Reproduction is an integral feature of all living beings. The process by which a living being produces its own like is called reproduction.
Types of Reproduction:
There are two main types, viz. asexual and sexual reproduction.
Asexual Reproduction: When a single parent is involved and no gamete formation takes place; the method is called asexual reproduction. No meiosis happens during asexual reproduction.
Sexual Reproduction: When two parents are involved and gamete formation takes; the method is called sexual reproduction. Meiosis happens during gamete formation; which is an important step of sexual reproduction.
Reproduction in Simple Organisms
Binary Fission: Most of the unicellular animals prefer this method for reproduction. These organisms reproduce by binary fission; especially when conditions are favourable, i.e. adequate amount of food and moisture is available. Binary fission is somewhat similar to mitosis. The mother cell divides into two daughter cells, and each daughter cell begins its life like a new individual. The parent generation ceases to exist, after binary fission. Amoeba is a very good example of the organism which reproduces by binary fission.
Multiple Fission: When conditions become unfavourable, i.e. food, moisture, proper temperature, etc. are not available; this is the preferred mode of reproduction by unicellular organisms. The organism develops a thick coating around itself. This is called cyst. The cyst helps the organism to tide over the bad phase. The nucleus divides into several nuclei and each daughter nucleus is surrounded by a membrane. All metabolic activities stop in the organism, after cyst formation. When favourable conditions return, the cyst dissolves or breaks down; releasing the daughter nuclei. The daughter nuclei; in turn; grow into new individuals. Plasmodium and entamoeba undergo cyst stage, when they are not in the body of their prime host, i.e. humans.
Budding: Yeast is an example of unicellular organism which reproduces by budding. Hydra is an example of multicellular organism which reproduces by this method.
Budding in Yeast: A small bud grows at any end of the yeast cell. Nucleus gets elongated and a part of it protrudes into the bud. The nucleus then divides into two nuclei. One of the nuclei goes into the bud. The bud grows to certain extent and gets detached from the mother cell.
Budding in Hydra: A bud grows anywhere on the main body of hydra. The bud grows to a certain size and gets detached from the mother hydra. This develops further to grow into a new individual.
Fragmentation: Reproduction by fragmentation is seen in filamentous algae, e.g. spirogyra. The filament of spirogyra divides into many pieces and each piece develops into a new individual.
Regeneration: Some simple organisms can regenerate a new individual from a part which has been accidentally cut off. Planaria show a good example of reproduction by regeneration. If a planarium is cut into several pieces; each piece develops into a new individual.
Spore Formation: Most of the fungi, bryophytes and pteridophytes reproduce by this method. Spores are produced in special spore-bearing organs; called sporangium. When spores mature; the sporangium bursts open to release them.
Vegetative Propagation: Vegetative propagation is a special case, as it happens in higher plants; which otherwise have the capability to reproduce sexually. When a vegetative part of a flowering plant reproduces a new plant, it is called vegetative propagation. Some examples of vegetative propagation are given below.
Tuber of Potato: The potato tuber is a modified stem. Many notches can be seen on the surface of potato. These are called ‘eyes’ of potato. Each ‘eye’ of a potato can give rise to a new potato plant. Farmers utilize this capability of potatoes to grow potato more quickly; which is not possible by using the seeds of potato.
Modified roots of Carrot and Sweet Potato: Carrot and sweet potato are examples of modifications of roots; for food storage. These roots can give rise to new plants; when kept under the soil.
Rhizomes of Ginger and Turmeric: Rhizomes are examples of modified underground stems for food storage. These contain nodes, internodes and scaly leaves. When buried under the soil, the rhizomes produce new plants.
Artificial Vegetative Propagation: Man has used artificial vegetative propagation to grow many plants. This has enabled farmers and horticulturists to grow many plants in shorter duration and has helped them to earn more profit. Artificial vegetative propagation has also helped in developing many new varieties of plants. Stem cutting, layering and grafting are the preferred means of artificial vegetative propagation.
Sexual Reproduction and Variations:
As discussed earlier, sexual reproduction involves two parents and gamete formation. Gametes are special cells which are formed after meiosis. There are two types of gametes, viz. male and female gametes. The number of chromosomes is haploid in the gametes. When gametes fuse during fertilization, the number of chromosomes becomes diploid. This is important for maintaining the unique identity of a particular species which reproduces by sexual method.
In sexual reproduction, the offspring gets sets of genes from two different individuals. This leads to subtle variation through subsequent generations. These variations accumulate over thousands of generations and finally may give rise to a new species. That is how all complex organisms have evolved from a common ancestor.
Sexual Reproduction in Flowering Plants:
Flower is a modified leaf which bears special organs and plays the role of the reproductive system in plant.
Structure of a typical Flower:
A typical flower is composed of four distinct whorls, viz. calyx, corolla, androecium and gynoecium.
Calyx: The outermost whorl of the flower is called calyx. It is composed of green leaf-like structures; called sepals.
Corolla: The second whorl of the flower is called corolla. It is composed of colourful leaf-like structures; called petals. Petals are colourful so that insects and birds can be attracted; to assist the flower in pollination.
Androecium: This is the third whorl in the flower. It is composed of stamens. Stamen is made of a slender stalk and anthers on top. Anthers produce the pollen-grains. Pollen grains are the male gametes.
Gynoecium: This whorl is at the centre of the flower. It is composed of a swollen base; called ovary. A slender style stands upright on the ovary. It has a flat top; called stigma. Ovules are inside the ovary. Ovules are the female gametes.
Pollination: The pollen grains need to be transferred to the stigma so that fertilization can take place. The transfer of pollen grains from anther to the stigma is called pollination. If the pollen grains from the same flower or the same plant are transferred to the stigma; it is called self-pollination. If pollen grains from a different plant are transferred to the stigma; it is called cross-pollination. Cross-pollination is better; from the perspective of variations. Many agents help plants in cross-pollination, e.g. insects, animals, air, water, etc. Insects are the main pollinators for the plant kingdom.
The fusion of male and female gametes is called fertilization. The product of fertilization is called zygote. Zygote undergoes several rounds of mitosis and develops into an embryo. Subsequently, the embryo develops into a new individual.
Fertilization in flowering plant:
After landing at the stigma, pollen grains absorb moisture and germinate. A pollen grain develops a pollen tube; which penetrates through the tissue of the style and reaches the ovule. Pollen nuclei are transferred through the pollen tube. After fertilization, zygote is formed; which finally develops into the embryo.
Changes in flower; After Fertilization: The calyx and corolla wither and fall off and so do the stamens. The ovary turns into the fruit. The embryo turns into seed. Once the seed becomes mature, fruit dries up so that dispersal of seeds can take place.
Reproduction in Human Beings
Male Reproductive System
The male reproductive system in human beings is composed of following parts:
Testis: There is a pair of testes; which lie in a skin pouch; called scrotum. The scrotum is suspended outside the body; below the abdominal cavity. This helps in maintaining the temperature of testes below the body temperature. This is necessary for optimum sperm production. Testis primarily serves the function of sperm production. Sperms are the male gametes. Apart from that, testis also produces testosterone. Testosterone is also called the male hormone, as it is responsible for developing certain secondary sexual characters in boys.
Vas Deferens: Vas deferens is the tube which carries sperms to the seminal vesicle.
Seminal Vesicle: This is the place where sperms are stored. Secretions from the seminal vesicle and prostate gland add up to make the semen.
Penis: It is a muscular organ which serves the genitor-urinary functions. The urethra works as the common passage for urine as well as for sperms.
Female Reproductive System:
The female reproductive system in human beings is composed of following parts:
Uterus: This is pear-shaped hollow muscular organ. Uterus is the place where the embryo gets implanted and develops into a newborn baby. The wall of the uterus provides safety and nutrition to the growing foetus.
Fallopian Tubes: One fallopian tube comes out from each side at the top of the uterus. The fallopian tubes end in finger-like structures; called fimbriae. Fertilization happens in the fallopian tube.
Ovary: There are two ovaries; one near each fallopian tube. Ovary produces the eggs or the female gametes. All the eggs are produced by the ovary when the female child is still in the womb. One egg matures in each ovulation cycle and is released from the ovary. The egg is caught by the flimbriae and transferred to the fallopian tube.
Vagina: The cervix (mouth of the uterus) opens into the vagina. Vagina is a muscular tube-like organ; which serves as the passage for the sperms and also as the canal during the child birth.
Human beings are different than other animals because they have the power of thinking. They have to obey certain moral values and need to behave sensibly in most of the aspects of life. Beginning of puberty does not mean that a person is psychologically ready for the process of reproduction. For a human being, reproduction involves more than just producing an offspring. As any act of sex has the potential of fertilization, so taking care of contraception becomes important. Moreover, the act of sex also has the potential of creating many sexually transmitted diseases. Examples of STDs are; gonorrhoea, Herpes, syphilis, AIDS, Hepatitis B. AIDS and Hepatitis B are incurable to date. Even the curable STDs are potentially dangerous; not only physically but also psychologically.
Reproductive health involves preventing the chances of STDs and preventing unwanted children. Reproductive health means a couple should be able to enjoy the reproductive phase of its life; without taking the burden of the gigantic family.