At each turn of a cell cycle, a bacterium or archaeon converts molecules taken from the environment into its own components and divides into two daughter cells. Each daughter cell receives a copy of the genome as well as copies of plasmids and nongenetic components. Mitochondria and chloroplasts proliferate by a similar division process, and, although they are organelles within eukarya, they reproduce rather than being made de novo by the cell. A population of genetically identical life forms made by this process is a clone.
Under poor or stressful environmental conditions, some bacteria form spores – specialized resting cells – by the process of sporulation. A few species of bacteria develop into multicellular organisms containing different cell types. Both sporulation and the formation of multicellular bacteria are simple examples of cell differentiation, which happens as a result of regulated changes in gene expression.
The cell cycle of bacteria has two phases, C (chromosome copying) and D (division). On average, cells double in size between divisions. The cell cycle is tightly regulated and division is precise, ensuring that the chromosomes, cytoplasm, and cell envelope are synthesized at the right rate and are equally apportioned between the two daughter cells. During the C phase the chromosome(s) and plasmids replicate, and components of the cytoplasm such as ribosomes, tRNAs, and enzymes are approximately doubled in number. During the D phase, the cell envelope grows, daughter chromosomes segregate (move apart), a contractile ring pinches the cell in two, and a septum forms between the incipient daughter cells.