NCERT Solution Class 12th Biology Chapter – 1 Reproduction in Organisms Notes

NCERT Solution Class 12th Biology Chapter – 1 Reproduction in Organisms

NCERT Solution Class 12th Biology Chapter – 1 Reproduction in Organisms Notes Menopause, Oogenesis, Reproduction, discuss Reproduction in Organisms Notes

NCERT Solution Class 12th Biology Chapter – 1 Reproduction in Organisms

Chapter – 1

Reproduction in Organisms

Notes

Reproduction is an important characteristic of the life cycle of living organisms. It involves producing offsprings from the existing (parent) organism. It occurs by asexual and sexual means.

Asexual reproduction generally takes place in protozoans, lower chordates, and lower invertebrates. It involves no formation and fusion of gametes. It occurs by various methods like budding, fission, fragmentation, plasmogamy, etc. Budding may be external (e.g. Hydra) or internal (e.g. Spongilla). Fission may be binary fission (e.g. Amoeba, Paramecium) or multiple fission (e.g. Amoeba, Plasmodium). Plasmotomy as in Opalina and fragmentation as in coelenterate colony and sponges.

Asexual reproduction is uniparental and does not produce variations in the next generation. The off-springs are identical to their parent and can be called clones.

Sexual reproduction takes place by the formation and fusion of gametes.
It involves various processes

1. gametogenesis i.e. formation of sex cells called the gametes. Gametes are produced by both male (male gamete) and female (female gamete) of the same species,
2. fertilization is the fusion of spermatozoa (male gamete) with the ovum (female gamete) to form a zygote,
3. development is the formation of a completely new individual from the zygote by repeated mitotic divisions.

Sexual reproduction involves the fusion of gametes called syngamy. It may be isogamous (e.g. Monocystis) or anisogamous (e.g. human beings). If the fusing gametes are similar (isogametes) .it is isogamous and if the fusing gametes are \ dissimilar, (heterogametes or isogametes) its anisogamy. In human beings, the sperm is microscopic, motile, and flagellated while the ovum is large, non-motile, spherical, and laden with food.

Syngamy may be exogamy if the fusing gametes are from two parents and endogamy if the fusing gametes are from the same parent.

Sperms and ova are produced by meiosis from spermatogonia and oogonia found in the testes of males and ovaries of females.

Sexual reproduction is biparental and introduces variations in the – offsprings. Sexual reproduction involving the fusion of male and female pronuclei is called conjugation e.g. ciliates.

Parthenogenesis is the process of the development of an egg into an offspring without fertilization. It may be natural (e.g. Apisindica) or artificial (Echinus). ,

Organisms may be unisexual or dioecious (e.g. human beings) or bisexual or monoecious (e.g. earthworm).

In sexually reproducing organisms the reproductive system consists of three types of organs:

1. primary sex organs, ‘viz. testes and ovaries, which produce haploid gametes by meiosis;
2. secondary sex organs, such as prostate and seminal vesicles in males and fallopian tubes, uterus, and vagina in the female, which are ducts to convey appropriate site for fertilization to the gametes and glands to provide useful secretions;
3. accessory sex organs, viz. facial hair, broad larynx, etc. in males and breasts in females, to distinguish between the two sexes by appearance.

The male reproductive system consists of a pair of testes suspended in the scrotal sac; a pair of ducts, each differentiated into an epididymis to store the sperms, a vas deferent for conduction of sperms and penis as an intermittent organ. A male urethra passing through an erectile penis, and 3 types of glands, viz., a pair of seminal vesicles, a prostate gland, and a pair of Cowper’s glands. The secretion of glands mix with sperms to form semen.

Testes consist of numerous coiled seminiferous tubules (crypts). Each is lined by germinal epithelium formed of germ cells and Sertoli (nurse) cells. Germ cells produce spermatozoa which obtain nutrition from the Sertoli cells. The interstitial cells (Leydig’s cells) of testes produce the male sex hormone, testosterone. Secondary sex glands of males include a prostrate, two seminal vesicles, and two Cowper’s glands. The secretion of these glands along with the sperms from the semen.

The female reproductive system is comprised of a pair of ovaries, two fallopian tubes (oviducts), a uterus, and a vagina. Each ovary is lined by the germinal epithelium of germ cells which form primary follicles. A primary follicle changes into Graafian follicles under the influence of FSH. Ovulation is controlled by the Luteinising hormone. Ovaries also secrete estrogens and progesterone hormones. ,

Fallopian tubes (oviducts) are differentiated into infundibulum, ampulla, isthmus, and uterine part. Fallopian tubes conduct the ovum towards the uterus.

The uterus is the site of fetal growth during pregnancy. The uterus (womb) is differentiated into the fundus, body, and cervix. The vagina acts as the birth canal. Vulva is the external genitalia. It comprises of vestibule, labiaminora, labia majora and clitoris. Breasts are accessory sex organs of females. Their size depends upon the estrogen of the ovary at puberty and lactogenic hormone after parturition.

Puberty is the period of sexual maturity. It is characterized by the development of secondary sexual characters. It comes between 13 to 16 years in the male and between 10 to 14 years in the female. It is controlled by testosterone in male and estrogens in the female.

Gametogenesis is the process of formation of haploid gametes (sperms and ova) in the primary sex organs, gonads (testes and ovaries).

Spermatogenesis occurs in seminiferous tubules of the testes. In it, diploid spermatogonium undergoes growth phase to form diploid primary spermatocyte which undergoes meiosis and form 4 haploid spermatids. Each spermatid transforms into sperm by a process called spermiogenesis. A mature sperm consists of a head, a neck, a middle piece, and a tail.

Oogenesis occurs in the ovary. A diploid oogonium undergoes a growth phase and forms a diploid primary oocyte which undergoes meiosis and forms a haploid ovum in 2 or 3 polar bodies. The human ovum is alecithal and is surrounded by many egg envelopes.

The menstrual cycle is cyclic changes in the ovaries and the reproductive tracts of primate females which culminate into the period vaginal bleeding called menstruation. It takes 28 days. It consists of four phases: Proliferative, ovulatory, luteal, and menstrual. The proliferative phase involves the growth and proliferation of the uterine endometrium, fallopian tubes, and vagina.

Ovulation occurs in the ovulatory phase. In the luteal phase, empty Graafian follicle changes into corpus luteum which secretes progesterone, further proliferation of the endometrium, and secretion of uterine milk. This is followed by menstruation if the ovum remains unfertilized. If fertilization occurs it is followed by implantation and normal growth of the fetus.

Menopause is the period when ovulation and the menstrual cycle stops. It generally occurs between 45 to 55 years.

The primate females show the menstrual cycle, the females of most of the mammals become sexually responsive during the heat period, they have an estrous cycle.

Asexual reproduction – Reproduction which involves no formation and fusion of gametes.

Binary fission – Type of asexual reproduction in which parent divides into two daughters.

Conjugation – Type of sexual reproduction found in ciliate protozoans. e.g. Paramecium.

Copulation – the sexual union of male and fèrnale. Also called coition.

Cytokinesis – Division of cytoplasm during cell division.

Epididymis – Secondary sex organs of males, which store sperms.

Gametogenesis – The formation of haploid gametes in the gonads.

Gestation period – The period of embryonic development between fertilization and delivery.

Hermaphrodite – Organisms with both testes and ovaries.

Isogamy – When two fusing gametes are morphologically and physiologically similar.

Karyokinesis – Division of the nucleus during the cell division.

Menopause – Period when ovulation and menstruation stop in the female.

Oogenesis – Formation of the ovum in the ovary.

Parthenogenesis – Development of an unfertilized ovum.

Reproduction – Method of producing progeny by the existing individuals for the continuation of the race.

Sexual reproduction – Method of reproduction that involves fusion of the gametes.

Spermatogenesis – Formation of haploid sperms in the testes.

Vitellogenesis – Formation of the yolk in the ovum.