Cell nucleus
The nucleus is the largest cellular organelle in animal cells.
In mammalian cells, the average diameter of the nucleus is approximately 6 micrometres (µm), which occupies about 10% of the total cell volume. The viscous liquid within it is called nucleoplasm (or karyolymph), and is similar in composition to the cytosol found outside the nucleus. It appears as a dense, roughly spherical or irregular organelle. The composition by dry weight of the nucleus is approximately: DNA 9%, RNA 1%, Histone Protein 11%, Residual Protein 14%, Acidic Proteins 65%.
Circular RNA
In the cell, circRNAs are predominantly found in the cytoplasm, where the number of circular RNA transcripts derived from a gene can be up to ten times greater than the number of associated linear RNAs generated from that locus. It is unclear how circular RNAs exit the nucleus through a relatively small nuclear pore. Because the nuclear envelope breaks down during mitosis, one hypothesis is that the molecules exit the nucleus during this phase of the cell cycle. However, certain circRNAs, such as CiRS-7/CDR1as, are expressed in neuronal tissues, where mitotic division is not prevalent.