Cell nucleus

Cell Nucleus

The cell nucleus is a membrane-bound organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The nucleus maintains the integrity of these genes and controls the activities of the cell by regulating gene expression.

Structure

Diagram of a human cell nucleus.

The nucleus is composed of several key structures:

Nuclear Envelope

The nuclear envelope is a double membrane that encloses the entire organelle and separates its contents from the cellular cytoplasm. The outer membrane is continuous with the endoplasmic reticulum.

Nuclear Pores

Diagram of a nuclear pore complex.

Nuclear pores are large protein complexes that cross the nuclear envelope, allowing the transport of molecules across the nuclear envelope. This transport includes the movement of RNA and ribosomal subunits from the nucleus to the cytoplasm and the import of proteins, carbohydrates, signaling molecules, and lipids into the nucleus.

Chromatin

Chromatin is the complex of DNA and protein found in the nucleus, which packages the DNA into a more compact, denser shape. It is divided into euchromatin and heterochromatin, which differ in their degree of condensation and transcriptional activity.

Nucleolus

The nucleolus is a region within the nucleus that is involved in the production of ribosomes. It is the site of rRNA synthesis and ribosome subunit assembly.

Nuclear Matrix

The nuclear matrix is a network of fibers found throughout the inside of a cell nucleus and is thought to provide structural support and may be involved in the regulation of gene expression.

Cajal Bodies

Overview of a Cajal body.

Cajal bodies are spherical sub-organelles found in the nucleus of proliferative cells. They are involved in the biogenesis of snRNPs and other RNA-related processes.

Function

The primary functions of the nucleus include:

• Gene Expression Regulation: The nucleus controls the expression of genes by regulating the transcription of DNA into RNA.
• DNA Replication: The nucleus is the site of DNA replication, where the cell's genetic material is duplicated before cell division.
• RNA Processing: The nucleus is involved in the processing of pre-mRNA into mature mRNA, which is then exported to the cytoplasm for translation.

Transport Mechanisms

Diagram of the Ran-GTP cycle involved in nuclear transport.

Transport across the nuclear envelope is mediated by the nuclear pore complexes and involves a variety of transport receptors. The Ran-GTP cycle is a key regulator of nucleocytoplasmic transport.

Chromosome Territories

Chromosome territories in a mouse cell nucleus.

Chromosome territories are regions of the nucleus preferentially occupied by particular chromosomes. This spatial organization is thought to play a role in the regulation of gene expression.

Transcription Factories

Diagram of a transcription factory.

Transcription factories are discrete sites within the nucleus where active transcription occurs. These factories contain clusters of RNA polymerases and associated transcription factors.

Related Pages

• Eukaryotic cell
• DNA replication
• Gene expression
• Ribosome
• Chromosome