Question

Mesokaryon is
(a) A nucleus like structure
(b) Nucleoid with histone protein
(c) A nucleus with condensed chromosomes
(d) A nucleoid with distinct chromosomes

Answer 1

Hint: Mesokaryon can be exclusively found in a unicellular organism which is neither plant nor an animal. They are eukaryotic and mostly aquatic. This class of organisms are an integral portion of phytoplanktons.

Complete answer:
Mesokaryon is a nucleus with a condensed chromosome. It is composed of ‘meso’ and ‘karyon’ which means ‘middle’ and ‘nucleus’ respectively. It is a special type of nucleus found only in dinoflagellates which consist of condensed chromosomes. Dodger gave the term ‘mesokaryon’ to the nucleus of dinoflagellates because they exhibit the characters of both prokaryotic as well as the eukaryotic nucleus. The nucleus of dinoflagellates is well organised like that of eukaryotes but the nuclear division process resembles that of prokaryotes. Hence, the term mesokaryon was used to denote the nucleus of dinoflagellates.
Kingdom Protista is divided into three classes - Photosynthetic protists, Consumer- decomposer protists (slime moulds) and Protozoan protists. Dinoflagellates are photosynthetic protists. They represent one of the main groups of phytoplankton. Phytoplanktons are any green photosynthetic organisms which can be passively drifted by the water current. These include bacteria, dinoflagellates, diatoms and algae. Some characteristic features of dinoflagellates are as follows:
- Habitat: Most of them are marine but some can be found in freshwater.
- Nutrition: They are photosynthetic.
- Structure: They are basically unicellular, motile, and biflagellate. Cells are covered by a rigid coat of well-articulated and sculptured plates made up of cellulose. This is the reason they are also known as armoured dinoflagellates.
- Flagellation: Heterokont flagellation can be observed wherein the two flagella are different i.e a transverse flagellum and a longitudinal flagellum. They each beat in dissimilar directions.
- Reserve food: The food is stored in the form of starch and oils.
Examples: Gymnodinium, Gonyaulax, Ceratium
So, the correct answer is ‘nucleus with condensed chromosomes’.

Note: Some dinoflagellates such as Gymnodinium and are responsible for a unique condition known as ‘red tides’ which can be witnessed on the coastal waters. This is due to the explosion of the population of dinoflagellates enough to discolour the water into red and brown. This has been reported harmful in some cases for various aquatic organisms due to a toxin released by the dinoflagellates.