Textbook Question
In terms of the polycistronic composition of mRNAs and the presence or absence of Shine–Dalgarno sequences, compare and contrast bacterial, archaeal, and eukaryotic mRNAs.
Ch. 9 - The Molecular Biology of Translation
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172
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Table of contents
- Ch. 1 - The Molecular Basis of Heredity, Variation, and Evolution64
- Ch. 2 - Transmission Genetics144
- Ch. 3 - Cell Division and Chromosome Heredity81
- Ch. 4 - Gene Interaction87
- Ch. 5 - Genetic Linkage and Mapping in Eukaryotes103
- Ch. 6 - Genetic Analysis and Mapping in Bacteria and Bacteriophages73
- Ch. 7 - DNA Structure and Replication71
- Ch. 8 - Molecular Biology of Transcription and RNA Processing79
- Ch. 9 - The Molecular Biology of Translation110
- Ch. 10 - Eukaryotic Chromosome Abnormalities and Molecular Organization100
- Ch. 11 - Gene Mutation, DNA Repair, and Homologous Recombination86
- Ch. 12 - Regulation of Gene Expression in Bacteria and Bacteriophage90
- Ch. 13 - Regulation of Gene Expression in Eukaryotes38
- Ch. 14 - Analysis of Gene Function via Forward Genetics and Reverse Genetics72
- Ch. 15 - Recombinant DNA Technology and Its Applications69
- Ch. 16 - Genomics: Genetics from a Whole-Genome Perspective49
- Ch. 17 - Organelle Inheritance and the Evolution of Organelle Genomes27
- Ch. 18 - Developmental Genetics43
- Ch. 19 - Genetic Analysis of Quantitative Traits66
- Ch. 20 - Population Genetics and Evolution at the Population, Species, and Molecular Levels105
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
Chapter 9, Problem 38b
Organisms of all three domains of life usually use the mRNA codon AUG as the start codon.
Despite AUG being the most common start codon sequence, very few proteins have methionine as the first amino acid. Why is this the case?
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Understand that AUG is the start codon in mRNA, and it codes for the amino acid methionine (Met) in eukaryotes and a modified form of methionine (formyl-methionine, fMet) in prokaryotes.
Recognize that during translation initiation, the ribosome identifies the AUG start codon and begins protein synthesis by incorporating methionine (or fMet) as the first amino acid in the polypeptide chain.
Note that after translation begins, post-translational modifications often occur. These modifications can include the removal of the initial methionine by specific enzymes, such as methionine aminopeptidases, which cleave the methionine from the N-terminus of the protein.
Understand that the removal of methionine is influenced by the identity of the second amino acid in the sequence and the structure of the protein, which determines whether the methionine is retained or removed.
Conclude that while AUG is the universal start codon and methionine is initially incorporated, post-translational modifications frequently result in proteins that do not retain methionine as the first amino acid.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Start Codon
The start codon is a specific sequence of nucleotides in mRNA that signals the beginning of protein synthesis. In most organisms, the codon AUG is recognized as the start codon, which codes for the amino acid methionine. This codon is crucial for initiating translation, as it sets the reading frame for the ribosome to assemble the corresponding amino acids into a polypeptide chain.
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Translation initiation
Post-Translational Modifications
Post-translational modifications are chemical changes that occur to a protein after its synthesis. These modifications can include the removal of certain amino acids, such as methionine, or the addition of functional groups. Such alterations can significantly affect the protein's function, stability, and localization, explaining why many proteins do not retain methionine as their first amino acid despite it being coded by the start codon.
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Alternative Start Codons
While AUG is the primary start codon, some organisms can utilize alternative start codons, such as GUG or UUG, under specific conditions. These alternative codons can lead to the incorporation of different amino acids at the beginning of the protein chain. This flexibility in translation initiation contributes to the diversity of protein structures and functions, further explaining the rarity of methionine as the first amino acid in many proteins.
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Related Practice
Textbook Question
Answer the following questions about the accompanying diagram.
Which end of the DNA is closest to A?
Textbook Question
The six nucleotides preceding the start codon and the first nucleotide after the start codon in eukaryotes exhibit strong sequence conservation as determined by the percentages of nucleotides in the to positions and the position (see Problem 34). Use the data given in the table for Problem 35 to determine the seven nucleotides that most commonly surround the start in vertebrates.
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Textbook Question
Answer the following questions about the accompanying diagram.
Is the DNA nearest A the template strand or the coding strand?
Textbook Question
Answer the following questions about the accompanying diagram.
What structure is closest to B?
Textbook Question
Organisms of all three domains of life usually use the mRNA codon AUG as the start codon.
Do organisms of the three domains use the same amino acid as the initial amino acid in translation? Identify similarities and differences.