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

(i) Nucleus

• The nucleus is a double-membrane bound organelle found in all eukaryotic cells and is the control center of the cell.

• Structural components:

  • Nuclear envelope: A double membrane that encloses the nucleus and separates it from the cytoplasm. The outer membrane is continuous with the endoplasmic reticulum.

  • Nuclear pores: Present in the nuclear envelope, these regulate the exchange of materials (like RNA and proteins) between the nucleus and cytoplasm.

  • Nucleoplasm (karyoplasm): The jelly-like fluid inside the nucleus that contains chromatin and the nucleolus.

  • Chromatin: A network of DNA and proteins (histones). During cell division, chromatin condenses into chromosomes.

  • Nucleolus: A dense, spherical structure involved in the synthesis of rRNA and the formation of ribosomal subunits.

(ii) Centrosome

• The centrosome is an organelle found only in animal cells, located near the nucleus. It plays a key role in organizing microtubules and in cell division.

• Structural components:

  • Centriole pair: The centrosome contains two cylindrical structures called centrioles, placed at right angles to each other. Each centriole is made up of nine triplets of microtubules arranged in a circular pattern.

  • Pericentriolar material: This amorphous region surrounds the centrioles and contains proteins required for microtubule nucleation.

• During cell division, centrosomes form the spindle fibers that help in chromosome separation.