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Transcript
Translation tRNA acceptor site of amino acid tyrosine Anticodon arm Anticodon: recognizes the codon of the mRNA. The recognition is facilitated by complementing base pairs. • Every tRNA carries only one specific amino acid • The third base in each codon may differ b/w two codons that code for the same a.a. – E.g, AAU and AAC both code for tyrosine. • If the tRNA’s anticodon is UUA , it can still bind to the codon AAC. • This flexibility makes it possible for the correct a.a. to be added in the growing polypeptide chain, even when errors were made in the gene sequence of mRNA • Aminoacyl-tRNA- is a tRNA molecule with its corresponding amino acid to its acceptor site at the 3’ end • Aminoacyl-synthetase – is the enzyme that adds the appropriate amino acid to each tRNA – there are at least 20 of them – each is specific for a particular amino acid and tRNA Ribosome • rRNA is produced from a DNA template in the nucleolus of nucleus • rRNA is packaged with a variety of proteins into ribosomal units, one larger than the other • Subunits move separately through nuclear envelope pores into cytoplasm where they combine to form ribosomes The ribosome brings the mRNA strand, the aminoacyl-tRNA, and the enzymes involved in building polypeptides together • They facilitate complementary base pairing b/w tRNA anticodons and mRNA codon. • Contains: • -one mRNA site • -two tRNA binding sites: • P site-holds the tRNA attached to the growing polypeptide • A site-holds the tRNA carrying the next amino acid to be added to polypeptide chain The Steps in Translation Polypeptide Chain Initiation • mRNA attaches to the ribosome • The start codon (AUG) is the first codon that is recognized by the ribosomes • AUG ensures that the correct reading frame is used by the ribosomes • AUG codes for methionine , therefore every protein initially starts with the amino acid methionine • Met enters the P site of the ribosome which signals the process to begin Polypeptide Chain Elongation • The next tRNA with the appropriate anticodon attaches to the codon on the mRNA, (carrying the required amino acid) enters the A site. • tRNA at the P site is released from its amino acid and the mRNA codon by enzyme action • Triplet at A site moves to P site bringing next triplet to A site • Amino acid at P site attaches to amino acid at A site (by a peptide bond) • tRNA that have been released are recycled by aminoacyltRNA synthesize adding a correct amino acid to them • Above process repeats itself until a STOP signal comes to the A site. Polypeptide Chain Termination • Stop signal does not bring any amino acid in as it moves to the P site and is released. • Stop Codons: UAG, UGA, and UAA • A protein which is known as a release factor recognizes that the ribosome has slowed down and aids in the release of the polypeptide chain from the ribosome • The ribosome subunits fall off the mRNA • Translation is over! At this point, modifications to the polypeptide may include: • sugars added to various amino acids through glycosylation • Phosphates may be added to various amino acids through phosphorylation • Enzymes may cleave the chain at various points