The structure of the nucleolus will be seen under the electron microscope as one or more basophil structures that are larger than the size of the chromatin grains. The nucleolus is the site of gene transcription from which rRNA molecules are obtained . rRNA is a type of RNA which is the building block of ribosomes. The newly formed rRNA molecule is immediately packaged with ribosomal proteins to be removed from the cell nucleus. Transcription of rRNA molecules in the nucleolus guarantees the formation of ribosomal molecules in the cytoplasm. For this need, in the daughter nucleus there are a number of pieces of DNA ( rDNA .).) which is transcribed into rRNA repeatedly and runs very quickly with the help of the RNA polymerase I enzyme . These pieces of DNA are called nucleolar organizers . The content of RNA in the daughter nucleus when compared with other parts of the cell nucleus is not fixed, which is estimated at 5%-20%.
Nucleolus (Source: ibiblio.org)