Ch. 11 - Gene Mutation, DNA Repair, and Homologous Recombination

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

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Sanders 3rd Edition

Ch. 11 - Gene Mutation, DNA Repair, and Homologous Recombination

Problem 30a

Chapter 11, Problem 30a

A fragment of a wild-type polypeptide is sequenced for seven amino acids. The same polypeptide region is sequenced in four mutants.

Use the available information to characterize each mutant.

Verified step by step guidance

Step 1: Compare the wild-type polypeptide sequence with each mutant sequence to identify the differences in amino acid composition and length. Note the positions where the sequences diverge.

Step 2: For Mutant 1, observe that the amino acid 'Gly' in the wild-type sequence is replaced by 'Val'. This suggests a single nucleotide substitution in the codon encoding 'Gly', resulting in a missense mutation.

Step 3: For Mutant 2, note that the sequence ends prematurely after 'Ser'. This indicates a truncation of the polypeptide, likely caused by a nonsense mutation introducing a premature stop codon.

Step 4: For Mutant 3, observe that the sequence starts with 'Thr' but diverges completely after that, with 'Thr–Leu–Asp'. This suggests a frameshift mutation, possibly due to an insertion or deletion altering the reading frame.

Step 5: For Mutant 4, note the significant divergence in sequence length and composition ('Thr–Gln–Leu–Trp–Ile–Glu–Gly'). This could be due to a large insertion or a complex rearrangement, such as a duplication or translocation event affecting the coding region.

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Key Concepts

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

Amino Acids and Polypeptides

Amino acids are the building blocks of proteins, linked together by peptide bonds to form polypeptides. Each polypeptide has a specific sequence of amino acids that determines its structure and function. In this context, understanding the sequence of amino acids in both the wild-type and mutant polypeptides is crucial for characterizing the mutations and their potential effects on protein function.

Mutations and Their Effects

Mutations are changes in the DNA sequence that can lead to alterations in the amino acid sequence of proteins. These changes can be classified as missense (where one amino acid is replaced by another), nonsense (leading to a premature stop codon), or frameshift (causing a shift in the reading frame). Analyzing the differences between the wild-type and mutant sequences helps in understanding how these mutations may affect the protein's function.

Protein Function and Structure

The function of a protein is closely tied to its three-dimensional structure, which is determined by the sequence of amino acids. Changes in the amino acid sequence due to mutations can lead to alterations in the protein's shape, stability, and activity. By characterizing the mutants in relation to the wild-type sequence, one can infer potential functional consequences of the mutations on the protein's role in biological processes.

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Related Practice

Textbook Question

Experiments by Charles Yanofsky in the 1950s and 1960s helped characterize the nature of tryptophan synthesis in E. coli. In one of Yanofsky's experiments, he identified glycine (Gly) as the wild-type amino acid in position 211 of tryptophan synthetase, the product of the trpA gene. He identified two independent missense mutants with defective tryptophan synthetase at these positions that resulted from base-pair substitutions. One mutant encoded arginine (Arg) and another encoded glutamic acid (Glu). At position 235, wild-type tryptophan synthetase contains serine (Ser) but a base-pair substitution mutant encodes leucine (Leu). At position 243, the wild-type polypeptide contains glutamine and a base-pair substitution mutant encodes a stop codon. Identify the most likely wild-type codons for positions 211, 235, and 243. Justify your answer in each case.

Textbook Question

A fragment of a wild-type polypeptide is sequenced for seven amino acids. The same polypeptide region is sequenced in four mutants.

Identify the mutation that produces each mutant polypeptide.

Textbook Question

A wild-type culture of haploid yeast is exposed to ethyl methanesulfonate (EMS). Yeast cells are plated on a complete medium, and 6 colonies (colonies numbered 1 to 6) are transferred to a new complete medium plate for further study. Four replica plates are made from the complete medium plate to plates containing minimal medium or minimal medium plus one amino acid (replica plates numbered 1 to 4) with the following results:

For colonies 1, 3, and 5, write '+' for the wild-type synthesis and '−' for the mutant synthesis of histidine and leucine.

Textbook Question

Identify any colonies that are his⁻, arg⁻, leu⁻.

Textbook Question

Are there any colonies for which genotype information cannot be determined? If so, which colony or colonies?

Textbook Question

A fragment of a wild-type polypeptide is sequenced for seven amino acids. The same polypeptide region is sequenced in four mutants.

Determine the wild-type mRNA sequence.

A fragment of a wild-type polypeptide is sequenced for seven amino acids. The same polypeptide region is sequenced in four mutants. Use the available information to characterize each mutant.

Verified video answer for a similar problem:

Key Concepts

Amino Acids and Polypeptides

Mutations and Their Effects

Protein Function and Structure

A fragment of a wild-type polypeptide is sequenced for seven amino acids. The same polypeptide region is sequenced in four mutants.

Use the available information to characterize each mutant.