The nucleus contains the cell’s genome. It is bounded by a 
membrane that consists of a pair of unit membranes sepa rated by a space of variable thickness. The inner membrane is 
usually a simple sac, but the outermost membrane is, in many 
places, continuous with the endoplasmic reticulum (ER). The 
nuclear membrane exhibits selective permeability because of 
pores, which consist of a complex of several proteins whose 
function is to import substances into and export substances 
out of the nucleus. The chromosomes of eukaryotic cells con tain linear DNA macromolecules arranged as a double helix. 
They are only visible with a light microscope when the cell is 
undergoing division and the DNA is in a highly condensed 
form; at other times, the chromosomes are not condensed 
and appear as in Figure 2-2. Eukaryotic DNA macromol ecules are associated with basic proteins called histones that 
bind to the DNA by ionic interactions.
A structure often visible within the nucleus is the nucle olus, an area rich in RNA that is the site of ribosomal RNA 
synthesis (see Figure 2-2). Ribosomal proteins synthesized in 
the cytoplasm are transported into the nucleolus and com bine with ribosomal RNA to form the small and large sub units of the eukaryotic ribosome. These are then exported to 
the cytoplasm, where they associate to form an intact ribo some that can function in protein synthesis