Cellular Biology

Protein Synthesis Transcription Traslation Gene Expression



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Protein synthesis, a way of gene expression in which gene produces a product, usually a protein. It involves two stages; Transcription and Translation.

Transcription is a process of RNA synthesis in which RNA molecules are synthesized on DNA template. As in eukaryotes DNA is restricted to nucleus does not participate in protein synthesis directly. An intermediate RNA molecule is involved in it which is synthesized in nucleus where genetic information stored in DNA is transferred to this RNA molecule. This RNA is called mRNA (messanger RNA) or transcript as it contains a message to direct the ribosomes to assemble amino acids for given kind of protein. During this process only one of the DNA strand is transcripted which is called sense strand and other strand is called antisense strand.

Transcription comprises three stages; Template binding and initiation, Elongation and Termination. Template binding involves a DNA molecule and RNA polymerase. RNA polymerase is a holoenzyme composed of a core enzyme and a sigma factor. Sigma factor recognises promoters along the sense strand. Promoters (initiation sites) are attachment points where initial binding of RNA polymerase and a DNA molecule occurs. In the absense of sigma factor, core enzyme is responsible for this binding but not specifically to the promoters. During elongation RNA polymerase passes along DNA molecule to unwind the double helix and a structure called open promoter complex is formed at the point of breakage. The synthesized portion of RNA molecule gradually dissociates from the strand allowing it to return to its original helical structure. Finally termination occurs at suitable positions after the ends of genes.

Translation, the final stage of gene expression in which RNA molecule is translated into polypeptides. This complex process involves ribosomes and three types of RNA molecule (mRNA,rRNA,tRNA produced during transcription). The main thing during translation process is that genetic code determines the sequence of nucleotides in mRNA molecule to specify the sequence of amino acids in a polypeptide. There must be a code for each of 20 amino acids found in polypeptide. Translation comprises four stages; tRNA charging (aminoacylation), Initiation, Elongation and finally Termination of polypeptide chain. tRNA charging involves linkage of tRNA molecules to its respective amino acids. This charging is controlled by a group of enzymes called aminoacyl synthetases. Each aminoacyl synthetases is specific for each amino acid. First amino acid become activated by reacting with ATP and then linked with itsrespective enzyme to form a complex. This reacted complex get linked with specific tRNA molecule through a covalent linkage.This process is called amino acid activation.

After this charging stage,initiation, elongation and termination of polypeptide chain is carried out respectively. During initiation process small ribosomal sub-unit binds to a mRNA molecule and then a charged tRNA (charged by Methionine, an amino acid coded by AUG) pairs with this small sub-unit to form a resultant structure called Initiation complex. After the formation of initiation complex elongation process starts. When large sub-unit of ribosome binds with this complex. The large sub-unit have two binding sites for charged tRNA molecule; P-site (peptdyl) and A-site (aminoacyl site). The tRNA binds to P-site and specific charged tRNA become attached to A-site.The attachment of tRNA to A-site is dictated by sequence of second triplet (anticodon) in mRNA. An enzyme peptidyl transferase, present in large ribosomal sub-unit, catalyses the formation of peptide bond to link two amino acids together. At the same time the covalent bond between amino acid and tRNA (on P-site) molecule is broken and a dipeptide is formed. This increase in the length of polypeptide chain by one amino acid is actually called elongation. Before elongation repeats itself another process called translocation occurs. During translocation tRNA (now uncharged) attached to P-site is released from the large sub-unit and A-site becomes vacant again for coming charged tRNA to repeat the process. The final stage termination is signalled when a termination codon UAG,UAA or UGA enters the A-site. Release factors enter the A-site and cleaves the polypeptide chain from tRNA. Ribosomes releases the polypeptide and mRNA and dissociates into its sub-units. Released polypeptide chain folds up into tertiary structure to begin its functional life.

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