In bacteria the genetic material is organized in a continuous strand of DNA. This circle of
DNA is localized in an area called the nucleoid, but there is no membrane surrounding a defined
nucleus as there is in the eukaryotic cells of protists, fungi, plants, and animals. In addition
to the nucleoid, the bacterial cell may include one or more plasmids, separate circular strands
of DNA that can replicate independently, and that are not responsible for the reproduction of the
organism. Drug resistance is often conveyed via plasmid genes.
Reproduction is chiefly by binary fission, cell division yielding identical daughter cells. Some
bacteria reproduce by budding or fragmentation. Despite the fact that these processes should produce
identical generations, the rapid rate of mutation possible in bacteria makes them very adaptable.
Some bacteria are capable of specialized types of genetic recombination, which involves the transfer
of nucleic acid by individual contact (conjugation), by exposure to nucleic acid remnants of dead
bacteria, by exchange of plasmid genes, or by a viral agent, the bacteriophages. Under unfavorable
conditions some bacteria form highly resistant spores with thickened coverings, within which the living
material remains dormant in altered form until conditions improve. Others, such as the
radioactivity-resistant Deinococcus radiodurans, can withstand serious damage by repairing their own DNA.
Autotrophic bacteria manufacture their own food by the processes of photosynthesis and chemosysthesis.
The photosynthetic bacteria include the green and purple bacteria and the cyanobacteria. Many of the
thermophilic archaebacteria are chemosynthetic autotrophs.
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