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NCERT Solutions for Class 12 Biology
Page No1 55
Q1. Fill in the blanks:
(a) Humans reproduce _ (asexually/sexually)
(b) Humans are _ (oviparous, viviparous, ovoviviparous)
(c) Fertilisation is _ in humans (external/internal)
(d) Male and female gametes are _ (diploid/haploid)
(e) Zygote is _ (diploid/haploid)
(f) The process of release of ovum from a mature follicle is called _
(g) Ovulation is induced by a hormone called _
(h) The fusion of male and female gametes is called _
(i) Fertilisation takes place in _
(j) Zygote divides to form _____________which is implanted in uterus.
(k) The structure which provides vascular connection between fetus and uterus is called ——
(g) luteinzing hormone
(i) fallopian tube
Q2. Draw a labeled diagram of male reproductive system.
Q3. Draw a labelled diagram of female reproductive system.
Q4. Write two major functions each of testis and ovary.
Answer: Functions of testis are
(i) It produces the male gamete or spermatozoa by the process of spermatogenesis.
(ii) It produces the male hormone testosterone which helps in spermatogenesis and development of male characteristics.
Function of ovary are
(i) It produces the female gamete or the ovum by oogenesis.
(ii) It produces the female hormone estrogen and progesterone which help develop female characteristics.
Q5. Describe the structure of a seminiferous tubule.
Answer: Structure of seminiferous tubules:
- Seminiferous tubules are found in the testicular lobules and are highly coiled structures. It is here that the production of sperms in the testes occurs
- Each of the seminiferous tubule has a lining of germinal epithelium
- On the inner side, it is lined by two types of cells – sertoli cells and spermatogonia
- Spermatogonia – they are the male germ cells that produce the primary spermatocytes through the process of meiotic divisions. The primary spermatocytes furthermore undergo meiotic divisions for the formation of secondary spermatocytes and ultimately spermatids. Lateron, spermatids metamorphosize into the male gametes termed as spermatozoa
- Sertoli cells are referred to as nurse cells of the testes. It is because they nourish the germ cells.
- Just adjacent to the seminiferous tubules, there are large polygonal cells referred to as leydig cells or interstitial cells which secrete testosterone – the male hormone.
Q6. What is spermatogenesis? Briefly describe the process of spermatogenesis.
Answer: Spermatogenesis is the process by which male spermatogonia develop into mature male gamete, spermatozoa. It starts at puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age. The process of spermatogenesis includes the formation of spermatogonia from germinal epithelium (primordial germ cell) through mitosis (multiplication phase).
Finally they stop undergoing mitosis, grow and become primary spermatocytes (growth phase). Each spermatocyte undergoes meiosis (maturation phase). First maturation division is reductional, and produces two secondary spermatocytes. The latter divides by equational division (second maturation division) to form four haploid spermatids. Spermatids receive nourishment from the Sertoli cells to form sperms. This step is called spermiogenesis.
Q7. Name the hormones involved in regulation of spermatogenesis.
Answer: Follicle-stimulating hormones (FSH) and luteinizing hormones (LH) are secreted by gonadotropin releasing hormones from the hypothalamus .These hormones are involved in the regulation of the process of spermatogenesis. FSH acts on sertoli cells, whereas LH acts on leydig cells of the testis and stimulates the process of spermatogenesis.
Q8. Define spermiogenesis and spermiation.
Answer: Spermiogenesis: It is the process of transforming spermatids into matured spermatozoa or sperms.
Spermiation: It is the process when mature spermatozoa are released from the sertoli cells into the lumen of seminiferous tubules.
Q9. Draw a labeled diagram of sperm.
Q10. What are the major components of seminal plasma?
Answer: Semen is the alkaline fluid ejaculated by man. It consists of sperms and seminal plasma. In the male reproductive system, its major components are mucus, spermatozoa, and various secretions of accessory glands. For the sperm to be active, the seminal plasma also contains fructose, calcium, ascorbic acid, and certain enzymes. Thus, nourishment and protection to sperms are provided.
Q11. What are the major functions of male accessory ducts and glands?
Answer: Male accessory ducts include rete testis, vasa efferentia, epididymis and vas deferens. These ducts store and transport sperms from the testis to the outside through urethra. The male accessory glands include paired seminal vesicles, a prostate and paired bulbourethral glands. Secretions of these glands constitute the seminal plasma which is rich in fructose, calcium and certain enzymes. The secretions of bulbourethral glands also helps in the lubrication of the penis.
Q12. What is oogenesis? Give a brief account of oogenesis.
Answer: Ooogenesis is the phenomena of formation of haploid female gametes known as ova from diploid oogonia in the ovary, Graffian follicles, to be precise. This process is discontinuous which is initiated during the period of foetal development that is terminated only after puberty sets in.
The process of Oogenesis takes place in three phases:
Multiplicative phase –
- Follicle cells are differentiated from the germinal epithelium of the ovary due to repeated mitosis division. Few follicle cells enlarge and are termed as egg mother cells and undergo mitosis to multiply which are referred to as oogonia.
- Growth phase – one of the oogonium of the egg nest differentiates while the rest change into surrounding nutritive follicular epithelium. There is an increase in the size of the differentiated isolated oogonium as it gets nourished from the girdling follicle cells thereby transforming into a diploid primary oocyte.
- Maturation phase – The diploid primary oocyte in this phase passes through two maturation divisions. Meiosis I – the first meiotic division splits the diploid primary oocyte into two haploid cells wherein the larger one is the secondary oocyte and the minor one is the polar body (polocyte). In meiosis II or the second meiotic division, the secondary oocyte splits to form one large ootid and a tiny second polar body. Furthermore, the first polar body splits through mitosis to form two polar bodies. The ootid matures into a functional haploid ovum. Therefore, one primary oocyte produces one large ovum and three polar bodies which inturn degenerate. They degenerate as they do no participate in reproduction, thus leaving behind one functional ovum.
Q13. Draw a labelled diagram of a section through ovary.
Q14. Draw a labelled diagram of a Graafian follicle?
Q15. Name the functions of the following:
(a) Corpus luteum
Answer: It acts as an endocrine gland and secretes progesterone hormone which is essential for endometrium of uterus.
Answer: It is innermost layer of uterus responsible for nutrition and development of the foetus. It undergoes cyclic changes during menstrual cycle and prepares itself for implantation of blastocyst and placentation.
Answer: Acrosome of sperm head contains hydrolytic enzymes that help in dissolving membranes of the ovum for fertilization.
(d) Sperm tail
Answer: It helps in the movement of sperm essential for fertilisation.
Answer: It help in collection of the ovum after ovulation.
Q16. Identify True/False statements. Correct each false statement to make it true.
(a) Androgens are produced by Sertoli cells. (True/False)
Answer: False, Androgens are produced by Leydig cells or interstitial cells.
(b) Spermatozoa get nutrition from Sertoli cells. (True/False)
(c) Leydig cells are found in ovary. (True/False)
Answer: False, Leydig cells are found in testis.
(d) Leydig cells synthesise androgens. (True/False)
(e) Oogenesis takes place in corpus luteum. (True/False)
Answer: False, Oogenesis takes place in Graafian follicles.
(f) Menstrual cycle ceases during pregnancy. (True/False)
(g) Presence or absence of hymen is not a reliable indicator of virginity or sexual experience. (True/False)
Q17. What is menstrual cycle? Which hormones regulate menstrual cycle?
- It is a cycle observed to be taking place in females, lasting for around 28 days on an average to complete
- It is a series of cyclic physiological changes occurring the female reproductive tract in primates, the end of which is combined with the collapsing of the uterine endothelium that is liberated in the form of blood and mucos through the vaginal opening, which is termed as menses.
- The different hormones that regulate the menstrual cycle are LH – leutinizing hormone, FSH – follicle stimulating hormone, progesterone, estrogen.
- During the follicular phase, the levels of LH and FSH that is secreted from the anterior pituitary gland increases. The FSH that is secreted under the effect of the releasing hormone (RH) from the hypothalamus, triggers the primary follicle to convert into a Gaarfian follicle.
- There is a gradual increase in the level of LH which causes the follicle to grow, also causing the secretion of estrogen.
- Estrogen obstructs the FSH secretion, triggering the secretion of the LH, which also results in the thickening of the uterine endometrium.
Q18. What is parturition? Which hormones are involved in induction of parturition?
- The process of delivering of the fully developed fetus or baby at the end of the pregnancy period through vigorous contraction of the uterus is called parturition.
- Estrogen (amount of estrogen is more than progesterone) and oxytocin are the hormones involved in the induction of parturition.
Q19. In our society the women are often blamed for giving birth to daughters. Can you explain why this is not correct?
Answer: All human beings have 23 pairs of chromosomes. Human males have 22 pairs of autosomes and contain one or two types of sex chromosome. They are either X or Y. On the contrary, human females have 22 pairs of autosomes and contain only the X sex chromosome. The sex of an individual is determined by the type of the male gamete (X or Y), which fuses with the X chromosome of the female. If the fertilizing sperm is X, then the baby will be a girl and if it is Y, then the baby will be a boy. Hence, it is incorrect to blame a woman for the gender of the child.
Q20. How many eggs are released by a human ovary in a month? How many eggs do you think would have been released if the mother gave birth to identical twins? Would your answer change if the twins born were fraternal?
Answer: An ovary releases an egg every month. When two babies are produced in succession, they are called twins. Generally, twins are produced from a single egg by the separation of early blastomeres resulting from the first zygotic cleavage. As a result, the young ones formed will have the same genetic make- up and are thus, called identical twins.
If the twins born are fraternal, then they would have developed from two separate eggs. This happens when two eggs (one from each ovary) are released at the same time and get fertilized by two separate sperms. Hence, the young ones developed will have separate genes and are therefore, called non-identical or fraternal twins.
Q21. How many eggs do you think were released by the ovary of a female dog which gave birth to six puppies?
Answer: In order to have given birth to six puppies, the ovary of the female dog released six eggs. Hence, six zygotes were formed for each to develop into a puppy.
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