Ribosomes

Ribosomes are small organelles made of ribosomal RNA (rRNA) and a variety of 80 different proteins. rRNA is synthesized in the nucleolus. Ribosomal subunits are assembled there made from rRNA and imported proteins made in the cytoplasm. Once assembled, the subunits pass through the nuclear pores to the cytoplasm where they take part in protein synthesis. Some ribosomes are free in the cytoplasm and can be recruited to a polyribosomal structure when a messenger RNA (mRNA) strand is to be translated into a cytoplasmic protein. Other ribosomes are attached to the endoplasmic reticulum where the protein is formed within the interior to the endoplasmic reticulum. These proteins are destined for secretion, storage or incorporation into membranes. Ribosomes are used in several areas of the cell and remain vital to its activity.