9.18 : Additional Subnuclear Structures

The eukaryotic nucleus is a double membrane-bound organelle that contains nearly all of the cell’s genetic material in the form of chromosomes. It is rightly called the “brain” of the cell as it shoulders the responsibility of responding to various physiological processes, stress, altered metabolic conditions, and other cellular signals.

The nucleus contains many membrane-less subnuclear organelles or nuclear bodies, such as nucleoli, Cajal bodies, speckles, paraspeckles, etc. These nuclear bodies help to spatially compartmentalize the nuclear environment and create distinct sites for some specific biological reactions. Besides, their membrane-less structure allows better exchange of their components with the nucleoplasm.

For example, Cajal bodies, which are spherical nuclear bodies inside the nucleus, contain various RNAs and proteins involved in the assembly and modification of small nuclear RNPs. It is inside these Cajal bodies that snRNPs undergo final maturation. The mature snRNPs then transit to the interchromatin granule clusters or nuclear speckles, where they are stored along with other splicing factors. The speckles are generally observed in close proximity to the active genes within the nucleus. This facilitates the recruitment of splicing factors to the sites of active transcription.

The nuclear bodies have also been shown to epigenetically regulate gene expression and hence, have implications in human diseases. For example, loss of Cajal bodies in Zebrafish embryos has been shown to arrest their development, illustrating their importance in animal development.