Each tRNA anticodon can base pair with one of the mRNA codons and add an amino acid or terminate translation, according to the genetic code. Of these 61, one codon (AUG) also encodes the initiation of translation. Of the 64 possible mRNA codons-or triplet combinations of A, U, G, and C-three specify the termination of protein synthesis and 61 specify the addition of amino acids to the polypeptide chain. Therefore, tRNAs are the molecules that actually “translate” the language of RNA into the language of proteins. Serving as adaptors, specific tRNAs bind to sequences on the mRNA template and add the corresponding amino acid to the polypeptide chain.
Depending on the species, 40 to 60 types of tRNAs exist in the cytoplasm. The tRNAs are structural RNA molecules that were transcribed from genes by RNA polymerase III. The complete mRNA/poly-ribosome structure is called a polysome. Each mRNA molecule is simultaneously translated by many ribosomes, all synthesizing protein in the same direction: reading the mRNA from 5′ to 3′ and synthesizing the polypeptide from the N terminus to the C terminus. The small subunit is responsible for binding the mRNA template, whereas the large subunit sequentially binds tRNAs. Mammalian ribosomes have a small 40S subunit and a large 60S subunit, for a total of 80S. coli, the small subunit is described as 30S, and the large subunit is 50S, for a total of 70S (recall that Svedberg units are not additive). Ribosomes dissociate into large and small subunits when they are not synthesizing proteins and reassociate during the initiation of translation. Mitochondria and chloroplasts also have their own ribosomes in the matrix and stroma, which look more similar to prokaryotic ribosomes (and have similar drug sensitivities) than the ribosomes just outside their outer membranes in the cytoplasm. Ribosomes exist in the cytoplasm in prokaryotes and in the cytoplasm and rough endoplasmic reticulum in eukaryotes. In eukaryotes, the nucleolus is completely specialized for the synthesis and assembly of rRNAs. A ribosome is a complex macromolecule composed of structural and catalytic rRNAs, and many distinct polypeptides. coli, there are between 10,000 and 70,000 ribosomes present in each cell at any given time. RibosomesĮven before an mRNA is translated, a cell must invest energy to build each of its ribosomes. Translation requires the input of an mRNA template, ribosomes, tRNAs, and various enzymatic factors.Ĭlick through the steps of this PBS interactive to see protein synthesis in action.
However, the general structures and functions of the protein synthesis machinery are comparable from bacteria to human cells. The composition of each component may vary across species for instance, ribosomes may consist of different numbers of rRNAs and polypeptides depending on the organism. In addition to the mRNA template, many molecules and macromolecules contribute to the process of translation. This reaction is catalyzed by ribosomes and generates one water molecule. Polypeptides are formed when the amino group of one amino acid forms an amide (i.e., peptide) bond with the carboxyl group of another amino acid (Figure 1). Each individual amino acid has an amino group (NH 2) and a carboxyl (COOH) group. Amino acids are covalently strung together by interlinking peptide bonds in lengths ranging from approximately 50 amino acid residues to more than 1,000. The process of translation, or protein synthesis, involves the decoding of an mRNA message into a polypeptide product. In turn, proteins account for more mass than any other component of living organisms (with the exception of water), and proteins perform virtually every function of a cell. The synthesis of proteins consumes more of a cell’s energy than any other metabolic process. The R and R’ designations refer to the rest of each amino acid structure. For simplicity in this image, only the functional groups involved in the peptide bond are shown. A peptide bond links the carboxyl end of one amino acid with the amino end of another, expelling one water molecule.
0 kommentar(er)
