Title SP-3_Ch-13_Reproduction in Organisms _ Human Reproduction.pdf ✅

Chapter Contents Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 LIFESPAN Life span can be defined as the period from birth to the natural death of an organism. Life span cannot necessarily be correlated with size of an organism. REPRODUCTION Reproduction is an essential feature of all living organisms. It is the process by which an individual multiplies in number by producing more individuals of its own type. There are two main categories of reproduction (a) asexual and (b) sexual. Nucleus Daughter cells Fig.: Binary fission in Amoeba (a) Asexual Reproduction is the process of multiplication of individuals without the formation or fusion of gametes. Asexual reproduction involves a single parent and the young ones produced through it are genetically similar to the parents and are called clones. Asexual reproduction can occur by binary fission, multiple fission, sporulation, budding and fragmentation. (i) Binary fission - It occurs in unicellular organisms. It involves the division of the nucleus followed by that of the cytoplasm, dividing the body into two young ones. It can be simple (irregular) e.g. Amoeba, transverse (e.g. Paramecium, Planaria) or longitudinal (Euglena, Vorticella). Chapter 13 Reproduction in Organisms & Human Reproduction Lifespan Reproduction The Male Reproductive System The Female Reproductive System Gametogenesis Menstrual Cycle Fertilisation and Implantation Pregnancy and Embryonic Development Parturition Lactation
2 Reproduction in Organisms & Human Reproduction NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 (ii) Multiple fission - It occurs in some protozoans. The nucleus divides into several daughter nuclei, followed by the subsequent division of cytoplasm, e.g., Plasmodium. (iii) Sporulation - In many protozoans, asexual reproduction occurs by the division of nucleus into several daughter nuclei and then each daughter nucleus gets enclosed by small amount of cytoplasm to form a spore. (iv) Budding - Bud is an outgrowth formed on the surface of the body which finally gets detached and develops into a young one. Budding can occur on the outer surface (exogenous) e.g., Hydra or internally (endogenous) e.g., sponges (gemmule formation) Budding and gemmule formation are the common methods of asexual reproduction seen in lower animals. (v) Fragmentation - In some organisms, if the body breaks into distinct pieces (fragments) each fragment grows into an adult capable of producing offspring (e.g., Hydra). This is also a mode of asexual reproduction called fragmentation. Note : Under unfavourable conditions the Amoeba withdraws its pseudopodia and secretes a three-layered hard covering or cyst around itself. This phenomenon is termed as encystation. When favourable conditions return, the encysted Amoeba divides by multiple fission and produces many minute amoeba or pseudopodiospores; the cyst wall bursts out, and the spores are liberated in the surrounding medium to grow up into many amoebae. This phenomenon is know as sporulation. (b) Sexual Reproduction - It is the process of reproduction which involves the formation and fusion of gametes. Sexual reproduction is usually biparental, i.e., involves a male and female individual producing male and female gametes respectively. Such animals are called unisexual or dioecious; e.g., frog, cockroach, human beings. In certain animals, the male and female sex organs are present in the same individual. Such species are called monoecious or bisexual, e.g., earthworm, leech. If the fusing gametes are similar, they are called isogametes. e.g., Monocystis. If they are dissimilar, the gametes are termed anisogametes. e.g., most of the animals. Events in Sexual Reproduction A. Pre-fertilization events B. Syngamy or fertilization C. Post-fertilization events A. Pre-fertilization Events Pre-fertilization events include two processes : (i) Formation of gametes (gametogenesis) (ii) Gamete transfer (i) Gametogenesis : The process of formation of gametes is known as gametogenesis. Male and female gametes and are always haploid. In animals, the male gametes called sperms are produced in the testes whereas the female gametes called eggs develop in the ovaries. In comparison to male gametes, female gametes are always produced in a much smaller number. Sexuality in organisms : Animals are either unisexual (e.g., ants, wasps, bees, mosquito, cockroach, frog, birds, rabbit, humans) or bisexual (i.e. hermaphrodites, e.g., earthworm, tapeworm, leech etc.)
NEET Reproduction in Organisms & Human Reproduction 3 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 It is interesting that most hermaphrodites do not reproduce by self-fertilization. For example, in earthworms, two animals copulate and each inseminates the other. In earthworm, male sex organs mature prior to female gonads (protandry condition), therefore, cross fertilization occurs. In some hermaphrodites, self-fertilization is prevented by development of testes and ovaries at different times. Meiosis is an essential feature of a sexual cycle. It results in the formation of daughter cells, each with half the number of chromosomes of the parent cell. During fertilization, the nuclei of two gamete cells fuse and the zygote thus formed has a fixed number of chromosomes for each species. In all organisms, this number of chromosomes represents the diploid condition (2n). If meiosis does not occur, fusion of gametes would result in doubling of the chromosomes for each successive sexually reproduced generation. This situation is prevented by reduction in the diploid number of chromosomes (2n) to haploid number (n) during gametogenesis. Table : Chromosome Numbers of Some Eukaryotes I. II. Fungi III. Protists Plants Haplopappus gracilis Ophioglossum Pencillium sp. Neurospora Saccharomyces Dictyostelium Amoeba proteus Garden pea Maize Rice Tomato Wheat Sugarcane (Adder’s tongue fern) (normally haploid) Yeast ( ) (normally haploid) Slime mold ( ) Organism Somatic Chromosome Number IV. Animals A. Vertebrates B. Insects C. Roundworm Frog Rat Man Chimpanzee Gorilla Cow Horse Dog Chicken Fruit fly ( ) Honey bee (male) Honey bee (female) (male) (female) Silkworm Geometrid moth (male) (female) Drosophila Caenorhabditis Periplaneta americana Periplaneta americana Caenorhabditis 4 14 20 24 24 42 80 1260 4 7 17 7 250 Somatic Chromosome Number 26 42 46 48 48 60 64 78 78 8 16(n) 32(2n) 33 34 56 224 12 11 (ii) Gamete transfer : In dioecious animals, male and female gametes are formed in different individuals. For fertilization to occur, sperm and egg must come together. Animals have evolved different strategies for this. For example, many animals secrete sex pheromones to attract their partners. Insects like the silkworm moth (Bombyx mori) produce tiny amounts of very volatile pheromones that diffuse very long distances. Only male moths of this species are attracted towards female moths by a distinctive alcohol (bombykol) produced by the female. Several marine invertebrates release their gametes into water, thus there is no need for the parents to make direct contact. In others, mating is important to bring male and female gametes in close contact.
4 Reproduction in Organisms & Human Reproduction NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 B. Syngamy and Fertilization The most important step in the process of sexual reproduction is the fusion of male and female gametes. Although the terms syngamy and fertilization are used synonymously but the actual act of fusion of gametes is syngamy whereas fertilization includes all the events that ultimately lead to syngamy. The result of syngamy and fertilization is the formation of a diploid zygote which is the vital link that ensures continuity of species between organisms of one generation to the next generation. C. Post-fertilization Events In all sexually reproducing organisms, fusion of male and female gametes results in the formation of a zygote. In organisms with external fertilization, zygote is formed outside the body, usually in water as in frogs, bony fish etc. In organisms exhibiting internal fertilization, zygote develops inside the body of the organism. Embryogenesis : Development of embryo from the diploid zygote is known as embryogenesis. Embryonic development is a complex process which involves cell division and cell differentiation. These events proceed according to the genetic information contained in the zygote and ultimately lead to the formation of mature animals. The developing embryo grows in size at the expense of food derived from outside. The mammalian embryo is nourished by the placenta. Depending upon the development of zygote inside or outside the body of the female parent, organisms have been classified into oviparous or viviparous. Oviparous organisms lay eggs (e.g., some species of sharks, skates, bony fishes, frog, lizards, birds), the yolk in the egg supplies food to the embryo. In viviparous organisms, on the other hand, development of fertilized egg into embryo takes place within the uterus of the female parent and the offspring is born as a juvenile. The embryo in viviparous organisms receives nourishment from the mother's blood through placenta. Many species of sharks are ovoviviparous. In such organisms, the eggs are incubated within a modified portion of the oviduct called uterus and the young ones are born alive after hatching. During development they depend on stored yolk for their nourishment. Parthenogenesis :- [Gk.-‘parthenos’ - virgin; ‘genesis’ = origin]. It is a specialized from of reproduction in which egg develops without fertilization. It may be complete (e.g., rotifers and certain wasps) where it is the only form of reproduction, or incomplete [e.g., bees, ants, wasps, aphids, Artemia (brine shrimp), Cypris etc] where it alternates between cyclic and non-cyclic. It is called arrhenotoky when only males are produced parthenogenetically e.g. honeybees, turkey (drones are always haploid), thelytoky if only females result from parthenogenesis (e.g. Lacerta, whiptail lizard) and amphitoky if eggs produce individuals of either sex. e.g. Aphis. (Plant lice). HUMAN REPRODUCTION SYSTEM The reproductive system of sexually reproducing animals consists of primary sex organs, i.e testes and ovaries which produce gametes; secondary sex organs such as prostate and seminal vesicles in the male and uterus and fallopian tubes in the female. The latter participate in reproduction but do not form gametes. The accessory sex organs help to distinguish the two sexes in appearance. THE MALE REPRODUCTIVE SYSTEM The male reproductive system is located in the pelvic region. It includes a pair of testes along with accessory ducts, glands and external genitalia. The Testes  The testes are situated outside the abdominal cavity within a pouch called scrotum. The scrotum helps in maintaining the low temperature of the testes (2 – 2.5°C lower than the normal internal body temperature), which is necessary for spermatogenesis. The slightly cooler temperature of the scrotum is necessary for the development of normal sperms.