The third type of RNA in the cell is ribosomal RNA, which constitutes about 60 percent of the ribosome. The remainder of the ribosome is protein, including about 75 types of proteins that are both structural proteins and enzymes needed in the manufacture of protein molecules.
The ribosome is the physical structure in the cytoplasm on which protein molecules are actually synthesized. However, it always functions in association with the other two types of RNA: tRNA transports amino acids to the ribosome for incorporation into the developing protein molecule, whereas mRNA provides the information necessary for sequencing the amino acids in proper order for each specific type of protein to be manufactured. Thus, the ribosome acts as a manufacturing plant in which the protein molecules are formed.
Formation of Ribosomes in the Nucleolus. The DNA genes for formation of ribosomal RNA are located in five pairs of chromosomes in the nucleus. Each of these chromosomes contains many duplicates of these particular genes because of the large amounts of ribosomal RNA required for cellular function.
As the ribosomal RNA forms, it collects in the nucleolus, a specialized structure lying adjacent to the chromosomes. When large amounts of ribosomal RNA are being synthesized, as occurs in cells that manufacture large amounts of protein, the nucleolus is a large structure, whereas in cells that synthesize little protein, the nucleolus may not even be seen. Ribosomal RNA is specially processed in the nucleolus, where it binds with “ribosomal proteins” to form granular condensation products that are primordial subunits of ribosomes. These subunits are then released from the nucleolus and transported through the large pores of the nuclear envelope to almost all parts of the cytoplasm. After the sub units enter the cytoplasm, they are assembled to form mature, functional ribosomes. Therefore, proteins are formed in the cytoplasm of the cell but not in the cell nucleus, because the nucleus does not contain mature ribosomes.
