An organism may be either a prokaryote or a eukaryote. In prokaryotes, illegitimate recombination results in a mutation of the genetic sequence of the prokaryote. PCC6803 was the third prokaryote and first photosynthetic organism whose genome was completely sequenced. The simplest known circadian clocks are bacterial circadian rhythms, exemplified by the prokaryote cyanobacteria. A prokaryote is defined as having no cell nucleus or other membrane bound-organelle. This prokaryote produces oxygen as a byproduct of its photosynthetic processes. A prokaryote has a very simple cell structure with no internal organelles. These sequences are derived from DNA fragments of bacteriophages that had previously infected the prokaryote. Even though there are a few prokaryote kexin-like peptidases, all kexins are eukaryotes. Several prokaryote serpins are found in extremophiles. It is Gram-positive, non-spore forming and non-motile prokaryote. The cellular structure of AFA is that of a simple prokaryote. "Salmonella typhimurium" is a prokaryote, therefore it is not a perfect model for humans.