Mammary glands are paired structures which contain glandular tissue and a variable amount of fat.

 

• Glandular tissue present in each breast is made up of 15 – 20 mammary lobes. These lobes have clusters of cells known as alveoli.
• Milk, stored in the cavities (lumens) of alveoli, is secreted from the cells of alveoli. The alveoli open in mammary tubules.
• Tubules from each lobe join to form a mammary duct.
• Many mammary ducts join to form a wider mammary ampulla.
• A mammary ampulla is connected to the lactiferous duct. Milk is sucked out through the lactiferous duct.

Ques 7. A human female experiences two major changes, menarche, and menopause during her life. Mention the significance of both the events.

Answers (7)

Menarche marks the onset of puberty (beginning of reproductive phase).

Menopause marks the end of the reproductive phase

Ques 8. a. How many spermatozoa are formed from one secondary spermatocyte?

b. Where does the first cleavage division of zygote take place?

Answers (8)

(a) One secondary spermatocyte forms four spermatids, all of which subsequently develop into spermatozoa via the process spermiogenesis.

(b) The first cleavage division of zygote takes place in the ampullary-isthmus junction.

Ques 9. Corpus luteum in pregnancy has a long life. However, if Fertilisation does not take place, it remains active only for 10-12 days. Explain.

Answers (9)

Corpus luteum is responsible for the secretion of progesterone, which is essential to maintain the endometrium and helps in the proper development of the foetus.

This is the reason for the long life of Corpus luteum when Fertilisation has occurred. However, when there is no fertilisation, it serves no purpose and thus remains active only for 10-12 days

Ques 10. What is a foetal ejection reflex? Explain how it leads to parturition?

Answers (10)

Foetal ejection reflex is the mild uterine contraction at the time of childbirth. It triggers the release of oxytocin from the maternal pituitary, which induces strong uterine contraction and finally leads to parturition.

Ques 11. Except for the endocrine function, what are the other functions of the placenta?

Answers (11)

The placenta is responsible for the following in addition to its endocrine function –

(A) Supply of oxygen and nutrient to fetus

(B) Removal of waste products from the fetus

Ques 12. Why do doctors recommend breastfeeding during the initial period of infant growth?

Answers (12)

Milk produced during the initial few days of lactation is known as colostrum and contains several antibodies, which are important for the development of disease resistance in the baby. This is the prime reason why doctors recommend breastfeeding during the initial period of growth of an infant.

Ques 13. What are the events that take place in the ovary and uterus during the follicular phase of the menstrual cycle?

Answers (13)

During the follicular phase of the menstrual cycle, the following things happen in the ovary and the uterus –

1. The primary follicle in the ovary grows into fully mature Graafian follicle
2. Endometrium in the uterus regenerates through the proliferation
3. Estrogen is secreted by growing follicle

Ques 14. Given below is a flow chart showing ovarian changes during the menstrual cycle. Fill in the spaces giving the name of the hormones responsible for the events shown.

Answers (14)

Hormones responsible for various changes during the menstrual cycle are:

1. Follicle Stimulating Hormone (FSH): It is responsible for the transformation of the primary follicle into Graafian follicle.
2. Follicle Stimulating Hormone (FSH): It is also responsible for ovulation. It works in coordination with LH and complement its function.
3. Luteinizing Hormone (LH): It is responsible for the development of corpus luteum from the Graafian follicle.

Ques 15. Give a schematic labelled diagram to represent oogenesis (without descriptions).

Answers (15)

Ques 16. What are the changes in the oogonia during the transition of a primary follicle to Graafian follicle?

Answers (16)

Following changes occur in the oogonia during the transition of a primary follicle to Graafian follicle:

(a) Each primary oocyte is surrounded by a layer of granulosa cells; called a primary follicle

(b) Primary follicle transforms into secondary follicle by getting surrounded by another layer of granulosa cells and a new theca.

(c) The secondary follicle then transforms into the tertiary follicle. The tertiary follicle is characterised by a fluid-filled cavity called the antrum. At this stage, the primary oocyte undergoes second meiotic division and forms a secondary oocyte and a tiny polar body.

(d) The tertiary follicle then changes into mature Graafian follicle. During this stage, a new layer; called zona pellucida; develops around the secondary oocyte.

Ques 17. Given below are the events in human reproduction. Write them in correct sequential order. Insemination, gametogenesis, Fertilisation, parturition, gestation, implantation

Answers (17)

Gametogenesis→Insemination→Fertilization→Implantation→Gestation→Parturition

Ques 18. The path of sperm transport is given below. Provide the missing steps in blank boxes.

Answers (18)

Ques 19. What is the role of cervix in the human female reproductive system

Answers (19)

Cervix and vagina together form the birth canal. Parturition happens through the birth canal.

Ques 20. Why are menstrual cycles absent during pregnancy?

Answers (20)

Gonadotropins, which are required for the development of new follicles, are suppressed due to the high levels of progesterone and estrogen during pregnancy. This inhibits the initiation of a new menstrual cycle.

NCERT Exemplar For Class 12 Science

Students must practise these additional questions for their own benefits, the ncert exemplar are curated by the best subject-matter experts to boost your knowledge on the presented topic. Students can easily access the ncert exemplar for class 12 science by visiting our website SimplyAcad and solve all the questions listed to secure maximum marks.

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