Chapter Eight : Reproduction

(1) There are two main theories about the origin of life. (2) Reproduction in living organisms takes place in two main levels: A- Molecular duplication in which the cellular molecules multiply. B- The whole organism. (3) There are two main methods of reproduction: A- A sexual reproduction that includes: 1- Binary fission e.g Amoeba; paramecium simple algae and bacteria. 2- Budding e.g yeast, hydra, sponge 3- Regeneration e.g: Sponge, Hydra star fish - planaria. 4- Sporogony e.g: Mushroon - some algae, bread mould, and some ferns. 5- Parthenogensis e.g some worms, crustaceans and insects. 6- Tissue culture: e.g carrot, tobacco. B- Sexual reproduction which may be: 1- By conjugation e.g spirogyra alga 2- By gametes e.g higher plants and animals. (4) Alternation of generation is the phenomenon through which some living organisms reproduce sexually followed by one or two cycles asexually. e.g plasmodium (protozoan), schistosoma (worms) and fern plants (Adiantum), from the benefits of alternation of generation is the production of great number of life stages because most of them are lost or damaged during the life cycle, the two types of reproduction enable the living organism to adapt with changes in environmental conditions. (5) The flower emerges from the axil of a leaf called bract (may be absent) (6) A typical flower has four floral whorls A- Calyx "Sepals" B- Corolla "petals" C- Androecium "Stamens" male organs D- Gynoecium "Carpals" female organs (7) Flowers of most monocot plants the calyx leaves are hardly differentiated from the corolla leaves so they are called perianth. (8) Monoecious plants are those that carry male and female flowers. (9) Diecious plants are those that male flowers exist on one plant and female ones exist on another plant. e.g palm tree (10) Flowers are divided according to their symmetry into: A- Actinomorphic e.g. onion. B- Zygomorphic e.g. petunia (11) The function of the flower is reproduction, the stages of reproduction in flowering plants are: 1- Formation of pollen grains 2- Formation of ovules 3- Pollination 4- Fertilization. (12) Pollination could be: A- Self pollination B - Cross pollination which takes place when: 1- The flowers are unisexual 2- Early maleness or early femaleness 3- Height of another is lower than the stigma. (13) The pollin grain germinates on the stigma forming a pollen tube which penetrates the stigma and passes through the style till it reaches the ovule micropyle, the tube nucleus degenerates and the generative nucleus divides forming two male nuclei, one of them fuses with the two polar nuclei (or embryo sac nucleus) forming the endosperm nucleus which divides to give the endosperm tissue, it supplies the embryo with food, the other male nucleus fuses with the egg cell forming the zygote. This process is called double fertilization. (14) We can divide seeds into: A- endospermic seeds e.g wheat, dates B- exoendospermic seeds e.g bean, pea (15) After fertilization a- The two integuments will form the testa b- The ovary wall becomes the pericarp c- The ovary will form the fruit d- Other floral whorls wither and fall down except in cases that they share in fruit formation. (16) If the two integuments are fused with the ovary wall "pericarp" This forms single seeded fruit or grain e.g. Maize - wheat. (17) Parthenocarpy means that some ovaries can develop naturally into seedless fruits without pollination or fertilization. E.g. Banana, pine, Apple, it can be carried out artificially e.g. tomato and cucumber by some stimulants e.g. indole acetic acid or naphthol acetic acid. (18) The male reproductive system in man has two functions: A- Production of sperms. (300 - 500 millions for each ejaculation) B- Production of male hormone which is responsible for the appearance of secondary sexual characters. (19) The position of testes outside the body exists in some mammals like primates and ungulates, because spermatogenesis needs lower temp. (20) The female reproductive system in human consists of: a- Two ovaries. b- Two oviducts c- The uterus d- the vagina. (21) The female reproductive system has the following functions: a- Production of ova ("about 400 ova" during the fecundity period) b- production of female hormones estrogen and progesterone. c- safe place for the development of the embryo. (22) The breeding cycle of placental mammals is a period where the ovary becomes active and the uterus becomes ready for fertilization, this cycle is periodic and coincide with mating and reproduction: a- Lion and tiger its annual (12 month) b- Cats and dogs biannual (6 month) c- Rabbits - rats every month. d- Human being every 28 days. (23) The menstrual cycle is differentiated into ovarian cycle and uterine cycle. (24) Menstruation cycle is divided into 3 phases: 1- Phase of proliferation (10 days) 2) Phase of ovulation (14 days) 3- Phase of menstruation (3 - 5 days) (25) Pregnancy occurs if the ovum is fertilized in the second phase (ovulation). (26) During pregnancy the corpus luteum remains to secrete the progesterone and estrogen, these two hormones inhibit the ovulation, preserve the endometrium and stimulate the mammary glands to secrete milk after delivary. (27) Contraceptive methods of birth control: A- The oral contraceptives "pills" which are combination of synthetic estrogen and progesterone. B- Intra uterine device "Coil" which is inserted into the lumen of the uterus and prevents the implantation of the ovum. (28) The inner membrane "amnion" and the outer membrane "chorion" are formed around the embryo in addition to the extension of the endometrium. (29) Both the chorionic villi and the endometrium intermingle to form the placenta. (30) After short time both margins of the amnion join forming the umbilical cord which extends from the placenta to the intestine of the embryo. (31) The mother's blood and foetus blood do not mix but their vessels are only adjacent to allow movement of substances by diffusion. (32) There are two types of twins: A- Fraternal "Two ova and two sperms" . B- Identical "one ovum and one sperm" - One mature ovum and one sperm, then dividing cells separate into two masses they are similar in all genetic characters, some time they are attached together form the head. Abdomen or chest this case is called siamese. - Extra - uterine fertilization "test tube babies" is done in 1977 when scientists succeeded to get a mature ovum from a wife's ovary and fertilized it externally with the husband's semen in a certain nutritive medium. - Embryo culture exp. have been conducted on rabbits and mice, the cells from their early embryonic stages were separated and cultured as separate masses of cells in a female's uterus to proudce complete individual. - Renucleation is a type of embryo cutlute through which the nucleus is removed from an embryo body cell of an Amphibian. Then the nucleus of an unfertilized ovum is destroyed by radiations, then the nucleus from the somatic cells of the embryo is implaned in the ovum which develops normally and give individuals identical to individual from which the cultured nucleus was taken. - Gamete banks are used to store gametes of selected animals to keep them available for external fertilization. - The gametes are stored in, frozen conditions up to 20 years. - Centrifugation or exposure to limited electric field can separate the sperms with X chromosomes from sperms with Y chromosomes.