Cell Nucleus - Introduction, History, Structure

Introduction to Cell Nucleus

The cell nucleus is a membrane-bound organelle present in eukaryotic cells and absent from prokaryotic cells. These eukaryotic cells mostly have a single nucleus (except mammalian red blood cells that lack nuclei) while others such as osteoclasts (bone cells) have many. The main structure of the nucleus is enclosed in a double membrane called a nuclear envelope that separates its content from the cellular cytoplasm and the nuclear matrix.

The cell nucleus contains nearly all of the cell’s genome i.e. nuclear DNA, which is organized into multiple chromosomes. The main function of the cell nucleus is to maintain the integrity of the genes, control the activities of the cell by regulating gene expression, and mediate the replication of DNA during the cell cycle.

History of cell nucleus

The cell nucleus was first described by Antonie van Leeuwenhoek in the 16th century, making it the first cell organelle to be discovered. He observed the nucleus in the red blood cells of salmon.

In 1804, Austrian microscopist and botanical artist Franz Bauer described the cell nucleus. It was detailed again in 1831 by Scottish botanist Robert Brown after studying orchids. Matthias Schleiden in 1838 concluded nucleus plays a role in generating cells and called the cell nucleus cytoblast.

Structure of cell nucleus

The cell nucleus contains mostly all of the cell’s DNA and is surrounded by a nuclear matrix – which is a network of fibrous intermediate filaments. It is covered by a double membrane called a nuclear envelope that separates fluid inside the nucleus from the rest of the cell.

The cell nucleus is the largest organelle in animal cells and its size is co-related to the size of the cell. In many eukaryotic cells, the cell nucleus occupies 10% of the cell volume.

The cell nucleus consists of the following structures:

Nuclear envelope

• Consists of two membranes – an inner membrane, and an outer membrane that is perforated by nuclear pores.

• Separates a cell’s genetic material from other cell components and maintains a distinct environment.

• The inner nuclear membrane surrounds nuclear content and provides a defining edge.

• The outer nuclear membrane encloses the inner nuclear membrane, is continuous with adjacent endoplasmic reticulum membrane, and is studded with ribosomes.

Nuclear pores

• Nuclear pore complexes (NPCs) are large protein complexes present in the nuclear envelope of eukaryotic cells.

• NPCs are composed of approximately thirty different proteins called nucleoporins.

• Provides selective transport of molecules between the nucleoplasm and the cytosol.

Nuclear lamina

• The nuclear lamina is organized meshwork on the internal face of the envelope or less organized support on the cytosolic face of the envelope.

• It provides the cell nucleus with mechanical support and anchor sites for chromosomes and nuclear pores.

• Composed of lamin proteins and is found on the cytosolic face of the membrane, such as emerin and nesprin.

Chromosomes

• The cell nucleus contains the majority of the cell’s genetic material known as chromosomes that are organized in the form of multiple linear DNA molecules.

• During most cell cycles, genetic materials are organized in a DNA-protein complex called chromatin.

• A small fraction of genes are located in the mitochondria.

• Two types of chromatin are present - Euchromatin and heterochromatin.

• Euchromatin contains genes that are frequently transcribed and are less compact in DNA form.

• Heterochromatin contains genes that are infrequently transcribed and are more compact in DNA form.

Nucleolus

• The nucleolus forms around tandem repeats of rDNA, DNA coding for ribosomal RNA (rRNA).

• The main function is to synthesize rRNA and assemble ribosomes.

Other nuclear bodies

• Cajal bodies and gems

• PIKA and PTF domains

• PML-nuclear bodies

• Paraspeckles

• Perichromatin fibrils

• Clastosomes

• Splicing speckles