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 aminoacyl- tRNA 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