Nucleus

By two membranes the nucleus is limited, the inner and outer nuclear membranes.  These two membranes fuse together at the nuclear pores through which molecules [messenger ribonucleic acid (mRNA), ribosomes, proteins etc.] may move between the cytosol and the nucleus.  Other proteins, for instance those involved in regulating gene expression, may pass through the pores from the cytosol to the nucleus. With the rough endoplasmic reticulum (RER) Outer nuclear membrane is frequently continuous. Within the nucleus the DNA is tightly coiled around histone proteins and organized into complicated called chromosomes. Visible under the light microscope is the nucleolus which is a sub region of the nucleus which is the site of ribosomal ribonucleic acid (rRNA) synthesis.