CHAPTER 3 HOW DO ORGANISMS REPRODUCE?
The basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions to build copies of their DNA. The DNA copies eventually separate, each with its own cellular apparatus. A cell, therefore, divides to give rise to two cells.
MODES OF REPRODUCTION USED BY SINGLE ORGANISMS
For unicellular organisms, cell division, or fission, leads to the creation of new individuals. Many different patterns of fission have been observed. Many bacterial and protozoa simply split into equal halves during cell division. In organisms such as amoeba, the splitting of the two cells during division can take place in any plane.
However in some unicellular organisms, binary fission occurs in a definite orientation in relation to these structures. Other unicellular organisms divide into many daughter cells simultaneously by multiple fission.
In multi-cellular organisms with relatively simple body organisation, simple reproductive methods can still work. For e.g. Spirogyra simply breaks up into smaller pieces upon maturation. These fragments grow into new individuals.
Many organisms have the ability to give rise to new individual organisms from their body parts. For e.g. Simple animals like Planaria can be cut into any number of pieces and each piece grows into a complete organism. This is called regeneration. Regeneration is carried out by specialized cells.
Organisms such as Hydra use regenerative cells for reproduction in the process of budding. In Hydra, a bud develops as an outgrowth due to repeated cell division at one specific site. These buds develop into tiny individuals and when fully mature, detach from the parent body and become new independent individuals.
5. Vegetative Propagation
In this method, parts like the root, stem and develop into new plants under appropriate conditions.
6. Spore formation
Spores are covered by thick walls that protect them until they come into contact with another moist surface and can begin to grow.
All these methods listed above are asexual mode of reproduction.
In sexual reproduction, two individuals are involved in the reproduction process. It incorporates the process of combining DNA from two different individuals during reproduction.
Sexual reproduction in flowering plants
Stamens and carpels are the reproductive parts of a flower which contain the germ cells. Stamen is the male reproductive part and it produces pollen grains that are yellowish in colour. Carpel is the female reproductive part and it is present in the centre of the flower. It is made up of three parts. The swollen bottom part is the ovary, middle elongated part is the style and the terminal part which may be sticky is the stigma. The pollen needs to be transferred from the stamen to the stigma where the male germ cell produced by the pollen grain fuses with the female gamete present in the ovule which is contained in the ovary.
Sexual reproduction in humans
Changes in the body during puberty are signs of sexual maturation. The male reproductive system in human beings consists of testes, vas deferens, seminal vessels, prostate glands, urethra and penis. The testes are responsible for producing sperms. The female reproductive system in human beings consists of ovaries, fallopian tubes, uterus and vagina. Sexual reproduction in human beings involves the introduction of sperm in the vagina of the female. Fertilisation occurs in the fallopian tube.
Contraception to avoid pregnancy can be achieved by the use of condoms, oral pills, copper-T and other methods.
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