Bio 201 1st Edition Lecture 37Outline of Last Lecture I. Cracking The CodeA. Is Genetic Code Overlapping?B. How many Nucleotides for an Amino Acid?Outline of Current LectureI. What is the code?II. TranslationA. tRNAB. 3 StepsCurrent LectureI. What is the code? -Marshall Nirenberg and Heinrich Matthei (1961)- Synthesized poly U RNA, mixed it with cell extract and radiolabeled amino acids. They purified the polypeptides in each tube measure radioactivity. RESULT: Phenylaline is the only tube containing radioactivity. UUU-phe CCC-pro AAA-lys. It was the easiest to find single letter codes but they eventually found all 20 amino acid codes. The code is degenerate- multiple codons code for a single amino acid. II. Translation- Ribosomes responsible for translating mRNA protein. Small ribosome subunit: mRNA binding- recognized by 5’ cap tail. Large ribosome subunit: tRNA binding and peptide bond formation (ribozyme). Bacteria have 70S ribosome while eukaryotes have 80S. A. tRNA- After its translation, every tRNA gets an amino acid attached to its 3’ end. Each anticodon binds to a codon in mRNA (reverse) bringing an amino acid into ribosome. These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.-tRNA’s are charged by tRNA synthetases. Aminoacyl-tRNA synthetase add an amino acid to the 3’ end of each RNA. This creates a high-energy bond. The amino acid added by synthetase is determined by the sequence of the anticodon in the tRNA. How is the correct tRNA selected during translation though?-Experiment: Convert ca on cytesinyl tRNA. Add polyUG RNA to ribosomes, amino acids and tRNA then analyze polypeptides. RESULT: polypeptides contain alamine, tRNA is selected based on its anicodon sequence, not the attached amino acid. Anticodon binds to RNA. B. 3 Steps- Translation has 3 steps, initiation, elongation, and termination which all requireenergy. 1. Initiation- (Eukaryotes) initiation factors binds to 5’ cap and poly A tail, small ribosomal subunit and initiations factors scan mRNA beginnings at 5’ cap and stops at first AUG. AUG encodes methionine (START), methionine RNA binds to codon. Large ribosomal subunit binds to complex so that methionyl-tRNA is in P site. (requires GTP)2. Elongation- tRNA #2 enters A site (GTP), M transferred onto amino acid #2, translocation: Elongation factors translocation mRNA (GTP), tRNA #1 now in E site, exits ribosome and repeats. Peptide bonds formation- carboxyl group of met forms a bond with amino group phe which remains attached to tRNA #2. tRNA #1 exits and ribosome translocations to the next codon. 3. Termination- STOP codon appears in A site, release factor enters ribosome and binds codon, polypeptide releases from last tRNA (GTP), ribosome disassembles andRNA
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