Cytology · MU Sofia

22. Nucleus. Structure of the nucleus. Chromatin. Chromosomes. Nucleolus.

Covers the nucleus as the cell's control centre, taking its three components in turn: the double-membraned nuclear envelope with its pore complexes and nuclear lamina, chromatin with its euchromatin, heterochromatin and Barr body types and its nucleosome packaging, and the nucleolus with its rRNA synthesis and ribosomal subunit assembly. It closes with chromosomes, sister chromatids, mitosis and meiosis, and karyotyping.

Inside: 3 sections · 5 labelled pictures · 16 flashcards · 3 min

Topic 22 of 127 in the MU Sofia syllabus for Cytology (Dental Medicine, Year 1). The free AceMed note covers it in 3 sections, about 371 words (3 min of reading), with 16 flashcards.

In this note

  1. Nucleus, its structure, chromatin, chromosomes, and the nucleolus:
  2. Nucleus
  3. Nucleolus:

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

  1. 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.

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