Completar frases FUNBIO.18 Translation: the prokaryotic ribosome; protein synthesisVersión en línea - Understand the steps involved in the process of RNA translation. - Discuss the role of the primary molecules involved in the translation of RNA to protein; RNA template, ribosomal structure, and transfer RNAs. - Describe in detail the events occurring at specific sites on the ribosome during the translation process; initiation, elongation, translocation and termination. por Zainab Nabeel 1 Translation requires : a ) A template - mRNA ( moves to the cytoplasm ) b ) A reaction surface - the ribosome c ) Activated precursors - - tRNAs : tRNA molecules coupled to specific amino - acids via energy - rich bonds The ribosome ( E coli . ) : - Composed of 55 distinct proteins plus 3 rRNAs - Two sub - units : Large ( ) 5S rRNA 23S rRNA 34 Proteins Small ( ) 16S rRNA 21 Proteins Combined : ribosome ~20 , 000 per cell ; equivalent to ~25% of cell mass The " S " stands for , a unit used to measure how fast molecules move in a centrifuge . Ribosomes have 3 pockets in the general structure : A - site , P - site , E - site rRNA : - Single - stranded rRNA molecules - Extensive - base - pairing - Complex structures tRNA's : - Fold to give an - shaped structure - The amino - acid is added to the ? end of the tRNA molecule to give an aminoacyl - tRNA - The " anti - codon loop " mediates recognition of codons within the mRNA Amino - acyl tRNA's : - Activated for protein synthesis - Also called ? charged tRNAs ? - Each amino - acid is coupled to its specific " carrier " tRNA molecule by a two - stage reaction , which requires energy in the form of ATP : 1 ) : Amino acid reacts with ATP and AMP is transferred to AAs 2 ) transfer of the aAA to the 3 ? of the tRNA and release of AMP - Catalysed by specific Aminoacyl - tRNA enzymes ( at least one Aminoacyl - tRNA Synthetase exists for each amino - acid type ) - The ribosome " reads " the mRNA in a 5 ? to 3 ? direction - Peptides / protein grows from the N - terminus to the C - terminus - Translated in groups of three bases termed - Ribosome moves along the mRNA until it locates the start codon ? ( Therefore all proteins begin with the amino - acid Methionine - AUG ) A triplet code read in a sequence of 3 , the ? Codon ? , specifies an amino - acid . The code contains : - 61 amino - acid coding codons - 3 Stop codons [ UAA , UAG , UGA ] - 1 Start codon [ AUG ] which also encodes Met stages of translation : 1 - Initiation : a - Assembly of small ribosome subunit along the mRNA b - Binding of the first amino acid carried on its tRNA to the ribosome . Initiator tRNA ? fMet . tRNA N - ( specific to bacteria ) c - then the large subunit around the beginning of the mRNA At initiation : A - site is vacant and P - site has fMet tRNA 2 - Elongation : a ) Second aminoacyl tRNA enters the A - site of the ribosome ( Codon - anticodon recognition ) b ) Peptidyl - transferase reaction ( Formation of peptide bond between amino acid ( P - site ) and second amino acid ( A - site ) ) c ) Uncharged tRNA enters E - site and leaves the ribosome ( The peptidyl - tRNA moves from the A - site to the P - site ) - The mRNA moves on by three bases . - The A site is now aligned with the next codon in the mRNA and ready to accept the next tRNA molecule 3 - : a ) mRNA contains 3 " Stop codons " : UAA , , UAG b ) These do not code for any amino acid . c ) Recognised by Factors ( Bind to the A - site and cause the peptide in the P - site to be cleaved from the tRNA and released ) post - translation : Proteins must fold up into their natural and biologically - active conformation . Mostly this is spontaneous . Chemical modification of a protein may occur after translation to extend the range of functions of the protein by attaching functional groups ( e . g . Phosphorylation and Glycosylation ) . Some proteins need helper proteins known as " Chaperones " to assist in the folding process . Each ribosome synthesizes just one polypeptide at a time . Many ribosomes can translate a single mRNA molecule ( Referred to as a " Polyribosome " or " Polysome " ) .
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