Camerini 2 - Lecture 2 The Genetic Code Today we will...

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1 Lecture 2: The Genetic Code Today, we will discuss the following: Demonstration of 3 bp reading frame (Brenner and Crick) using frameshift mutations Completion of the codon table by Nirenberg and Khorana The Start codon Discovery of Stop Codons The Universal Genetic and Variants Codon bias Overview of protein synthesis tRNA structure
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2 What is the biochemical basis for mutation? Mutation and alteration of the sequence of DNA. Chemical carcinogens and ionizing radiation can result in changes in the nuc leotide sequence of DNA . These mutations fall into three classes: 1. Changes in the identity of a single base pair; Base substitution: 2. Insertion or deletion of a single base; Frameshift mutation: 3. Deletion of t wo or more bases; Deletion mutation: Spontaneous reversion can occur. Spontaneous reversion is extremely rare. Spontaneous reversion is extremely rare.
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3 The genetic code in brief… How is the information coded? If we only have 4 bases in DNA (and in mRNA), then how do we get 20 different amino acids (AAs)? Codon length Permut Codons AAs encoded 1 nucleotide 4 1 A,C,G,U 4 AAs 2 nucleotides 4 2 AU, AG 16 AAs 3 nucleotides 4 3 AUG 64 AAs 4 nucleotides 4 4 AGAG 256 AAs Does the code overlap? Not likely (recall co-linearity experiments)
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4 How is the code read? Crick and Brenner Occam’s Razor: “Keep It Simple Stupid!” Does the code overlap? Not likely (recall co-linearity experiments; overlapping code = multiple AAs changed for single base mutation)
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5 Crick and Brenner: T4 rII frameshift mutants Purpose: determine how the “code” is read Approach: deletion and insertion mutants in T4 rII locus (two genes important for E. coli infection); look for revertants Concept: Original: THE FAT CAT ATE THE BIG RAT Deletion: THE FAT AT ATE THE BIG RAT With punctuation, only the CAT codon is missing Original: THE FAT CAT ATE THE BIG RAT Deletion: THE FAT AT ATE THE BIG
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