CONTENTS The Ribosomes, Protein Factories

The Ribosomes, Protein Factories Ribosomes, which were discovered in the mid-1950's, are extremely tiny--less than 30 nanometers in diameter. However, due to their crucial role in protein manufacture, ribosomes can also be extremely numerous. In E. coli, for example, ribosomes account for one-fourth of the cell's mass. A ribosome is made of two unequally sized subunits, each of which is composed of at least 40 different proteins and a form of RNA called ribosomal RNA. Click images for larger size In the nucleus, DNA's instructions are transcribed into a messenger molecule of ribonucleic acid (RNA). During translation, a strand of mRNA moves between the two parts of a ribosome like a piece of thread being pulled through the eye of a needle. The ribosome reads the message of the mRNA not one nucleotide at a time, but rather in groups of three. These groups, called codons, are like words. Each word specifies one of the 20 different amino acid subunits of a protein or is a signal to start or stop making a protein. For example, the codon AGC in mRNA is translated into the amino acid serine, whereas nucleotides in a different order, say GCA, code for alanine. The amino acids called for by the mRNA are brought from the cytoplasm to the ribosome by a third kind of RNA, transfer RNA (tRNA). This small molecule is a connector: One end carries three nucleotides, known as the anticodon, which will join to a codon in the mRNA according to the rules of base pairing (A with U, and G with C). The molecule's other end carries an amino acid. As the mRNA passes through the ribosome, tRNA brings the correct amino acids in and they are linked together by chemical bonds to form a long chain. When all the amino acids for a protein are joined, the chain is released. Each strand of mRNA can be read many thousands of times. Indeed, at any one moment a strand of mRNA containing the instructions for a protein may be attached to as many as 30 ribosomes. Moreover, ribosomes work very quickly to connect the required amino acids into a protein. Each ribosome in a single E. coli, for example, can link 15 amino acids in a second. The speed and efficiency of translation means that each gene is capable of directing the manufacture of very large quantities of protein. For instance, in each cell of a silkworm's silk gland there is a single gene that codes for the protein fibroin, the chief component of silk. Each time it is activated, the gene can make 10,000 copies of its specific mRNA, and each copy of mRNA can direct the synthesis of 100,000 molecules of fibroin. In 4 days, a silk gland cell can manufacture a billion molecules of fibroin! The code in a strand of messenger RNA is translated into a protein in tiny organelles, called ribosomes, in the cytoplasm. Ribosomes fall into two categories: those that are free in the cytoplasm and those that are bound to membranes. The two kinds of ribosomes play similar roles in the manufacture of proteins. But while the free ribosomes leave the proteins equally free to float in the cytoplasm, the bound ribosomes transfer their finished proteins into a large, cobwebby organelle--the endoplasmic reticulum.