Ribosome

Ribosomes (/ˈraɪbəˌsoʊm, -boʊ-/) comprise a complex macromolecular machine, found within all living cells, that serves as the site of biological protein synthesis (translation). Ribosomes link amino acids together in the order specified by messenger RNA (mRNA) molecules. Ribosomes consist of two major components: the small ribosomal subunits, which read the mRNA, and the large subunits, which join amino acids to form a polypeptide chain. Each subunit consists of one or more ribosomal RNA (rRNA) molecules and a variety of ribosomal proteins (r-protein or rProtein). The ribosomes and associated molecules are also known as the translational apparatus.The sequence of DNA, which encodes the sequence of the amino acids in a protein, is copied into a messenger RNA chain. It may be copied many times into RNA chains. Ribosomes can bind to a messenger RNA chain and use its sequence for determining the correct sequence of amino acids for generating a given protein. Amino acids are selected, collected, and carried to the ribosome by transfer RNA (tRNA) molecules, which enter one part of the ribosome and bind to the messenger RNA chain. It is during this binding that the correct translation of nucleic acid sequence to amino acid sequence occurs. For each coding triplet in the messenger RNA there is a distinct transfer RNA that matches and which carries the correct amino acid for that coding triplet. The attached amino acids are then linked together by another part of the ribosome. Once the protein is produced, it can then fold to produce a specific functional three-dimensional structure although during synthesis some proteins start folding into their correct form.Ribosomes were first observed in the mid-1950s by Romanian-American cell biologist George Emil Palade, using an electron microscope, as dense particles or granules. The term 'ribosome' was proposed by scientist Richard B. Roberts in the end of 1950s:During the course of the symposium a semantic difficulty became apparent. To some of the participants, 'microsomes' mean the ribonucleoprotein particles of the microsome fraction contaminated by other protein and lipid material; to others, the microsomes consist of protein and lipid contaminated by particles. The phrase 'microsomal particles' does not seem adequate, and 'ribonucleoprotein particles of the microsome fraction' is much too awkward. During the meeting, the word 'ribosome' was suggested, which has a very satisfactory name and a pleasant sound. The present confusion would be eliminated if 'ribosome' were adopted to designate ribonucleoprotein particles in sizes ranging from 35 to 100S.The ribosome is a highly complex cellular machine. It is largely made up of specialized RNA known as ribosomal RNA (rRNA) as well as dozens of distinct proteins (the exact number varies slightly between species). The ribosomal proteins and rRNAs are arranged into two distinct ribosomal pieces of different size, known generally as the large and small subunit of the ribosome. Ribosomes consist of two subunits that fit together (Figure 2) and work as one to translate the mRNA into a polypeptide chain during protein synthesis (Figure 1). Because they are formed from two subunits of non-equal size, they are slightly longer in the axis than in diameter.Ribosomes are minute particles consisting of RNA and associated proteins that function to synthesize proteins. Proteins are needed for many cellular functions such as repairing damage or directing chemical processes. Ribosomes can be found floating within the cytoplasm or attached to the endoplasmic reticulum. Basically, their main function is to convert genetic code into an amino acid sequence and to build protein polymers from amino acid monomers. Ribosomes are classified as being either 'free' or 'membrane-bound'.In bacterial cells, ribosomes are synthesized in the cytoplasm through the transcription of multiple ribosome gene operons. In eukaryotes, the process takes place both in the cell cytoplasm and in the nucleolus, which is a region within the cell nucleus. The assembly process involves the coordinated function of over 200 proteins in the synthesis and processing of the four rRNAs, as well as assembly of those rRNAs with the ribosomal proteins.The ribosome may have first originated in an RNA world, appearing as a self-replicating complex that only later evolved the ability to synthesize proteins when amino acids began to appear. Studies suggest that ancient ribosomes constructed solely of rRNA could have developed the ability to synthesize peptide bonds. In addition, evidence strongly points to ancient ribosomes as self-replicating complexes, where the rRNA in the ribosomes had informational, structural, and catalytic purposes because it could have coded for tRNAs and proteins needed for ribosomal self-replication. Hypothetical cellular organisms with self-replicating RNA but without DNA are called ribocytes (or ribocells).Many textbooks suggest that there are only two kinds of ribosomes, namely prokaryotic and eukaryotic ribosomes. However, ribosomes are surprisingly heterogeneous with different compositions in different species. Heterogeneous ribosomes have different structures and thus different activities compared to typical ribosomes in major model organisms.  This article incorporates public domain material from the NCBI document 'Science Primer'.