Nucleus

Introduction

The nucleus is a membrane-bound organelle found in eukaryotic cells. It stores most of the cell’s genetic material as DNA, organized into chromosomes. Functions include DNA replication, transcription, RNA processing, and regulation of gene expression. It acts as the command center, coordinating cell growth, metabolism, and reproduction.

"The nucleus is the control room of the cell, where genetic instructions are stored and executed." -- Bruce Alberts

Structure

General Morphology

Typically spherical or oval. Size: 5–10 μm diameter. Number varies by cell type (usually one nucleus per cell).

Components

Encased by nuclear envelope; contains nucleoplasm, chromatin, nucleolus. Contains nuclear matrix for structural support.

Physical Properties

Viscous nucleoplasm: aqueous solution with ions, enzymes, nucleotides. Dynamic compartment with selective transport.

Nuclear Envelope

Structure

Double lipid bilayer membrane: inner and outer membranes. Outer membrane continuous with rough endoplasmic reticulum.

Function

Separates nucleoplasm from cytoplasm. Regulates molecular traffic via nuclear pores. Maintains nuclear integrity.

Associated Proteins

Lamina: intermediate filament network providing mechanical support. Nuclear membrane proteins anchor chromatin and nuclear pore complexes.

Chromatin

Composition

DNA + histone proteins + non-histone proteins. Forms chromosomes during mitosis.

Types

Heterochromatin: dense, transcriptionally inactive. Euchromatin: loose, transcriptionally active.

Organization

Nucleosomes: DNA wrapped around histone octamers. Higher-order folding controls gene accessibility.

Nucleolus

Definition

Dense, spherical structure within nucleus. Not membrane-bound.

Function

Synthesis and assembly of ribosomal RNA (rRNA) and ribosomal subunits.

Organization

Composed of fibrillar centers, dense fibrillar components, granular components. Site of rDNA transcription and ribosome biogenesis.

Nuclear Pores

Structure

Large protein complexes embedded in nuclear envelope. Diameter ~120 nm. Composed of ~30 nucleoporins.

Function

Selective bidirectional transport of macromolecules: RNA, proteins, ribosomal subunits.

Transport Mechanism

Passive diffusion for small molecules (<40 kDa). Active transport for larger molecules via nuclear localization/export signals and transport receptors.

Transport Cycle:1. Cargo binds importin/exportin.2. Complex translocates through pore.3. RanGTP/GDP regulates cargo release.4. Transport receptor recycles.

DNA Replication

Timing

Occurs during S-phase of cell cycle within nucleus.

Process

Semiconservative mechanism: DNA helicase unwinds strands; DNA polymerase synthesizes complementary strands.

Replication Origins

Multiple origins per chromosome. Replication forks proceed bidirectionally.

Transcription

Definition

Synthesis of RNA from DNA template.

Enzymes Involved

RNA polymerase II transcribes mRNA. RNA polymerase I and III transcribe rRNA and tRNA.

Regulation

Transcription factors bind promoters/enhancers. Chromatin remodeling modulates accessibility.

Transcription Steps:1. Initiation: RNA polymerase binds promoter.2. Elongation: RNA strand synthesis 5’→3’.3. Termination: RNA polymerase releases transcript.

RNA Processing

Pre-mRNA Modifications

5’ capping, splicing out introns, 3’ polyadenylation.

Spliceosome

Complex of snRNPs catalyzing intron removal.

Quality Control

Processed mRNA exported to cytoplasm. Defective RNAs degraded by nuclear surveillance.

Role in Cell Cycle

Interphase

Nucleus intact. DNA replication and transcription active.

Mitosis

Nuclear envelope breaks down during prophase. Chromosomes condense. Reforms in telophase.

Regulation

Cell cycle checkpoints monitor DNA integrity. Nuclear events coordinate progression.

Molecular Regulation

Gene Expression Control

Epigenetic modifications: DNA methylation, histone acetylation. Control accessibility and transcriptional activity.

Signal Transduction

Nuclear receptors respond to external signals. Modify transcriptional programs.

Non-coding RNAs

Regulate chromatin structure, mRNA stability, translation.

Clinical Significance

Nuclear Abnormalities

Altered nuclear morphology indicates cancer, laminopathies, and other diseases.

Genetic Disorders

Mutations in nuclear proteins cause progeria, muscular dystrophies.

Therapeutic Targets

Nuclear transport inhibitors and epigenetic drugs used in cancer therapy.

References

• Alberts B., Johnson A., Lewis J., et al. Molecular Biology of the Cell. 6th ed. Garland Science; 2014. pp. 589-630.
• Lodish H., Berk A., Kaiser C.A., et al. Molecular Cell Biology. 8th ed. W. H. Freeman; 2016. pp. 345-370.
• van Steensel B. Chromatin: the interface between genome and nuclear organization. Trends Cell Biol. 2011;21(9):517–525.
• D’Angelo M.A., Hetzer M.W. Structure, dynamics and function of nuclear pore complexes. Trends Cell Biol. 2008;18(10):456–466.
• Dechat T., Adam S.A., Taimen P., Shimi T., Goldman R.D. Nuclear lamins. Cold Spring Harb Perspect Biol. 2010;2(11):a000547.