Core equivalent

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Comparison of the cell structure of eukaryotes and prokaryotes

The area in a prokaryotic cell ( protocyte ) that is filled with DNA is referred to as the core equivalent or nucleoid (also nucleoid , literally "the nucleus-like", sometimes the term nuclear sphere ) . In prokaryotes ( bacteria and archaea ), the DNA is a self-contained, tightly packed molecule that lies freely in the cytoplasm and is not surrounded by a nuclear envelope as in eukaryotic cells ( eucytes ) .

The nucleoid is functionally equivalent to the nucleus of the eucytes because it controls the growth , development and metabolism of the cell through gene expression . An important difference, however, is that the cell nucleus spatially separates transcription from translation , while these two processes occur in a coupled manner in the nucleoid of the bacteria. Nucleoids stand out clearly from the surrounding cytoplasm on electron microscopic images.

Nucleoids in organelles

A number of organelles of the eukaryotic cells - such as the plastids ( chloroplasts , leucoplasts , rhodoplasts etc.) on the one hand, or the mitochondria and hydrogenosomes on the other hand - have their own DNA genome , unless it is completely transferred to the nucleus of the eucyte in the course of evolution (such as the mitosomes from the mitochondrial relationship and almost all hydrogenosomes). This genome is also condensed in a nucleoid (or several), one then speaks of cp nucleoids (in plastids) or mt nucleodes (in mitochondria). The contour length of the DNA (the actual length of the unrolled molecular strand) in the chloroplasts of terrestrial plants is, for example, about 30 to 60  μm , with a total diameter of these organelles of about 4 to 8 μm.

Histone-like proteins

The DNA of bacteria, as well as of plastids and mitochondria, is not associated with real histones (these are only found in the cell nuclei of eukaryotes and in precursors in archaea). The dense packing of DNA in a nucleoid is accomplished in these groups by proteins whose function is therefore histonähnlich ( English histone like protein , HLP -.. D h analogous to histones). These are HU in bacteria (. Eg H-NS ), ABF2 in mitochondria and HC ( acronym for English h iStone-like protein of c hloroplast ) in the chloroplasts of red algae such as Cyanidioschyzon merolae ( Cyanidiales ). The histone-like proteins of these three groups are, in accordance with the endosymbiotic theory, homologous to one another ; H. a common evolutionary origin of HU, Abf2 and HC can be assumed. Between the nucleoids of the bacteria and the organelles derived from them (if they exist) on the one hand there is only one functional similarity to the cell nuclei of the eucytes on the other hand (analogy). The histone-like proteins found in archaea, on the other hand, show structural similarities (homology) with the real histones of the eucytes , which suggests that the eucrayons are derived from the archaea ( eocyte hypothesis ).

Individual evidence

  1. Mineralienatlas: Biological Taxonomy / Nomenclature
  2. ^ Leighton Then: Bioscience — Explained: Green DNA - Simple isolation, restriction and electrophoresis of chloroplast DNA . BIOSCINCE EXPLAINED, Science and Plants for Schools, Homerton College, Cambridge 2002. (via WebArchive)
  3. Jeremy Burgess: An introduction to plant cell development . Cambridge university press, Cambridge 1989, ISBN 0-521-31611-1 , p. 62.
  4. ^ Biology, 8th Edition, Campbell & Reece . Benjamin Cummings (Pearson), 2009, ISBN 978-0-321-54325-7 , p. 516.
  5. John M. Archibald: The Puzzle of Plastid Evolution . In: Current Biology . 19, No. 2, 2009, pp. R81-8. doi : 10.1016 / j.cub.2008.11.067 . PMID 19174147 .
  6. T. Kobayashi, M. Takahara, SY Miyagishima, H. Kuroiwa, N. Sasaki, N. Ohta, M. Matsuzaki, T. Kuroiwa: Detection and Localization of a Chloroplast-Encoded HU-Like Protein That Organizes Chloroplast Nucleoids . In: The Plant Cell Online . 14, No. 7, 2002, pp. 1579-1589. doi : 10.1105 / tpc.002717 . PMID 12119376 . PMC 150708 (free full text).
  7. Bram Henneman, Clara van Emmerik, Hugo van Ingen, Remus T. Dame: Structure and function of archaeal histones , in: PLOS Genetics, September 13, 2018, doi: 10.1371 / journal.pgen.1007582

See also