Mesocaryotes

Mesocaryotes include dinophytes - dinoflagellates .

Mesocaryotes already have a clearly differentiated nucleus, however, in its structure some primitive features inherent in the nucleoid have been preserved. A similar duality manifests itself in other features of cell organization.

The mesocaryotic nucleus, called the dinocaryon , contains from 5 to 284 “chromosomes” and is characterized by a significant DNA content (3-200 pg), which in kinetic parameters resembles eukaryotic, but enriched with 5-hydroxymethyluracil (3-19 mol.%).

"Chromosomes" are constantly condensed, that is, molecular genetic processes are carried out in these morphologically stable structures. No histones and nucleosome organization were found in them, although a small number of histone-like proteins were found that were not homologous to either histones or histone-like prokaryotic proteins (the protein / DNA ratio was 0.1, while in other eukaryotes it was close to 1). The distribution of "chromosomes" during cell division is apparently mediated by their contact with the intact nuclear membrane .

There is no evidence of any period of DNA synthesis similar to the S phase of eukaryotic interphase. It is possible that transcriptional activity is limited by the peripheral diffuse region of the “chromosomes” of mesocaryotes. The type of organization of the genetic apparatus of mesocaryotes can be considered evolutionarily not only as a transition from prokaryotes to eukaryotes, but also as an independent branch of development from common ancestors, such as ancient archaebacteria, with eukaryotes.

According to molecular phylogeny, dinoflagellates do not represent a group that separated early from common ancestors with other eukaryotes. They are the closest relatives of ciliates and sporozoans, forming with them the monophyletic taxon of the alveolate (Alveolata). In recent years, genes encoding two histones, H3 and H2A, have been found in dinoflagellate cells ^[1] .

• Rizzo PJ Those amazing dinoflagellate chromosomes // Cell Research. - 2003. - Vol. 13. - P. 215-217. - DOI : 10.1038 / sj.cr.7290166 .