[Biology Class Notes] on Organisms Reproduction Pdf for Exam

All living organisms reproduce to maintain continuity of life. Reproduction is a biological process by which living organisms produce new organisms. It is the way by which generations of species continue to live. Without reproduction, the species will go extinct. It ensures continuity of genes over the next generations and also introduces new gene variations, which is crucial for the survival of organisms over a long period. Reproduction is an important process for the existence of life on Earth.

Two different modes of reproduction shown by the various living organism on Earth are-

• Asexual reproduction- The type of reproduction through which a single individual produces copies of itself without any genetic contribution.

• Sexual reproduction-  The type of reproduction in which a new organism is created by a combination of genetic material from two parents. 

Asexual Reproduction

A single parent gives rise to a new individual in asexual reproduction. No formation of gametes or genetic materials is involved in this process. The resulting organism is an identical copy of its parent. The organism which reproduces asexually is generally capable of splitting on their own. Multicellular organisms that reproduce asexually inherit the genes from a single parent.

The types of asexual reproduction are as follows-

Fission

The parent cell divides in this process and the resultant cell/cells regenerate the parts to become an individual. Fission is of two types, depending on the number of cells forming.

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• Binary Fission- The parent cell divides into two cells in binary fission. The parent cell duplicates the genetic material and divides it into two parts before dividing it. Each resultant cell has its nucleus and the potential to grow into an individual. Binary fission occurs under favorable conditions like the availability of food and water.

• Multiple Fission- It is the process in which a single parent cell produces multiple copies. The nuclei of the parent cell divide into several nuclei each surrounded by a cell membrane. When the cytoplasm separates, multiple copies are created giving rise to multiple daughter cells. It mostly occurs under unfavorable conditions. Example- Algae.

Budding

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New organisms are formed as a result of bud or outgrowth formation from cell division on certain sites on the body of an organism. The resultant individuals remain attached to the parent organism until they mature. They detach on maturity and leave scar tissue behind. Examples- Yeast, Hydra, Coral, Jellyfish, etc.

Vegetative Propagation

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Plants reproduce by vegetative propagation wherein; a new plant grows from the fragment or special reproductive structure of the parent plant. It is the type of reproduction in which the reproducing plants give rise to clones of itself. This type of reproduction may take place naturally or can be induced artificially. Artificial means include grafting, cutting, layering, naturally by means of runners, suckers, bulbs, etc.

Spore Formation

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Under certain unfavorable conditions, organisms form spores. Spores can be dormant or reproductive. Dormant spores help the organisms survive in worst conditions, whereas, the reproductive spores develop into new individuals without fusion with any reproductive cell. The organisms which develop a sporangium under unfavorable conditions spores inside the sporangium. When conditions get favorable again these spores develop into individuals. Example- fungi, protozoa, etc. 

Regeneration

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Few organisms have the capability to develop into a new individual from part of a parent individual. When parts of these organisms are cut into several pieces, each part has the capability to develop into a new individual. The organisms have specialized cells that proliferate and give rise to many cells. These cells undergo development and form tissues and thus give rise to a new individual. Example- Starfish, Hydra, Planaria, etc.

Sexual Reproduction 

Sexual reproduction is the method of reproduction shown by all multicellular organisms. In this type, the fusion of gametes takes place to give rise to a new individual. The gametes are from two different parents, mostly male and female. There is a mixing of genetic material in this process and the resultant individual has copies of parental DNA and a mixed genome. This type of reproduction introduces variations at the genetic level. 

Sexual Reproduction in Plants

Flowering plants or angiosperms reproduce by sexual reproduction. A flower is the reproductive organ of such a plant. It has male gametes in the form of pollen grains that fuse with the female gamete or egg. This process is called gametogenesis. Pollen grains travel from anther to the stigma of the flower by the process of pollination and reach the female gamete present in the ovule. The fusion of gametes leads to fertilization and formation of a zygote which develops into an embryo. The embryo turns into seeds inside fruits which when germinated grows into a new plant. 

Sexual Reproduction Animals

Sexual reproduction in animals also involves the fusion of gametes and the formation of a zygote. Male reproductive organ testes form sperms and female reproductive organ ovaries form an egg. Sperms fertilize the egg by fusing with it and give rise to a zygote. The zygote then develops into an embryo. Through various embryonic stages, the embryo develops into a young one. This process involves the mixing of genetic material from both the parental genes. 

Therefore, the resultant offspring has a different set of genes for every trait called alleles. The offspring has one allele for each trait from each parent. The mixing of genes is the key to evolution. 

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