The basic steps of binary fission that we just saw—replication of DNA, segregation of replicated DNA to daughter cells, and division of one cell into two—occur in all forms of cell division. However, cell division in eukaryotes (by mitosis) is more complicated than cell division in prokaryotes (by binary fission). Compared with the single, relatively small, circular DNA molecule that is the genome of prokaryotic cells, the genome of eukaryotic cells is typically much larger and is organized into one or more linear chromosomes, each of which must be replicated and separated into daughter cells. And whereas the DNA of prokaryotes is attached to the inside of the plasma membrane, allowing replicated DNA to be separated into daughter cells by cell growth, the DNA of eukaryotes is located in the nucleus. As a result, eukaryotic cell division requires first the breakdown and then the re-formation of the nuclear envelope, as well as mechanisms other than cell growth to separate replicated DNA. As we saw in Chapter 10 and discuss in more detail in section 11.2, chromosomes of dividing eukaryotic cells attach to the mitotic spindle, which separates them into daughter cells.

Interestingly, some unicellular eukaryotes exhibit forms of cell division that have characteristics of binary fission and mitosis. For example, dinoflagellates, like all eukaryotes, have a nucleus and linear chromosomes. However, unlike most eukaryotes, the nuclear envelope does not break down but stays intact during cell division. Furthermore, the replicated DNA is attached to the nuclear envelope. The nucleus then grows and divides in a manner reminiscent of binary fission. These and other observations of intermediate forms of cell division in additional organisms strongly suggest that mitosis evolved from binary fission.