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Ch. 11 - Gene Mutation, DNA Repair, and Homologous Recombination
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 11 - Gene Mutation, DNA Repair, and Homologous RecombinationProblem 30c
Chapter 11, Problem 30c

A fragment of a wild-type polypeptide is sequenced for seven amino acids. The same polypeptide region is sequenced in four mutants.
Table comparing a wild-type polypeptide sequence with four mutant sequences, highlighting amino acid changes.
Identify the mutation that produces each mutant polypeptide.

Verified step by step guidance
1
Step 1: Analyze the wild-type polypeptide sequence and compare it to each mutant sequence. Note the differences in amino acid composition and length for each mutant.
Step 2: For Mutant 1, observe that Glycine (Gly) is replaced by Valine (Val). This suggests a missense mutation, where a single nucleotide change alters the codon to encode a different amino acid.
Step 3: For Mutant 2, note that the sequence is truncated after Serine (Ser). This indicates a nonsense mutation, where a nucleotide change introduces a premature stop codon, halting translation.
Step 4: For Mutant 3, observe that the sequence starts with Thr–Thr instead of Thr–His, and ends with Leu–Asp. This suggests a frameshift mutation, likely caused by an insertion or deletion that shifts the reading frame.
Step 5: For Mutant 4, note the significant change in sequence and length, with Thr–Gln–Leu–Trp–Ile–Glu–Gly replacing the original sequence. This suggests a frameshift mutation due to a larger insertion or deletion, altering the reading frame and producing a completely different polypeptide.

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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 amino acid has a specific side chain that determines its properties and role in protein structure and function. Understanding the sequence of amino acids in a polypeptide is crucial for identifying mutations, as even a single change can significantly affect the protein's characteristics.
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Proteins

Mutations

Mutations are changes in the DNA sequence that can lead to alterations in the amino acid sequence of a polypeptide. They can be classified as substitutions, deletions, or insertions. Identifying the type of mutation in the context of the wild-type and mutant sequences is essential for understanding how these changes impact protein function and can lead to different phenotypes.
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Mutations and Phenotypes

Comparative Sequence Analysis

Comparative sequence analysis involves examining the differences between the wild-type and mutant polypeptide sequences to identify specific mutations. By comparing the sequences side by side, one can pinpoint where the amino acid changes occur, which is critical for determining the nature of each mutation and its potential effects on the protein's function.
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Sequencing Overview
Related Practice
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:

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

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Textbook Question

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.

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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.

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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.

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Textbook Question

Alkaptonuria is a human autosomal recessive disorder caused by mutation of the HAO gene that encodes the enzyme homogentisic acid oxidase. A map of the HAO gene region reveals four BamHI restriction sites (B1 to B4) in the wild-type allele and three BamHI restriction sites in the mutant allele. BamHI utilizes the restriction sequence 5′-GGATCC-3′. The BamHI restriction sequence identified as B3 is altered to 5′-GGAACC-3′ in the mutant allele. The mutation results in a Ser-to-Thr missense mutation. Restriction maps of the two alleles are shown below, and the binding sites of two molecular probes (probe A and probe B) are identified.

DNA samples taken from a mother (M), father (F), and two children (C1 and C2) are analyzed by Southern blotting of BamHI-digested DNA. The gel electrophoresis results are illustrated.

Using A to represent the wild-type allele and a for the mutant allele, identify the genotype of each family member. Identify any family member who is alkaptonuric.

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Textbook Question

Alkaptonuria is a human autosomal recessive disorder caused by mutation of the HAO gene that encodes the enzyme homogentisic acid oxidase. A map of the HAO gene region reveals four BamHI restriction sites (B1 to B4) in the wild-type allele and three BamHI restriction sites in the mutant allele. BamHI utilizes the restriction sequence 5′-GGATCC-3′. The BamHI restriction sequence identified as B3 is altered to 5′-GGAACC-3′ in the mutant allele. The mutation results in a Ser-to-Thr missense mutation. Restriction maps of the two alleles are shown below, and the binding sites of two molecular probes (probe A and probe B) are identified.

DNA samples taken from a mother (M), father (F), and two children (C1 and C2) are analyzed by Southern blotting of BamHI-digested DNA. The gel electrophoresis results are illustrated.

In a separate figure, draw the gel electrophoresis band patterns for all the genotypes that could be found in children of this couple.

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