Ch. 15 - Gene Mutation, DNA Repair, and Transposition

Problem 30

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Chapter 15, Problem 30

Shown below are two homologous lengths of the alpha and beta chains of human hemoglobin. Consult a genetic code dictionary (Figure 13.7), and determine how many amino acid substitutions may have occurred as a result of a single nucleotide substitution. For any that cannot occur as a result of a single change, determine the minimal mutational distance.

Alpha: ala val ala his val asp asp met pro
Beta: gly leu ala his leu asp asn leu lys

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Key Concepts

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

Homologous Sequences

Homologous sequences are DNA, RNA, or protein sequences that share a common ancestry. In the context of the question, the alpha and beta chains of hemoglobin are homologous proteins, meaning they evolved from a common ancestral gene. Analyzing these sequences helps identify similarities and differences that may arise from mutations, such as single nucleotide substitutions.

Amino Acid Substitution

Amino acid substitution occurs when a single nucleotide change in the DNA sequence leads to the replacement of one amino acid with another in the protein sequence. This can affect the protein's structure and function. Understanding how these substitutions arise from nucleotide changes is crucial for determining the potential impact of mutations on protein function.

Mutational Distance

Mutational distance refers to the number of changes required to transform one sequence into another. In this context, it involves calculating how many nucleotide changes are necessary to convert the alpha chain sequence into the beta chain sequence. This concept is essential for understanding the genetic variations and evolutionary relationships between homologous proteins.

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Mutations and Phenotypes

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Textbook Question

With the knowledge that radiation causes mutations, many assume that human-made forms of radiation are the major contributors to the mutational load in humans. What evidence suggests otherwise?

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Textbook Question

Among Betazoids in the world of Star Trek®, the ability to read minds is under the control of a gene called mindreader (abbreviated mr). Most Betazoids can read minds, but rare recessive mutations in the mr gene result in two alternative phenotypes: delayed-receivers and insensitives. Delayed-receivers have some mind-reading ability but perform the task much more slowly than normal Betazoids. Insensitives cannot read minds at all. Betazoid genes do not have introns, so the gene only contains coding DNA. It is 3332 nucleotides in length, and Betazoids use a four-letter genetic code. The following table shows some data from five unrelated mr mutations. Mutation Description of Mutation Phenotype _ mr-1 Nonsense mutation in codon 829 Delayed-receiver mr-2 Missense mutation in codon 52 Delayed-receiver mr-3 Deletion of nucleotides 83–150 Delayed-receiver mr-4 Missense mutation in codon 192 Insensitive mr-5 Deletion of nucleotides 83–93 Insensitive For each mutation, provide a plausible explanation for why it gives rise to its associated phenotype and not to the other phenotype. For example, hypothesize why the mr-1 nonsense mutation in codon 829 gives rise to the milder delayed-receiver phenotype rather than the more severe insensitive phenotype. Then repeat this type of analysis for the other mutations. (More than one explanation is possible, so be creative within plausible bounds!)

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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.
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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.

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Textbook Question

It has been noted that most transposons in humans and other organisms are located in noncoding regions of the genome—regions such as introns, pseudogenes, and stretches of particular types of repetitive DNA. There are several ways to interpret this observation. Describe two possible interpretations. Which interpretation do you favor? Why?

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Textbook Question

Mutations in the IL2RG gene cause approximately 30 percent of severe combined immunodeficiency disorder (SCID) cases in humans. These mutations result in alterations to a protein component of cytokine receptors that are essential for proper development of the immune system. The IL2RG gene is composed of eight exons and contains upstream and downstream sequences that are necessary for proper transcription and translation. Below are some of the mutations observed. For each, explain its likely influence on the IL2RG gene product (assume its length to be 375 amino acids).

Nonsense mutation in a coding region