Ribosomes (World of Microbiology and Immunology)
Ribosomes are organelles that play a key role in the manufacture of proteins. Found throughout the cell, ribosomes are composed of ribosomal ribonucleic acid (rRNA) and proteins. They are the sites of protein synthesis.
Although Robert Hooke first used a light microscope to look at cells in 1665, it was only during the last few decades that the cell's organelles were discovered. This is primarily because light microscopes do not have the magnifying power required to see these tiny structures. Using an electron microscope, scientists have been able to see most of the cells substructures, including the ribosomes.
Ribosomes are composed of a variety of proteins and rRNA. They are organized in two functional subunits that are constructed in the cell's nucleolus. One is a small subunit that has a squashed shape, while another is a large subunit that is spherical in shape. The large subunit is about twice as big as the small unit. The subunits usually exist separately, but join when they are attached to a messenger RNA (mRNA). This initiates protein synthesis.
Production of a protein begins with initiation. In this step, the ribosomal small subunit binds to the mRNA along with the first transfer RNA (tRNA). The next step is elongation, where the ribosome moves along the mRNA and strings together the amino acids one by one. Finally, the ribosome encounters a stop sequence and the two subunits release the mRNA, the polypeptide chain, and the tRNA.
Protein synthesis occurs at specific sites within the ribosome. The P site of a ribosome contains the growing protein chain. The A site holds the tRNA that has the next amino acid. The two sites are held close together and a chemical reaction occurs. When the stop signal is present on the mRNA, protein synthesis halts. The polypeptide chain is released and the ribosome subunits are returned to the pool of ribosome units in the cytoplasm.
Ribosomes are found in two locations in the cell. Free ribosomes are dispersed throughout the cytoplasm. Bound ribosomes are attached to a membranous structure called the endoplasmic reticulum. Most cell proteins are made by the free ribosomes. Bound ribosomes are instrumental in producing proteins that function within or across the cell membrane. Depending on the cell type, there can be as many as a few million ribosomes in a single cell.
Because most cells contain a large number of ribosomes, rRNA is the most abundant type of RNA. rRNA plays an active role in ribosome function. It interacts with both the mRNA and tRNA and helps maintain the necessary structure. Transfer RNA is the molecule that interacts with the mRNA during protein synthesis and is able to read a three amino acid sequence. On the opposite end of the tRNAs, amino acids are bonded on a growing polypeptide chain. Generally, it takes about a minute for a single ribosome to make an average sized protein. However, several ribosomes can work on a single mRNA at the same time. This allows the cell to make many copies of a single protein rapidly. Sometimes these multiple ribosomes, or polysomes, can become so large that they can be seen with a light microscope.
The ribosomes in eukaryotes and prokaryotes are slightly different. Eukaryotic ribosomes are generally larger and are made up of more proteins. Since many diseases are caused by prokaryotes, these slight differences have important medical implications. Drugs have been developed that can inhibit the function of a prokaryotic ribosome, but leave the eukaryotic ribosome unaffected. One example is the antibiotic tetracycline.
See also Protein synthesis