In terms of metabolic impact and numbers, prokaryotes dominate the biosphere. They outnumber all eukaryotes combined. They live in a diversity of environments and a teaspoon of common dirt can harbor 100 million or more bacteria. Not only are bacteria plentiful in total numbers, but species diversity is high as well. Although recognizing distinct species of bacteria is a challenge for microbiologists, modern approaches using DNA diversity analysis suggest that new species of bacteria evolve quite rapidly. Recent studies suggest that in that same teaspoon of soil there could reside up to 1 million different species.
In some cases, prokaryotes can divide in as little as 20 minutes (although much slower rates are also observed). Generally, prokaryotes have three factors that enable them to grow and reproduce rapidly. First, prokaryotes have a small genome (genetic material). Second, prokaryotes have relatively simple structure. Third, prokaryotes reproduce via binary fission (cell division in which a prokaryotic chromosome replicates and the mother cell pinches in half to form two new daughter cells) as shown in this figure.
Binary fission is simply cell division in which two identical offspring each receive a copy of the original, single, parental chromosome. Therefore, binary fission is a type of asexual reproduction (reproduction that does not require the union of two reproductive cells, and that produces offspring genetically identical to the parent cell). A population of rapidly growing prokaryotes can synthesize their DNA almost continuously, which aids in their fast generation times. Even as a cell is physically separating, its DNA can be replicating for the next round of cell division, as seen in the animation below.
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These three factors (small genome, simple morphology, and binary fission) allow for a short generation time when compared to eukaryotic cells. This short generation time means that evolutionary changes occur relatively quickly when compared to longer-lived species.
Compared to eukaryotes, prokaryotes usually have much smaller genomes. On average, a eukaryotic cell has 1000 times more DNA than a prokaryote. This means that less DNA must be replicated (copied) with each cell division in prokaryotes.
The DNA in prokaryotes is concentrated in the nucleoid . The prokaryotic chromosome is typically a double-stranded DNA molecule that is arranged in a single large ring. What shape are eukaryotic chromosomes?
Prokaryotes often have smaller rings of extrachromosomal DNA termed plasmids (these are also found in a few eukaryotes). Most plasmids consist of only a few genes. Plasmids are not required for survival in most environments because the prokaryotic chromosome programs all of the cell's essential functions. However, plasmids may contain genes that provide resistance to antibiotics, metabolism of unusual nutrients, plasmids that confer virulence, and other special functions. Plasmids replicate independently of the main chromosome, and many can be readily transferred between prokaryotic cells.