Question

Describe the structure of the following with the help of labelled diagram :
(i) Nucleus
(ii) Centrosome

Answer 1

Nucleus: Nucleus is relatively a large organelle controlling all the activities of the eukaryotic cells. Nucleus is a specialized double membrane bound protoplasmic body which contains all the genetic information of controlling cellular metabolism and transmission to the posterity.

A nucleus in non-dividing phase is called interphase nucleus and a typical interphase nucleus is differentiated into nuclear envelope, nucleoplasm or nuclear matrix, nucleolus and chromatin.

• Nuclear envelope: It bounds the nucleus on the outside and separates it from the cytoplasm. It is made up of two membranes-outer and inner. The inner membrane is smooth whereas the outer membrane may be smooth or its cytoplasmic surface may bear ribosomes like the RER. The outer membrane is often connected to ER. These two membranes of the nuclear envelope are separated by a space known as perinuclear space. The space is 10 to 50 nm in width.

Nuclear envelope contains a large number of complex pores which are formed by the fusion of its two membranes. The nuclear pores control the passage of substances to inside or outside of the nucleus e.g., RNA, ribosomes, proteins.

• Nucleoplasm: It is a transparent, semi-fluid and colloidal substannce which fills the nucleus. It contains nucleolus and highly extended and elaborate nucleoprotein fibres called chromatin.
• Nucleolus: It is spherical structure found in the nucleoplasm. It is not separated from rest of the nucleoplasm as it is not bound by a membrane. It is the site for ribosomal RNA synthesis.
  
• Chromatin: The interphase nucleus contains a loose, extended and diffused network of nucleoprotein fibres called chromatin. Chromatin fibres condense to form chromosomes.

Centrosome: An organelle that serves as the main microtubule organizing center (MTOC) of the animal cell is known as centrosome. It is also found in cells of some lower plants that takes part in formation of spindle and basal bodies of cilia or flagella. Centrosomes are composed of two orthogonally arranged centrioles which are surrounded by an amorphous mass of protein termed as pericentriolar material. A centrosome lies near the nucleus. It has a non-granular cytoplasmic region called centrosphere and two cylindrical structures named centrioles. The two centrioles lie at right angles to each other. Each centriole has a whorl of nine triplet fibrils of fused microtubules that run parallel to one another but at an angle of 40 degree. The centre of the centriole has a proteinaceous rod called hub. Hub is connected to peripheral triplets by means of proteinaceous strands known as spokes. A thickening called X is present on the spoke near the triplet. Another thickening Y is found nearby. Y is attached to X as well as C-A linker between the adjacent triplets. The whole complex gives a cart-wheel appearance.