Ribosomes are tiny, uniform, basophilic granular particles of ribonucleoprotein that engage in protein synthesis. They consist of two unequal sized subunits. The larger subunit has two binding sites, an aminoacyl site and a peptidyl site.
Ribosomes leave the nucleolus and cluster together to form polyribosomes.
They attach their peptidyl sites to membrane surfaces of the RER and project outward, or disperse themselves freely in the cytoplasm. Immature and rapidly proliferating cells usually contain free ribosomes in their cytoplasm accounting for their diffuse basophilia. Generally, RER ribosomes synthesize protein for cell secretion while free ribosomes synthesize protein for cell proliferation. When mRNA finds the ribosomes, the ribosomes line up along its filament reading its complimentary codons and attaching the correct amino acids to its peptidyl site. Each of the 20 amino acid has its own type of tRNA and codons.