Mesokaryon is

(a) A nucleus like structure

(b) Nucleoid with histone protein

(c) A nucleus with condensed chromosomes

(d) A nucleoid with distinct chromosomes

Mesokaryon refers to a type of nucleus characterized by condensed chromosomes. The term is derived from “meso” (meaning middle) and “karyon” (meaning nucleus). This nucleus is unique to dinoflagellates and was named by Dodger, since it displays features of both prokaryotic and eukaryotic nuclei.

• Like eukaryotes, the nucleus of dinoflagellates is well-organized.
• However, their nuclear division process resembles that of prokaryotes.

Because of this dual nature, the term mesokaryon was introduced to describe the dinoflagellate nucleus.

Dinoflagellates and Their Features

Dinoflagellates belong to the Kingdom Protista, specifically the class of photosynthetic protists, and they form an important component of phytoplankton. Phytoplankton includes green photosynthetic organisms like bacteria, diatoms, dinoflagellates, and algae that drift passively with water currents.

Characteristic Features of Dinoflagellates:

• Habitat: Mostly marine, though some occur in freshwater.
• Nutrition: Primarily photosynthetic.
• Structure: Unicellular, motile, and biflagellate organisms. Their cells are enclosed by a rigid coat of cellulose plates, giving rise to the name armoured dinoflagellates.
• Flagellation: Show heterokont flagellation, with two different types of flagella—one transverse and one longitudinal—beating in different directions.
• Reserve Food: Stored in the form of starch and oils.
• Examples: Gymnodinium, Gonyaulax, Ceratium.

Therefore, the correct answer is: “nucleus with condensed chromosomes.”