The principal mode of reproduction is an asexual separation of one bacterium into two. There are, in addition, several mechanisms that make possible the exchange of genetic material, the DNA, among and between bacterial cells. None, however, are as elaborate as the mitosis—meiosis choreography of gene exchange in the eukaryotes.
Three common types of asexual reproduction are present:
- Binary fission (the most common): The “chromosome” replicates and the two genomes move to opposite ends of the cell. The old cell walls then grow inward between the two, pinching the cell apart—no mitosis, no microtubules. The whole process is over within 30 minutes to three hours.
- Fragmentation: This occurs when filaments of cells break into separate pieces or fragments.
- Budding: An outgrowth ( bud) pushes out from the cell, pinches off, and then enlarges into a new cell.
With no nuclei, there can be no sexual reproduction in the prokaryotes, but there is an exchange of DNA. In one type, conjugation, conjugation (“sex”) pili (bridges of cytoplasm) form between cells, and some DNA is passed from the donor to the recipient cell.
In bacterial communities some DNA exists outside of cells, presumably left when the cells die, or more probably, excreted into the environment by living cells. This “free” DNA is picked up bacterial cells in another kind of gene exchange, transformation.
A third type of exchange— transduction—results when bacteriophages (special kinds of viruses) bring into bacterial cells the DNA from their previous viral host. (Researchers in biotechnology use the same method to introduce new genes into host organisms.)
Random changes in the DNA are common. These mutations of the genetic code alter the response of the individual to its environment. If the mutation is deleterious, the individual dies, but favorable mutations spread rapidly as the cells divide repeatedly and often.