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Chapter 13 The Genetic Code and Transcription
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13.1 The Genetic Code Uses Ribonucleotide Bases as “Letters”
triplet unambiguous degenerate start and stop signals commaless nonoverlapping almost universal
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13.2 Early Studies Established the Basic Operational Patterns of the Code
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The Triplet Nature of the Code
The code is non-overlapping
GTACA
If overlapping: “GTA, TAC, ACA”
Protein sequence is limited Not actually limited
Also, one basepair change results in only one amino acid change
The code is degenerate
20 amino acids and 64 possible codons 44 extra codons
More than one codon causes insertions of the same amino acid
The code is unambiguous
Each codon only codes for one amino acid (invariant)
How were codons matched with their amino acids?
Cell free synthesis with polynucleotide phosphorylase
Initiation & Stop codons
Start: AUG – Methionine
Stop: UAG, UAA or UGA No tRNA binds
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13.3 Studies by Nirenberg, Matthaei, and Others Led to Deciphering of the Code
Synthesizing Polypeptides in a Cell-Free System
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Homopolymer Codes
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A mixed copolymer experiment in which a ratio of 1A:5C is used
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The Triplet Binding Assay
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Repeating Copolymers
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13.4 The Coding Dictionary Reveals Several Interesting Patterns among the 64 Codons
The wobble hypothesis Base-pairing is less constrained for the third letter of
each codon
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13.6 The Genetic Code Is Nearly Universal
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13.8 Transcription Synthesizes RNA on a DNA Template
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13.9 Studies with Bacteria and Phages Provided Evidence for the Existence of mRNA
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Evidence for RNA as the intermediate Phage infection of bacteria
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13.10 RNA Polymerase Directs RNA Synthesis
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13.11 Transcription in Eukaryotes Differs from Prokaryotic Transcription in Several Ways
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Heterogeneous Nuclear RNA and Its Processing: Caps and Tails
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13.12 The Coding Regions of Eukaryotic Genes Are Interrupted by Intervening Sequences
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RNA Editing May Modify the Final Transcript
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13.13 Transcription Has Been Visualized by Electron Microscopy