The Nucleus

The nucleus is a prominent feature of eukaryotic cells that stores the genetic material, DNA. It is enclosed by a double-membrane structure called the nuclear envelope.

Inside the nucleus, DNA wraps around histone proteins to form chromatin, a DNA-protein complex, which allows DNA to be compactly stored in the nucleus. Chromatin is suspended in the nucleoplasm, a gel-like substance that fills the nucleus.

When the cell is not dividing, chromatin fibers are loosely arranged. They resemble tangled strings and allow easier access of RNA polymerase to DNA during transcription.

When the cell prepares to divide, chromatin fibers condense into chromosomes.

Within the nucleus, a distinct region called the nucleolus exists, which is responsible for the assembly of ribosomes. Here, ribosomal RNA is produced and combined with ribosomal proteins to form large and small ribosomal subunits.

A double-membrane nuclear envelope surrounds the nucleus, enclosing and protecting structures such as chromatin and the nucleolus. This envelope contains nuclear pore complexes made of many proteins. These complexes regulate the flow of ions, molecules, ribosomal subunits, and messenger RNA between the nucleus and the cytoplasm.

While most cells in the human body have a single nucleus, some cells, such as mature red blood cells, are anucleated. They lack a nucleus.

Other cells, such as skeletal muscle cells, are multinucleated. They have more than one nucleus to support structural and functional needs