This agency does exist in the form of mRNA. Therefore, some sort of agency must exist to carry instructions from the DNA to the ribosomes. The DNA, that controls protein synthesis, is located in the chromosomes within the nucleus, whereas the ribosomes, on which the protein synthesis actually occurs, are placed in the cytoplasm. All the three kinds of RNAs play a role in protein synthesis. The three types of RNAs are transcribed from different regions of DNA template, RNA chain is complementary to the DNA strand which produces it. There are three types of RNA in every cell: messenger RNA or mRNA, ribosomal RNA or rRNA and transfer RNA or tRNA. The various components are linked up as in DNA. The bases in RNA are adenine, guanine, uracial and cytosine. Each nucleotide unit is composed of three smaller molecules: a phosphate group, a 5- carbon ribose sugar, and a nitrogen-containing base. RNA molecule is a long, un-branched, single-stranded polymer of ribonucleotides (Fig. Since the DNA is more or less stable, the proteins formed in a cell are exactly like the preexisting proteins. Each base triplet codes for a specific amino acid. The specificity control is exercised by DNA through mRNA sequences of 3 consecutive nitrogenous bases in the DNA double helix form the biochemical or genetic code. Thus, protein synthesis requires specificity control to provide instructions about the exact sequence in which the given numbers and kinds of amino acids should be linked to get the desired polypeptides. DNA as Specificity Control:Ī cell, in order to maintain its own special characteristics, must manufacture proteins exactly similar to those present already in it. These are available in the cytoplasmic matrix as an amino acid pool. The proteins of living organisms need about 20 amino acids as building blocks or monomers. Therefore, amino acids form the raw material for protein synthesis. §Note: these signal sequences get cleaved and will not be part of the mature proteins.Proteins are the polymers of amino acids. This process is covered in more detail here: In contrast, if a protein lacks a signal sequence it will (usually) be translated in the cytosol - many if not most of these proteins will remain in the cytosol, but some will end being imported into mitochondria, chloroplasts, peroxisomes, or the nucleus. These proteins will eventually be exported, sent to some types of organelles, or remain associated with a cell membrane. The ER-bound ribosomes are thus tethered to the ER by the growing polypeptide during its synthesis. The proteins produced by ER-bound ribosomes start with what are known as a signal sequence§ and are initiated within the cytosol - the signal sequence then directs the complex of peptide, mRNA, and ribosome to dock with the ER. The ribosomes are reversibly attached to the outer surface of the membrane rather than being inserted into the membrane. How each RNA carries out its specific task is discussed in this section, while the biochemical events in protein synthesis and the required protein factors are described in the final section of the chapter. The three types of RNA participate in this essential protein-synthesizing pathway in all cells in fact, the development of the three distinct functions of RNA was probably the molecular key to the origin of life. Translation is the whole process by which the base sequence of an mRNA is used to order and to join the amino acids in a protein. Ribosomes are composed of a large and small subunit, each of which contains its own rRNA molecule or molecules. They also bind tRNAs and various accessory molecules necessary for protein synthesis. These complex structures, which physically move along an mRNA molecule, catalyze the assembly of amino acids into protein chains. Ribosomal RNA (rRNA) associates with a set of proteins to form ribosomes. The correct tRNA with its attached amino acid is selected at each step because each specific tRNA molecule contains a three-base sequence that can base-pair with its complementary code word in the mRNA.ģ. Each type of amino acid has its own type of tRNA, which binds it and carries it to the growing end of a polypeptide chain if the next code word on mRNA calls for it. Transfer RNA (tRNA) is the key to deciphering the code words in mRNA. Messenger RNA (mRNA) carries the genetic information copied from DNA in the form of a series of three-base code “words,” each of which specifies a particular amino acid.Ģ.
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