Final answer:
The nucleolus is the nonmembranous structure within the nucleus responsible for rRNA formation and ribosome assembly. RNA polymerase I in the nucleolus synthesizes rRNA, which combines with proteins to form ribosomes that are crucial for protein synthesis.
Step-by-step explanation:
The nonmembranous body within the nucleus that is the site of rRNA formation is called the nucleolus. The primary function of the nucleolus involves the assembly of ribosomes, which are critical for protein synthesis in the cell. The nucleolus is where ribosomal RNA (rRNA) is transcribed, processed, and assembled into ribosomes. RNA polymerase I, located within the nucleolus, is responsible for the synthesis of all rRNA molecules except for 5S rRNA. These rRNAs are structural and not translated into proteins, but they play an essential role in the process of translation, which occurs in the cytoplasm.
Ribosomes consist of two subunits, each made up of rRNA and proteins, and function as the site where mRNA is translated into a polypeptide chain that will become a functional protein. After the ribosomal subunits are assembled in the nucleolus, they are then transported out to the cytoplasm through the nuclear pores. Ribosomes can either be found as free ribosomes in the cytoplasm or associated with the rough endoplasmic reticulum (RER), depending on the destination of the proteins they produce.