A mesokaryote or mesokaryotic organism is a single-celled

eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

that shows intermediate resemblance to both

prokaryote A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...

s and 'higher' eukaryotes. The term originates from a 1965 hypothesis by John David Dodge, who proposed that certain eukaryotes (mainly

dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...

s) with

closed mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

and other traits considered 'primitive' were an intermediate step between prokaryotes and the remaining eukaryotes. This idea originated in the late 20th century, and was later disproven by more detailed ultrastructural studies in the following decades.

History

The first investigations of the

nucleus Nucleus (: nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucleu ...

, during the 1950s-1960s, revealed a fine nucleus and chromosome structure that was completely different from other nucleated organisms or

s, lacking

histone In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei and in most Archaeal phyla. They act as spools around which DNA winds to create structural units called nucleosomes ...

s and with a permanently condensed

chromatin Chromatin is a complex of DNA and protein found in eukaryote, eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important r ...

. Based on these findings, the phycologist John David Dodge proposed in 1965 the concept of Mesocaryota (or mesokaryotes) under the hypothesis that these features were an intermediate nuclear organization between

s and eukaryotes. This hypothesis led to the theory that dinoflagellates were the first to evolve from the split with prokaryotes, followed by the remaining eukaryotes. The traits considered by Dodge to define Mesocaryota were: lack of detectable histones; absence of a

mitotic spindle In cell biology, the spindle apparatus is the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a process ...

; continuous

DNA synthesis DNA synthesis is the natural or artificial creation of deoxyribonucleic acid (DNA) molecules. DNA is a macromolecule made up of nucleotide units, which are linked by covalent bonds and hydrogen bonds, in a repeating structure. DNA synthesis occu ...

; chromatin fibrils arranged in arched swirls as in

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

nucleoid The nucleoid (meaning '' nucleus-like'') is an irregularly shaped region within the prokaryotic cell that contains all or most of the genetic material. The chromosome of a typical prokaryote is circular, and its length is very large compared to ...

s; and chromosomes permanently condensed persistently adhered to the

nuclear envelope The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus, which encloses the genetic material. The nuclear envelope consists of two lipid bilayer membran ...

, which remains intact throughout

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

(i.e., it is a

). The mesokaryote hypothesis was disproven in the following decades through more detailed observations of the criteria listed above. For example, detailed studies on the parasitic ''

Syndinium ''Syndinium'' is a cosmopolitan genus of parasitic dinoflagellates that infest and kill marine planktonic species of copepods and radiolarians.Chatton, E. 1910: The existence of coelom Dinoflagellate parasites. The Syndinium in pelagic copepods. ...

'' demonstrated the presence of an unconventional type of basic histone-like proteins and of an extranuclear mitotic spindle in dinoflagellates, similarly to 'higher' eukaryotes. Dinoflagellates remained considered a group of ancient but true eukaryotes. With the improvement of

molecular phylogenetics Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

, dinoflagellates, like other groups that exhibited closed mitosis, were instead revealed to be derived, branching within the Alveolata, whose members have conventional nuclei. Thus, these traits were reinterpreted as highly derived. Due to its short lifespan, the mesokaryote hypothesis has had little impact.

References

{{reflist, 30em Eukaryotes Obsolete biology theories