Question

Describe the structure of the following with the help of labelled diagrams.
(i) Nucleus

(ii) Centrosome

Answer 1

(i) Nucleus

The nucleus is a double wall membrane structure. It regulates the cellular activities . It constitutes the following parts-

Nuclear membrane : It is a double membrane separating the contents of the nucleus from the cytoplasm. The narrow space between the two membranes is called the perinuclear space. Nuclear membrane has tiny holes called nuclear pores. These holes allow specific substances to be transferred into a nucleus and out from it.

Nucleoplasm/Nuclear matrix : It is a homogenous granular fluid present inside the nucleus. It contains the nucleolus and chromatin. The nucleolus is a spherical structure that is not bound by any membrane. It is rich in protein and RNA molecules and is the site of ribosome formation. Chromatin is an entangled mass of thread-like structures. It contains DNA and some basic proteins called as histones.

(ii) Centrosome

Centrosome has two cylindrical structures called centrioles. Each has a cartwheel-like organisation. A centriole is made up of microtubule triplets in a ring. The adjacent triplets are linked together. There is a proteinaceous hub in the central part of a centriole. The hub is connected to the triplets via radial spokes. These centrioles help in organising the spindle fibres and astral rays during cell division. They form the basal body of cilia and flagella as well.