Nucleus
The nucleus is the cell's command centre, storing genetic information, and is vital for DNA replication and RNA synthesis and processing. It is typically the largest organelle within a cell
- I. Structure of the Nucleus The nucleus comprises three main components:
- nuclear envelope,
- chromatin (DNA and associated proteins)
- nucleolus
- Nuclear Envelope:
- A double membrane that selectively separates the nucleoplasm from the cytoplasm.
- The outer membrane is continuous with the rough endoplasmic reticulum (RER) and has polyribosomes attached.
- Nuclear Pore Complexes (NPCs) regulate the bidirectional movement of macromolecules between the nucleus and cytoplasm.
- The nuclear lamina, a meshwork of intermediate filaments, stabilises the envelope and associates with chromatin
Chromatin:
- A complex of DNA, protein (primarily histones), and RNA.
- Functions: To package DNA into a smaller volume, reinforce it for mitosis, prevent DNA damage, and control gene expression and DNA replication.
- Types:
- Euchromatin: Finely dispersed, lightly stained, contains actively transcribed DNA.
- Heterochromatin: Coarse, dark-stained, represents tightly coiled and inactive DNA.
- Sex Chromatin (Barr Body): A dense mass of heterochromatin found in female mammalian cells, representing an inactive X chromosome.
- Packaging: DNA wraps around histone molecules to form nucleosomes. These nucleosomes then form a more compact 30-nm chromatin fibre, leading to higher-level packaging.