Dinokaryon
![Nucleus with a dark nucleolus. The cell usually measures between 20 and 30 micrometers.](/Images/godic/202501/09/Oxyrrhis_marina0353.jpg")
A dinokaryon is an eukaryotic nucleus present in dinoflagellates in which the chromosomes are fibrillar in appearance (i.e. with unmasked DNA fibrils) and are more or less continuously condensed.
The nuclear envelope does not break down during mitosis, which is thus termed closed mitosis. The mitotic spindle is extranuclear.