4-5-6 Nucleus

Cell Nucleus Overview

  • The cell nucleus is a membrane-enclosed structure found in eukaryotic cells.

  • Functions:

    • Contains hereditary information.

    • Controls cell growth and reproduction.

  • Size: Approximately 10 microns.

Structure of the Nucleus

  • Composed of:

    • Nuclear envelope: Two membranes with nuclear pores.

    • Chromatin: DNA-protein complex.

    • Nucleolus: Site of RNA synthesis.

Nuclear Envelope

  • Definition: Also known as nuclear membrane.

  • Composition:

    • Two parallel membranes separated by 10-50 nanometers (nm).

    • Contains a perinuclear space.

  • Function:

    • Provides a selectively permeable barrier between nucleus and cytoplasm.

    • Encloses chromatin.

Inner and Outer Membrane

  • Outer membrane:

    • Continuous with rough endoplasmic reticulum (rER).

    • Studded with ribosomes.

  • Inner membrane:

    • Supported by intermediate protein filaments called nuclear lamina.

    • Perinuclear space connects with the rER.

Nuclear Pores

  • Function:

    • Extensive transport of materials between nucleus and cytosol.

    • Selectively imports proteins (e.g., histones, DNA/RNA polymerases) and exports ribonucleoproteins, tRNAs, mRNAs.

  • Structure:

    • Consists of 50-100 proteins (nucleoporins).

    • Diameter of 100 nm.

    • Typical mammalian cell has 3000-4000 pores.

Nuclear Transport Processes

  • Import:

    • Proteins with nuclear localization signals (NLS) are imported.

    • Import complexes (importin α and β) transport proteins through nuclear pore.

    • GTP hydrolysis releases proteins in the nucleus.

  • Export:

    • Proteins targeted for export by nuclear export signals.

    • Exportins recognize signals and bind to Ran/GTP to facilitate transport.

Nuclear Matrix

  • Structure:

    • 3-dimensional filamentous network (skeleton) in the interphase nucleus.

    • Composed of:

      1. Lamina (outer matrix).

      2. Interchromatin network (inner matrix).

  • Function:

    • Organizes nuclear functional domains (e.g., DNA replication, mRNA transport).

Chromatin

  • Composition: Complex of DNA and proteins, responsible for the nucleus's basophilia.

  • Forms:

    • Heterochromatin: Densely packed, often transcriptionally silent.

    • Euchromatin: Loosely packed, associated with active transcription.

Types of Heterochromatin

  • Constitutive: DNA sequences never transcribed.

  • Facultative: DNA sequences transcribed in some cell types but not in others (e.g., X-chromosome inactivation forms Barr body).

DNA Packing in Chromatin

  • Histones: Major DNA-binding proteins maintaining chromatin structure.

  • Nucleosome: Basic unit, consisting of DNA wrapped around histones.

  • Hierarchy of packing:

    • Nucleosomes → solenoid structures (30 nm) → looped domains → condensed chromatid during mitosis.

Chromosome Structure

  • Composed of two chromatids joined at the centromere.

  • Telomeres: Caps on the ends of chromosomes.

  • Contains 46 chromosomes in human somatic cells (22 pairs of autosomes + 1 pair of sex chromosomes).

Types of Chromosomes

  • Metacentric: Centromere in the middle.

  • Submetacentric: Centromere divides chromosome into unequal arms.

  • Acrocentric: Centromere near one end.

  • Telocentric: Centromere at the end.

Nucleolus

  • Most obvious structure in the nucleus.

  • Function: Site of rRNA transcription, processing, and ribosome assembly.

  • Composition: Large aggregate of macromolecules including rRNA genes and partially assembled ribosomes.

Content and Function of Nucleolus

  • Contains multiple rRNA gene clusters distributed across five different chromosomes in human cells.

  • Pre-rRNA is processed in dense fibrillar component and assembled into ribosomal subunits in the granular component.

  • Eukaryotic ribosomes: Composed of 5S, 5.8S, 18S, and 28S rRNAs.

Structural Dynamics of Nucleolus

  • Varies in size; well-developed in protein-synthesis-active cells.

  • Processing of pre-rRNA continues during cell cycle events such as mitosis, influencing the nucleolus's activity.

Summary of Nucleolar Activity

  • In mitosis: rRNA genes gather at nucleolar organizing region (NOR).

  • Following mitosis, nucleolus resumes rRNA transcription activity.