Lecture Notes on Eukaryotic Cell Nucleus

Overview

• The nucleus is a membrane-enclosed organelle in eukaryotic cells.
• It stores, protects, and expresses most genetic information (DNA).
• A small portion of DNA is also found in mitochondria.

Nuclear Structure and Composition

Nuclear Envelope

• Encloses the nucleus and is known as the nuclear membrane or nuclear envelope.
• Consists of an inner and outer lipid bilayer.
• Perinuclear Space: Fluid space between the inner and outer membranes.
• Nuclear Pores: Protein complexes that allow transport of materials into and out of the nucleus.

Outer and Inner Membrane

• Outer Membrane: Connected to the rough endoplasmic reticulum, facilitating RNA transport.
• Inner Membrane: Faces the nucleoplasm and is covered by intermediate filaments.
  • Nuclear Lamina: A network of proteins stabilizing the nucleus and involved in gene expression.

Nuclear Pores

• Protein complexes extending throughout the nuclear membrane.
• Facilitate transport of biomolecules (e.g., RNA, ribosomal units, polymerase).

Nucleolus

• Central region of the nucleus, occupying ~25% of its space.
• Contains RNA molecules and proteins for synthesizing ribosomal RNA (rRNA).

DNA Composition in the Nucleus

• Nucleus stores, protects, and expresses the DNA (deoxyribonucleic acid).
• Linear DNA wound around proteins called histones.
  • Eight histones form a nucleosome.
  • Nucleosomes coil and supercoil into chromatin.
• Chromatin: Condensed form of DNA, which is unwound when the DNA is actively used for transcription.

Summary

• Most DNA in the nucleus is in a condensed supercoiled state (chromatin) when not in use.
• Transcription involves unwinding small sections of DNA.