All living things on Earth can be divided into two major groups. One, the prokaryotes, came into being close to three and a half billion years ago and includes two domains of organisms, the Bacteria and the Archaea. These are simple, mostly single-celled organisms that have only a small amount of genetic material and reproduce asexually, meaning that there is no systematic genetic diversity in a given prokaryote species in the absence of chance mutations; all of a given prokaryote's descendants are genetically identical. They reproduce using a process called binary fission.
The domain Eukaryota, in contrast, includes animals, plants and fungi, and is made of mostly multicellular creatures. Their genetic material is divided into units called chromosomes that are contained in a membrane-bound nucleus, and they are rich in specialized internal structures called organelles. Eukaryotic cells feature a cell cycle and reproduce sexually using the processes of mitosis and cytokinesis. A few exceptions to the "only prokaryotes undergo binary fission" rule, however, do exist.
Prokaryotic Cells vs. Eukaryotic Cells
Prokaryotic cells have only a small amount of genetic material, which in all known life forms is DNA (deoxyribonucleic acid). This DNA often assumes the form of a circular chromosome that sits in the cytoplasm, or the jelly-like matrix that makes up the substance of the cell inside its external cell membrane and the wall exterior to the membrane. The cytoplasm also contains ribosomes, which make proteins upon instructions from the DNA.
Eukaryotic cells have, in addition to the nucleus, a wealth of other membrane-bound organelles. These include mitochondria, Golgi bodies, an endoplasmic reticulum and (in plants) chloroplasts. Unlike prokaryotic cells, these cells make use of aerobic ("with oxygen") respiration as well as anaerobic ("without oxygen") respiration, which accounts for the vastly greater size of eukaryotic organisms.
Prokaryotic cell division is characterized by the fact that the segregation of DNA occurs in concert with the splitting of the whole cell (and hence the organism, in almost all cases). In eukaryotes, the DNA is replicated, or copied. and is then divided in mitosis, while the cell itself divides afterward in cytokinesis.
Binary Fission Examples
While the term "binary fission" most often refers to the splitting in two of an entire single-celled organism, it refers more generally to any cellular process that results in the simple non-sexual duplication of an entity within a cell. When eukaryotes prepare for cell division, they first replicate everything but their DNA, in addition to growing larger in general.
Mitosis and the Cell Cycle
A eukaryotic cell begins its life as one of two daughter cells formed in cytokinesis. It then undergoes several phases, collectively termed the cell cycle:
- G1, in which the cell replicates all of its organelles and grows larger.
- S, in which the chromosomes in the nucleus replicate.
- G2, in which the cell checks its work.
- M, which includes mitosis and cytokinesis.
Mitosis of the M phase itself includes distinct stages: prophase, metaphase, anaphase and telophase. Here, the nuclear membrane dissolves, the replicated chromosomes are pulled apart and new membranes form around the identical daughter nuclei. Cytokinesis, which actually starts during anaphase, is completed soon after the telophase of mitosis, and the cell cycle is complete.
Binary Fission in Eukaryotes
A class of single-celled eukaryotes called protozoans, which include the amoeba and the paramecium, are very "prokaryote-like" except for the presence of organelles, although not all of the organelles are present. These organisms often reproduce by binary fission rather than mitosis.
This fission can take a number of forms. Among these is budding, in which there are two daughter cells markedly unequal in size; intracellular budding, in which the daughter arises inside the organism rather than simply splitting off; and multiple fission (also called segmentation), which features a number of sequential cycles of nuclear replication not followed by cytokinesis, resulting in a multinucleated cell that can then give rise to multiple progeny at the same time.