- Formation of new organism of same species by earlier existing organism is called as reproduction.
- It is also one of the various reasons responsible for evolution of each species.
- In living organisms, reproduction occurs mainly by two methods.
- Those two methods are-
- Asexual and
- Sexual reproduction.
Asexual reproduction :
A. Asexual reproduction in unicellular organisms
1. Binary Fission :
- Prokaryotes (Bacteria), Protists (Amoeba,Paramecium, Euglena, etc.) and eukaryotic cell-organelle like mitochondria and chloroplasts perform asexual reproduction by binary fission.
- In this process, the parent cell divides to form two similar daughter cells.
- Binary fission occurs either by mitosis or amitosis.
- Axis of fission / division is different in different protists.
- Ex.: Amoeba divides in any plane due to lack of specific shape; hence it is called as ‘simple binary fission’.
- Paramecium divides by ‘transverse binary fission’.
- Euglena by ‘longitudinal binary fission’.
- Binary fission is usually performed by living organisms during favourable conditions i.e. availability of abundant food material.
2. Multiple Fission :
- Asexual reproduction by multiple fission is performed by Amoeba and other similar protists.
- Amoeba stops the formation of pseudopodia.
- It becomes rounded and forms protective covering around plasma membrane.
- Such encysted Amoeba or any other protist is called as ‘Cyst’.
- Many nuclei are formed by repeated nuclear divisions in the cyst.
- It is followed by cytoplasmic division and thus, many amoebulae are formed.
- They remain encysted till there are adverse conditions.
- Cyst breaks open on arrival of favourable conditions and many amoebulae are released.
3. Budding:
- yeast cells performing budding i.e. a small bud coming out of many parent cells.
- Asexual reproduction occurs by budding in yeast- a unicellular fungus.
- Yeast Cell produces two daughter nuclei by mitotic division, so as to reproduce by budding. This Yeast cell is called as parent cell.
- A small bulge appears on the surface of parent cell.
- This Bulge is actually a bud.
- One of the two daughter nuclei enters this bud.
- After sufficient growth, bud separates from the parent cell and starts to live independently as a daughter yeast cell.