Unicellular organism

Unicellular organism is an organism that consists of a single cell. This includes most life on Earth, with bacteria being the most numerous. Unicellular organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. All life that has a cellular structure with a nucleus falls into the eukaryotic category, and life forms without a nucleus fall into the prokaryotic category.

Characteristics

Unicellular organisms are usually microscopic, less than one tenth of a millimeter long. There are exceptions, such as the giant amoeba Chaos carolinense, which can reach lengths of 5 mm, and the even larger Valonia ventricosa, a species of algae which can reach 1 to 4 cm.

Classification

Unicellular organisms are divided into two types: prokaryotic and eukaryotic organisms. Prokaryotes include bacteria and archaea, two of the three domains of life. Protozoans, most algae, and yeast are examples of eukaryotes.

Reproduction

Unicellular organisms reproduce through a process called binary fission. During binary fission, the organism duplicates its genetic material, or deoxyribonucleic acid (DNA), and then divides into two parts (cytokinesis), with each new organism receiving one copy of DNA.

Evolution

Unicellular organisms are believed to be the first form of life on Earth, with early examples including the bacteria-like fossils found in Western Greenland that are about 3.5 billion years old.

See also

• Cell (biology)
• Multicellular organism
• Colonial organism
• Superorganism

References

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