Unicellular organisms reproduce asexually mainly by binary fission, multiple fission and budding.
- Binary fission: the parent cell divides into two similar daughter cells (by mitosis or amitosis). It occurs in favourable conditions. Bacteria, Amoeba, Paramecium and Euglena reproduce this way. Amoeba has no fixed shape, so it divides in any plane (simple binary fission); Paramecium shows transverse binary fission and Euglena longitudinal binary fission.
- Multiple fission: in adverse conditions Amoeba stops forming pseudopodia, becomes round and forms a protective covering called a cyst. Its nucleus divides repeatedly, followed by division of cytoplasm, forming many amoebulae. When favourable conditions return, the cyst bursts and the amoebulae are released.
- Budding: in yeast, a unicellular fungus, the nucleus divides by mitosis and a small bulge (bud) appears on the parent cell. One daughter nucleus enters the bud; the bud grows and separates to live as a new yeast cell.