DNA Lecture 5 PDF

mRNA Translation PROF.DR: KHALID SALAH EL DEIN The Genetic Code  The genetic code is the same for all organisms (UNIVERSAL)  A codon is a “word” in DNA/RNA language. It is formed by three nucleotides..  The genetic code is not overlapping  AUG is always the start codon for tr...

mRNA Translation PROF.DR: KHALID SALAH EL DEIN The Genetic Code  The genetic code is the same for all organisms (UNIVERSAL)  A codon is a “word” in DNA/RNA language. It is formed by three nucleotides..  The genetic code is not overlapping  AUG is always the start codon for translation and there are three stop codons to end translation. Fig. 17-16a Growing polypeptide Exit tunnel tRNA molecules Large E PA subunit Small subunit 5 mRNA 3 (a) Computer model of functioning ribosome Fig. 17-16b P site (Peptidyl-tRNA binding site) A site (Aminoacyl- E site tRNA binding site) (Exit site) E P A Large subunit mRNA Small binding site subunit (b) Schematic model showing binding sites Amino end Growing polypeptide Next amino acid to be added to polypeptide chain E tRNA mRNA 3 5 Codons (c) Schematic model with mRNA and tRNA  A ribosome has three binding sites for tRNA:  The P site holds the tRNA that carries the growing polypeptide chain  The A site holds the tRNA that carries the next amino acid to be added to the chain  The E site is the exit site, where discharged tRNAs leave the ribosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings  Initiation of Translation  The initiation stage of translation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits  First, a small ribosomal subunit mRNA and a special initiator tRNA  Then the large subunit binds with the complex Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 17-17 Large ribosomal 3 U A C 5 subunit P site 5 A U G 3 Initiator tRNA GTP GDP E A mRNA 5 5 3 3 Start codon Small mRNA binding site ribosomal Translation initiation complex subunit  Elongation of the Polypeptide Chain  During the elongation stage, amino acids are added one by one to the preceding amino acid  Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 17-18-1 Amino end of polypeptide E 3 mRNA P A 5 site site Fig. 17-18-2 Amino end of polypeptide E 3 mRNA P A 5 site site GTP GDP E P A Fig. 17-18-3 Amino end of polypeptide E 3 mRNA P A 5 site site GTP GDP E P A E P A Fig. 17-18-4 Amino end of polypeptide E 3 mRNA P A Ribosome ready for site site 5 next aminoacyl tRNA GTP GDP E E P A P A GDP GTP E P A  Termination of Translation  Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome  This reaction releases the polypeptide, and the translation assembly then comes apart Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 17-19-1 Release factor 3 5 Stop codon (UAG, UAA, or UGA) Fig. 17-19-2 Release Free factor polypeptide 3 3 5 5 2 GTP Stop codon 2 GDP (UAG, UAA, or UGA) Fig. 17-19-3 Release Free factor polypeptide 5 3 3 3 5 5 2 GTP Stop codon 2 GDP (UAG, UAA, or UGA) In Prokaryotes