Ribosomes consist of two major subunits—the small ribosomal subunit reads the mRNA, while the large subunit joins amino acids to form a polypeptide chain. Each subunit is composed of one or more ribosomal RNA (rRNA) molecules and a variety of proteins.Eukaryotes have 80S ribosomes, each consisting of a small (40S) and large (60S) subunit. Their small subunit has a 16S RNA sub-unit (consisting of 1540 nucleotides) bound to 21 proteins.ach subunit is composed of one or more ribosomal RNA (rRNA) molecules and a variety of proteins. Eukaryotes have 80S ribosomes, each consisting of a small (40S) and large (60S) subunit. Their small subunit has a 16S RNA sub-unit (consisting of 1540 nucleotides) bound to 21 proteins.