In This Post we are providing Chapter-3 HUMAN REPRODUCTION NCERT MOST IMPORTANT QUESTIONS for Class 12 BIOLOGY which will be beneficial for students. These solutions are updated according to 2021-22 syllabus. These MCQS can be really helpful in the preparation of Board exams and will provide you with a brief knowledge of the chapter.
NCERT MOST IMPORTANT QUESTIONS ON HUMAN REPRODUCTION
1. Differentiate between spermatogenesis and oogenesis.
Ans.
| Spermatogenesis | Oogenesis |
| 1. It occurs inside the testes. | 1. It occurs inside the ovary. |
| 2. All the stages are completed inside the testes. | 2. Majority occurs inside the ovary but last stages occur in the oviduct. |
| 3. Spermatogonia develop from the germinal epithelium lining in the seminiferous tubules. | 3. Oogonia develop from the germinal epithelium overlying the ovary. |
| 4. All spermatogonia give rise to spermatocytes. | 4. Only few oogonia give rise to oocytes. |
| 5. Primary spermatocytes divide by meiosis I to give rise to two secondary spematocytes | 5. Primary oocyte undergoes meiosis I to give rise to one secondary oocyte and a polar body. |
| 6. Secondary spermatocyte divides by meiosisII to give rise to two spermatids. | 6. Secondary oocyte divides by meiosisII to form the ovum and the second polar body. |
| 7. Each spermatid differentiates into spermatozoan or sperm. | 7. No differentiation is required after meiosisII. |
| 8. The sperms formed are motile. | 8. The ovum or egg is non- motile. |
2. ‘A fertilized egg is the blue print of future development’. Explain
Ans. The sperm carries the genetic information from the father in form of 23 chromosomes (including the male sex chromosome X or Y) while the egg bears the genetic information from the mother (including the female sex chromosome X). Thus during fertilization the fusion of the male and the female gametes produce new genetic combination which introduces variation in the progeny. The zygote or the fertilized egg contain the genetic information which accordingly controls the development of the embryo.
3. Briefly describe the stages of spermatogenesis in human?
Ans. Spermatogenesis consists of two phases:-
I. FORMATIDN OF SPERMATIDS :- It further consist of 3 phases
- Multiplication phase :- undifferentiated germ cells undergo repeated division to produce sperm mother cell or spermatogonia.
- Growth phase :- Spermatogonia increase in volume & is now called PRIMARY SPERMATOCYTES.
- Maturation phase: – primary spermatocyte undergoes meiosis I to produce small size haploid secondary spermatocyte secondary spermatocyte divides by meiosis – II & forms haploid Spermatids.
II.FORMATION OF SPERMS :- The tramsformation or differentiation of spermatids into spermatozoa or sperm is called spermiogenesis & occurs under the influence of FSH
4. Describe the hormonal control of human male reproduction system with the help of a flow chart & highlight the inhibitory & stimulatory directions in it?
Ans. i) Spermatogenesis is initiated due to an increase in the secretion of Gonadotropin releasing hormone from hypothalamus at the age of puberty.
- The increased levels of GnRH act on anterior pituitary& stimulate the secretion of two gonadotropins i-e. leuteinizing hormone (LH) & follicle stimulating hormone (FSH)
- LH acts on leydig cells & stimulate them to secrete testosterone
- FSH acts on sertoli cells & stimulate secretion of some factors help in spermiogenesis
5. A sperm has just fertilized a human egg in the fallopian tube. Trace the events that the fertilized eggs will undergoes upto implantation of blastocyst in the uterus.
Ans. 1. CLEAVAGE :-Fertilized egg starts dividing lay specific mitotic divisions called cleavage. The zygotes undergoes mitotic division in the isthmus of oviduct to form daughter cell the cells formed as a result of cleavage called blastomere
2. BLASTOCYST :- 3-4 days after fertilization, the morula twins into large mass of cells called blastocyst Outer peripheral cells enlarge & flatten further & form trophoblast. Trophoblast cells secretes a fluid into interior & form a cavity called blastocoel. The embryonic stage with blastocoels is called blastula.
6. Where oogenesis does takes place. Describe the stages of this process?
Ans. The process of formation & maturation of ovum is called oogenesis. It takes place in ovary & is initiated during embryonic development of female foetus. It consists of 3 phases :–
- Multiplication phase :- The primordial germ cells divide by meiosis to produce oogonia. These oogonia divide lay repeated mitotic divisions forming clusters. In each cluster only one of them enters into growth phase & is called primary oocyte.
- Growth phase :- Growth phase occurs only after attainment of puberty. It involves – increase in size of oocyte to many folds & synthesis of you.
- Maturation phase :- The first division is meiotic as a result two haploid (n) cells are produced. In this division, cytobinesis is unequal, large daughter cell with almost all cytoplasm is called secondary oocyte & smaller me with less cytoplasm is called polar body. The secondary oocyte then undergoes second meiotic division to form an ovum & second polar body.
7.
Ans. (i) ’D’ Spermatids = undergo spermiogenesis
(ii) ‘A’= Spermatogonium; B = Primary spermatocyte
(iii) ‘B’ = Diploid E = Haploid
(iv) ‘F’ = Sertoli cells – Nutrition to germ cells
(v) Mitosis in Cell ‘A’, Meiosis in cell ‘B’
8. Explain the development of human embryo with diagrams.
Ans. The Fusion of the sperm and the egg in humans result into formation of the diploid structure called zygote. The zygote starts dividing mitotically as it moves through the oviduct into the uterus to form 2,4,8,16 daughter cells called blastomeres. The stage is called morula. The Morula divides further and differentiates into blastocysts. The outer layer of blastomeres called trophoblast gets attached to the endometrial layer of the uterus.
The uterine wall divides and encloses the blastocysts and this is referred to as implantation.
The inner layer of blastomeres in the blastocysts gives rise to the embryo.
9. What is menstruation? What are the specific actions of FSH, LH, estrogen & progesterone in menstrual cycle?
Ans. During menstrual phase of menstrual cycle which starts on 28th day the endometrial lining of female genital tract break down due to lack of progesterone As a result bleeding occurs. This monthly flow of blood is caller menstruation.
During menstrual cycles, the various changes occurs in the ovary under the influence of various hormones :-
- Menstrual phase :- The levels of hormones LH ,FDH estrogen & progesterone is very less which results in breakdown of endometrial lining of uterus.
- Follicular phase :- In this phase , the levels of pituitary hormones FSH & LH increases which causes ovarian hormone estrogen to release,. FSH controls the follicular phase , it stimulates the growth of follicles. Both FSH & LH reach their peak level in middle of cycle (14th day)
- OVULATORY PHASE :- The level of LH hormones reaches its peak (called LH swing) induces the ruptures of mature Graffian follicle & there by release of ovum
- Luteal phase :- The LH & FSH hormones begins to decline. After ovulation, the follicle becomes to ruptures & is transformed into corpus Luteum which secretes large quantities of progesterone
10. A woman has conceived & implantation has occurred within her uterus. Discuss the sequence of changes up to parturition which will take place within her body under the influence of various hormones.
Ans. The following changes takes place in the body of women after implantation :-
- The trophoblast differentiates into two layers outer layer secretes enzymes to dissolve the endometrium of uterus.
- The inner layer grows out as finger – like projections called chorionic villi into uterine stoma. The chorionic villi & the uterine tissue become inter digitated to form structural & functional unit called placenta.
- Placenta secretes hormones like HCG, HPL , estrogen & progesterone that are necessary to maintain pregnancy
- Umbilical cord, the structure that connects the placenta with the foetus is formed.
- Simultaneously, inner cell mass differentiates into outer layer called ectoderm & inner layer called endoderm. & a middle layer called mesoderm appears between ectoderm & endoderm.
- The primary germ layers give rise to all the tissues & organs of the adults e.g. after one month heart is formed & after second month digits & limbs are formed.
- By the end of ninth month of pregnancy, foetus is completely developed & is ready for delivery.
- During parturition, ovary secretes a hormone called relaxin that facilitates parturition which softens the connective tissue. Mild contraction called foetal ejection reflex is induced. This triggers release of oxytocin from posterior pituitary. Oxytocin induces stronger leads to expulsion of baby from uterus, through birth canal.
