Biologists classify all cells into two broad categories: prokaryotes, which include the bacteria, and eukaryotes, which include all plant, animal, and fungi cells. The main distinction between the two categories of cells is the presence or absence of a nucleus. Prokaryotes do not have a nucleus; all of their genetic material is contained in a single circular strand of DNA that floats freely in the cytoplasm. Eukaryotes, in contrast, have a nucleus, which contains long, complex chains of DNA called chromosomes.

The presence or absence of a nucleus is important when it comes to cell division. Prokaryotic cells, like all single-celled organisms, divide in order to reproduce. Because all of a prokaryote's genetic material is confined to a single ring of DNA, the process of replication is relatively simple. It begins at a fixed location on the ring called the replication origin. Here, the DNA molecule begins to "unzip," forming two half-segments of DNA, called strands. Just as quickly, replacement nucleotides are added to the strands, creating two identical molecules of DNA. The two identical DNA molecules then attach to different areas of the interior of the cell membrane. When the cell divides, each resulting daughter cell receives one of the two molecules.

The process of replicating and dividing genetic material is much more complex in eukaryotes than it is in prokaryotes. A typical eukaryotic cell contains about a thousand times more DNA than a prokaryotic cell. And instead of being neatly contained in a single ring, eukaryotic DNA forms many distinct molecules, called chromosomes. Human cells have 46 chromosomes. To ensure that each daughter cell receives one and only one of each chromosome, eukaryotic cells undergo a complex, multistep process called mitosis. Before mitosis can begin, eukaryotes must first replicate their DNA. This process is similar to but takes much longer than DNA replication in prokaryotes. Then, during mitosis, the replicated chromosomes line up along the center line of the cell. Tiny fibers, called spindle fibers, attach to the individual chromosomes and pull them apart, toward opposite poles of the cell, where they remain until the cell divides.

The simplicity of DNA replication and cell division in prokaryotes allows these organisms to reproduce very quickly. Growing under optimal conditions, the common bacterium Escherichia coli, for example, can double its population size in just 20 minutes. (Contrast that with humans, who take on average 20 years to produce a single offspring.) Within weeks, a population of E. coli bacteria can virtually reinvent itself. And given that the time required for one individual to pass its genes to the next generation is one of the most important factors in the evolutionary process, this puts prokaryotes on an evolutionary fast track.

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