7.4 adapted from John Burrell http://click4biology7.4.1 Activation of tRNA.Activation specificity: How does the tRNA attach to the correct amino acid? The shape of each tRNA (defined by the loop and the helical sections. ) is different. An activating enzyme adds a specific amino acid to the CCA base sequence (at 3 end of the tRNA) this requires ATP (energy). Each amino acid has one or more tRNA molecules. (This again reflects a degenerate code.) a) Amino acid which is specific to each tRNA. (b) CCA base sequence to which the amino acid is attached by the Activating Enzyme. (c) Complementary base pairing sequence. Helical in shape. (d) 8 free bases non-pairing giving one loop of RNA. (e) 7 free bases non-pairing giving second loop of RNA. (f) Small open loop of RNA which is variable in shape between different tRNA. (g) Anti-codon (3 bases) which binds to the mRNA codon (3 bases) this is specific to the amino acids being carried. The anti-codon is complementary to the sense DNA.7.4.2 Ribosome structure. Proteins and Ribosomal RNA combine in the structure Large sub-unit and a small sub-unit Large sub-unit has 3 binding sites for tRNA molecules ( E, P and A site). Small sub-unit has a binding site for mRNA Ribosome Function: Ribosomes contain enzymes. They catalyze the translation of mRNA into a polypeptide.7.4.3 Stages of translation. Translation has three parts (just as in transcription)Initiation overview: The ribosome, tRNA and mRNA come together to begin the translation of the mRNA.Elongation overview: tRNA molecules attach to the mRNA based on the codon-anticodon recognition. Amino acids are brought together and polymerized into the primary structure of the polypeptide.Termination overview: mRNA and the ribosomes detach from one another. The polypeptide is released and the tRNA returns to be charged with more amino acid.
7.4.4 Translation direction Translation of the mRNA takes place from the 5 free end to the free 3 end. Ribosomes move along the mRNA in this direction. The genetic code is translated from the 5 free end to the 3 free end.7.4.5 Peptide bonds between amino acids. During translation amino acids are joined together to form polypeptides. The specific sequence of amino acids is called the primary structure. Between each amino acid a peptide bond forms to join them together. In this example the amino acids are both alanine in which the R group is a single hydrogen. The carboxyl acid end on the first amino acid is orientated to the amino group of the second amino acid. The -OH group and -H are removed to form water (condensation reaction). The bond forms between the terminal carbon on the first amino acid and the nitrogen on the second amino acid. The backbone of the molecule has the sequence N-C-C-N-C-C Polypeptides maintain this sequence no matter how long the chain. The R groups project from the backbone. As the amino acids are added in translation the polypeptide folds up into it specific shape. The end of the codon sequence in mRNA has been reached. The ribosome encounters a termination sequence signaling the end of translation. The ribosome moves the alanine tRNA to the P site. The polypeptide is released from the translation process. The ribosome has no new codons read. The two sub units move and separate. The protein will now be further modified in either the endoplasmic reticulum, Golgi or secreted in a vesicle
7.4.6 Translation process.The tRNA charged with Methionine has theanti-codon UAC. This is complementary tothe start codon (mRNA) of AUG.The small sub unit of the ribosomeassociates with the Methionine tRNA.The small unit of the ribosome moves overthe START codon.The large unit of the ribosome moves overthe mRNA.There are three binding sites for tRNA on thelarge sub unit.A-(Amino acid) is the position which the newtRNA codon-anticodon binds making surethat the correct amino acid is in position.P-(Polypepide) is the position in which theamino acid on the tRNA adds to thepolypeptide.E-( Exit) is the position the tRNA (withoutamino acid) locates and is the released fromthe ribosome to become re-activated.The START codon (AUG) occupies the Psite.The A site is free for the complementarytRNA to bind.Specificity is maintained by the codon-anticodon binding which is a majorfeature of the ribosome function.In this sequence the A site has the codonCCG.The tRNA anticodon GGC which carriedProline hydrogen bonds with the codonbases.The codon -anticodon binding has placedthe two amino acids methionine andproline beside each other.(a)The bond between the tRNA andmethionine is broken.This releases free energy.(b)The free energy is used to form thepeptide bond between methionine andproline.The large sub-unit then moves to threebases (one codon) towards the 3 end ofthe mRNA.
The anticodon tRNA for Alanine complementary base pairs with the A site codon. The ribosome checks that this is the correct tRNA and therefore amino acid. The bond between the tRNA and Proline is broken. Free energy is released. A peptide bond is formed between Proline and Alanine. The peptide chain will be folding and shaping 7.4.7 Free and membrane bound ribosomesFree ribosomes:Free ribosomes in the cytoplasm are associated with the synthesis of proteins forinternal use in the cell.Ribosomes which are attached to the wall of the endoplasmic reticulum are associatedwith proteins which will be placed into vesicles and secreted form the cell.