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Bio 1010 Dr. Bonnie A. Bain 92 CHAPTER 10 DNA Structure and Function Part 3 Translation (translate the mRNA code into a polypeptide) The Players *mRNA *tRNA *Ribosomes (rRNA + protein) The Process Initiation Elongation Termination Initiation of Translation 1. An mRNA molecule binds to a small ribosomal subunit A special “initiator” tRNA then binds to the Start codon on the mRNA The “initiator” tRNA carries the amino acid, methionine, on its amino acid attachment site Anticodon on tRNA is UAC Start codon on mRNA is AUG Initiation of Translation 2. A large ribosomal subunit fits on to the small subunit The “initiator” tRNA fits in to the P site on the ribosome Figure 10.18 Translation: Elongation Once initiation is complete, then amino acids are added one by one Each addition to the amino acid chain requires 3 steps (Fig. 10.18, p. 185): 1. Codon Recognition 2. Peptide Bond Formation 3. Translocation 1. Codon Recognition The anticodon of the tRNA pairs with the codon of the mRNA Occurs at the A site of the ribosome Figure 10.19 2. Peptide bond formation The polypeptide (on the tRNA at the P site) attaches to the new amino acid on the tRNA at the A site The polypeptide is attached to the “new” amino acid by a peptide bond Figure 10.19 3. Translocation The P site tRNA leaves the ribosome The ribosome translocates (moves) the other tRNA from the A site over to the P site This movement then exposes the next mRNA codon to be translated (at the A site) and the process then repeats itself Figure 10.19 Termination of Translation Elongation continues until a stop codon reaches the A site Stop codons: UAA, UAG, UGA When the stop codon appears, translation stops The completed polypeptide breaks off and leaves the ribosome Figure 10.19 Translation Rate: very fast 1 ribosome can make 1 polypeptide in less than 1 minute As it is being made, the polypeptide bends and folds into its secondary and then tertiary shape After it is made, several may come together to form a protein with a quaternary structure Figure 10.20 Mutations Any change in the nucleotide sequence of the DNA molecule Types of mutations: 1. Base Substitutions 2. Base Insertions or Base Deletions 1. Base Substitutions Replacement of one base or nucleotide with another Can result in: no change to a protein or insignificant change (Missense mutation) NOTE: SOME MISSENSE MUTATIONS, LIKE SICKLE CELL ANEMIA CAN ALSO HAVE MAJOR EFFECTS—textbook is misleading here or major, life-threatening change (Nonsense mutations) 1. Base Substitutions Can result in: no change to a protein Example GAA changes to GAG No change in the protein because both codons code for the same amino acid (Glu) 1. Base Substitutions Or they can result in insignificant change (but see sickle cell anemia) Called Missense Mutations Example GGC changes to AGC GGC codes for Glycine AGC codes for Serine Missense Mutations May or may not have a major effect on the protein Example of a major effect: One base change causes sickle-cell anemia see next slide (p. 186 in textbook) Figure 10.21 1. Base Substitutions Nonsense mutations These turn an amino acid codon into a Stop codon Causes the polypeptide to be terminated prematurely Example: AGA changes to UGA AGA codes for arginine UGA is a Stop codon 2. Base Insertions or Base Deletions Cause Reading Frame errors (frameshift mutations) Since mRNA is read as a series of nucleotide triplets, any shift in where to start reading will cause errors 2. Base Insertions or Base Deletions Example of a base deletion: AAG-UUU-GGC-GCA Remove the first U AAG-UUG-GCG-CA Figure 10.22b Stopped here on Monday Nov. 7, 2011 AAG-UUU-GGC-GCA Lys – Phe – Gly- Ala Remove the first U AAG-UUG-GCG-CA Lys – Leu – Ala- ? Insertions of extra nucleotides can also cause these problems (frameshift mutations) But, if 3 nucleotides are added (or if 3 are taken away), then it is no longer a frame-shift mutation Point Mutations: A change in a single nucleotide Frameshift Mutations: An insertion or deletion of 1 or 2 nucleotides Remember: Mutations are the ultimate source of diversity of life on our planet Figure 10.23