 Because the nuclear membrane is <unk> to large molecules , nuclear pores are required that regulate nuclear transport of molecules across the envelope . The pores cross both nuclear membranes , providing a channel through which larger molecules must be actively transported by carrier proteins while allowing free movement of small molecules and ions . Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes . Although the interior of the nucleus does not contain any membrane @-@ bound sub compartments , its contents are not uniform , and a number of sub @-@ nuclear bodies exist , made up of unique proteins , RNA molecules , and particular parts of the chromosomes . The best @-@ known of these is the nucleolus , which is mainly involved in the assembly of ribosomes . After being produced in the nucleolus , ribosomes are exported to the cytoplasm where they translate mRNA . 
