Nucleoplasm

Nucleoplasm, also known as karyoplasm, is the semi-fluid substance that fills the nucleus of eukaryotic cells, excluding the nucleolus. It lies within the nuclear envelope and surrounds the chromatin and various nuclear bodies, providing a solvent-like matrix for nuclear activities.

The nucleoplasm contains water, ions (such as potassium, magnesium, and calcium), nucleotides, and small metabolites, along with a network of proteins. It includes histones and other chromatin-associated proteins, enzymes involved in DNA replication, transcription, and RNA processing, and the nuclear matrix that helps organize chromatin. Nuclear bodies such as Cajal bodies and speckles reside in the nucleoplasm and participate in RNA maturation and processing.

Chromatin fibers are suspended within the nucleoplasm, enabling regulated access to DNA for transcription and replication. The nucleoplasm provides the environment for RNA synthesis by RNA polymerases, RNA processing, and ribonucleoprotein assembly. It also participates in the dynamic organization of chromatin through histone interactions and scaffolding structures, influencing gene expression and genome stability. The exchange of molecules between the nucleoplasm and the cytoplasm occurs via nuclear pores in the surrounding nuclear envelope.

The nucleolus is a distinct substructure within the nucleus and is not part of the nucleoplasm, although it interacts with nucleoplasmic components. The term nucleoplasm emphasizes the soluble nuclear content, distinct from the membrane boundary and the nucleolus, and contrasts with the cytoplasm outside the nucleus.