Ch. 12 - The Genetic Code and Transcription

Klug - Essentials of Genetics 10th Edition

Klug10th EditionEssentials of GeneticsISBN: 9780135588789Not the one you use?Change textbook

Klug 10th Edition

Ch. 12 - The Genetic Code and Transcription

Problem 23c

Chapter 12, Problem 23c

Shown here are the amino acid sequences of the wild-type and three mutant forms of a short protein.
____
Wild-type: Met-Trp-Tyr-Arg-Gly-Ser-Pro-Thr
Mutant 1: Met-Trp
Mutant 2: Met-Trp-His-Arg-Gly-Ser-Pro-Thr
Mutant 3: Met -Cys-Ile-Val-Val-Val-Gln-His

Use this information to answer the following questions:
The wild-type RNA consists of nine triplets. What is the role of the ninth triplet?

Verified step by step guidance

Step 1: Understand that the wild-type protein sequence is composed of eight amino acids, but the RNA has nine triplets (codons). Each codon corresponds to one amino acid or a special signal during translation.

Step 2: Recall that the last codon in an mRNA sequence often does not code for an amino acid but instead serves as a stop codon, signaling the end of translation.

Step 3: Identify that the ninth triplet in the wild-type RNA likely functions as a stop codon, which terminates protein synthesis and ensures the protein is the correct length.

Step 4: Compare the mutant sequences to see if any changes affect the length or composition of the protein, which can help confirm the role of the ninth triplet as a stop signal.

Step 5: Conclude that the ninth triplet's role is to act as a termination signal (stop codon) in the mRNA, preventing further addition of amino acids beyond the eighth residue.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Genetic Code and Codons

The genetic code consists of triplets of nucleotides called codons, each specifying a particular amino acid or a stop signal during protein synthesis. Understanding how codons translate into amino acids is essential for interpreting RNA sequences and their corresponding protein products.

Stop Codons and Termination of Translation

Stop codons are specific triplets in mRNA (UAA, UAG, UGA) that do not code for amino acids but signal the end of translation. The ninth triplet in the wild-type RNA likely functions as a stop codon, terminating protein synthesis and defining the protein's length.

Effects of Mutations on Protein Sequence

Mutations can alter the amino acid sequence by changing codons, causing premature stops or extensions. Comparing wild-type and mutant sequences helps identify the role of specific codons, such as how the absence or change of the ninth triplet affects protein length and function.

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Related Practice

Textbook Question

In a mixed copolymer experiment, messages were created with either 4/5C:1/5A or 4/5A:1/5C. These messages yielded proteins with the following amino acid compositions.

Using these data, predict the most specific coding composition for each amino acid.

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Textbook Question

Shown here are the amino acid sequences of the wild-type and three mutant forms of a short protein.

___________________________________________________

Wild-type: Met-Trp-Tyr-Arg-Gly-Ser-Pro-Thr

Mutant 1: Met-Trp

Mutant 2: Met-Trp-His-Arg-Gly-Ser-Pro-Thr

Mutant 3: Met-Cys-Ile-Val-Val-Val-Gln-His _

Use this information to answer the following questions:

Using the genetic coding dictionary, predict the type of mutation that led to each altered protein.

1047

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Textbook Question

Shown here are the amino acid sequences of the wild-type and three mutant forms of a short protein.

___________________________________________________

Wild-type: Met-Trp-Tyr-Arg-Gly-Ser-Pro-Thr

Mutant 1: Met-Trp

Mutant 2: Met-Trp-His-Arg-Gly-Ser-Pro-Thr

Mutant 3: Met-Cys-Ile-Val-Val-Val-Gln-Hi

___________________________________________________

Use this information to answer the following questions:

For each mutant protein, determine the specific ribonucleotide change that led to its synthesis.

478

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Textbook Question

Shown here are the amino acid sequences of the wild-type and three mutant forms of a short protein.

___________________________________________________

Wild-type: Met-Trp-Tyr-Arg-Gly-Ser-Pro-Thr

Mutant 1: Met-Trp

Mutant 2: Met-Trp-His-Arg-Gly-Ser-Pro-Thr

Mutant 3: Met-Cys-Ile-Val-Val-Val-Gln-Hi

___________________________________________________

Use this information to answer the following questions:

Of the first eight wild-type triplets, which, if any, can you determine specifically from an analysis of the mutant proteins? In each case, explain why or why not.

566

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Textbook Question

Shown here are the amino acid sequences of the wild-type and three mutant forms of a short protein.

___________________________________________________

Wild-type: Met-Trp-Tyr-Arg-Gly-Ser-Pro-Thr

Mutant 1: Met-Trp

Mutant 2: Met-Trp-His-Arg-Gly-Ser-Pro-Thr

Mutant 3: Met -Cys-Ile-Val-Val-Val-Gln-His

___________________________________________________

Use this information to answer the following questions:

Another mutation (mutant 4) is isolated. Its amino acid sequence is unchanged from the wild type, but the mutant cells produce abnormally low amounts of the wild-type proteins. As specifically as you can, predict where this mutation exists in the gene.

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Textbook Question

Recent observations indicate that alternative splicing is a common way for eukaryotes to expand their repertoire of gene functions. Studies indicate that approximately 50 percent of human genes exhibit alternative splicing and approximately 15 percent of disease-causing mutations involve aberrant alternative splicing. Different tissues show remarkably different frequencies of alternative splicing, with the brain accounting for approximately 18 percent of such events [Xu et al. (2002). Nucl. Acids Res. 30:3754–3766].

Why might some tissues engage in more alternative splicing than others?

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