Why is the RNA in the cytosol much smaller than the RNA in the nucleus?

Answer 1

In the nucleus, RNA is transcribed from the entire gene. The genes contain both introns and exons. Only the exons contain the coding information for the protein. After transcription these long RNAs (called hnRNA) are processed to remove the introns. After adding a poly A tail, and a cap on the 5' end, the now much smaller RNA is exported into the cytosol where it can be translated.

Answer 2

RNA molecules in the cytosol are typically smaller because they have completed processing, such as splicing and capping, which removes non-coding regions and introns, resulting in a smaller mature RNA molecule. In contrast, RNA in the nucleus still includes these non-coding regions and introns, contributing to their larger size.