Question

Microtubules do not occur in:
A. Mitochondria
B. Centrioles
C. Spindle fibres
D. Flagella

Answer 1

Hint:-Microtubules are hollow, non-branched microscopic structures of tubules with variable lengths. They are made up of protein tubulin and are present on specific nucleating sites. They are found in almost all the eukaryotic cells.

Complete step-by-step solution:-
Microtubules are nucleated and are composed of many sub-units (polymer) of the globular tubulin protein, and are present not only in the cytoplasm, but also in many other specialized structures. They are present as a single structure or in pairs, or even in bundles. Their main function is to maintain the shape of the cells and provide cytoskeleton to the cells. It also helps in transport of materials intracellularly, such as of secretory vesicles, organelles, etc. They are present in cilia and flagella, and make up its internal structure.
Microtubules are also present in centrioles and are involved in the process of cell division (both mitosis and meiosis). They are present in the spindle fibres that help in pulling the chromosomes apart during the cell division. Microtubules are also present in basal bodies, sensory hair, chromosome, fibres, spindle apparatus, etc. Thus it is clear that centrioles, spindle fibres and flagella contain microtubules while mitochondria do not.
So the correct option is (A) Mitochondria.

Note:- In centriole, a set of microtubules are present in 9 groups in a particular way. The microtubules extend from the centrioles and spindle fibres, and spread in all the directions possible.
In flagella, microtubules are present in a 9+2 arrangement in which nine pairs are present at the periphery and are fused together while the other two are present in the centre, and are not fused.