Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

11. Translation

Translation

Genetics

11. Translation

Translation

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3:01 minutes

Problem 38b

Textbook Question

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?

Verified step by step guidance

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.

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.

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

The flow of genetic information from DNA to protein is mediated by messenger RNA. If you introduce short DNA strands (called antisense oligonucleotides) that are complementary to mRNAs, hydrogen bonding may occur and 'label' the DNA/RNA hybrid for ribonuclease-H degradation of the RNA. One study [Lloyd et al. (2001). Nucl. Acids Res. 29:3664–3673] compared the effect of different-length antisense oligonucleotides upon ribonuclease-H–mediated degradation of tumor necrosis factor (TNFα) mRNA. TNFα exhibits antitumor and pro-inflammatory activities. The following graph indicates the efficacy of various-sized antisense oligonucleotides in causing ribonuclease-H cleavage. What factors other than oligonucleotide length are likely to influence antisense efficacy in vivo?

The flow of genetic information from DNA to protein is mediated by messenger RNA. If you introduce short DNA strands (called antisense oligonucleotides) that are complementary to mRNAs, hydrogen bonding may occur and 'label' the DNA/RNA hybrid for ribonuclease-H degradation of the RNA. One study [Lloyd et al. (2001). Nucl. Acids Res. 29:3664–3673] compared the effect of different-length antisense oligonucleotides upon ribonuclease-H–mediated degradation of tumor necrosis factor (TNFα) mRNA. TNFα exhibits antitumor and pro-inflammatory activities. The following graph indicates the efficacy of various-sized antisense oligonucleotides in causing ribonuclease-H cleavage. Describe how antisense oligonucleotides interrupt the flow of genetic information in a cell.

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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.

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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.

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Textbook Question

Answer the following questions about the accompanying diagram.

Does the diagram depict molecular activity in a bacterium or a eukaryote? Explain the reasoning for your answer.

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