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Ch. 15 - Gene Mutation, DNA Repair, and Transposition
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of GeneticsISBN: 9780135564776Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoKlug 12th EditionCh. 15 - Gene Mutation, DNA Repair, and TranspositionProblema 31a
Capitolo 15, Problema 31a

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

Guida verificata passo dopo passo
1
Step 1: Understand what a nonsense mutation is. A nonsense mutation is a point mutation in the DNA sequence that changes a codon encoding an amino acid into a stop codon, which signals termination of translation prematurely.
Step 2: Recognize that the IL2RG gene codes for a protein of 375 amino acids. A nonsense mutation in the coding region will cause the ribosome to stop translating the mRNA earlier than normal, resulting in a truncated protein that is shorter than 375 amino acids.
Step 3: Consider the functional consequences of a truncated protein. Since the protein is shorter, it may lack important functional domains necessary for cytokine receptor activity, potentially leading to loss of function or a nonfunctional protein.
Step 4: Reflect on the impact on the immune system. Because the IL2RG protein is essential for immune system development, a truncated, nonfunctional protein caused by a nonsense mutation could disrupt cytokine signaling and contribute to severe combined immunodeficiency disorder (SCID).
Step 5: Summarize that a nonsense mutation in the coding region of the IL2RG gene likely results in a shortened, nonfunctional protein product, impairing immune system function.

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Nonsense Mutation

A nonsense mutation is a change in the DNA sequence that introduces a premature stop codon within the coding region. This results in early termination of translation, producing a truncated and usually nonfunctional protein. In the IL2RG gene, such mutations can severely disrupt the protein's role in immune signaling.
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Point Mutations

Gene Structure and Exons

Genes are composed of exons, which are coding sequences, and introns, which are non-coding. The IL2RG gene has eight exons that together encode the full-length protein of 375 amino acids. Mutations within exons directly affect the amino acid sequence and thus the structure and function of the protein.
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Ribosome Structure

Protein Function in Cytokine Receptors

The IL2RG gene encodes a protein component of cytokine receptors critical for immune system development. Proper protein structure is essential for receptor function and signaling. Mutations that alter the protein can impair immune cell development, leading to disorders like SCID.
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Proteins
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Domanda del libro di testo

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).

Insertion in Exon 1, causing frameshift

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Domanda del libro di testo

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).

Insertion in Exon 7, causing frameshift

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Domanda del libro di testo

Skin cancer carries a lifetime risk nearly equal to that of all other cancers combined. Following is a graph [modified from K. H. Kraemer (1997). Proc. Natl. Acad. Sci. (USA) 94:11 14] depicting the age of onset of skin cancers in patients with or without XP, where the cumulative percentage of skin cancer is plotted against age. The non-XP curve is based on 29,757 cancers surveyed by the National Cancer Institute, and the curve representing those with XP is based on 63 skin cancers from the Xeroderma Pigmentosum Registry.

Provide an overview of the information contained in the graph. 

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Domanda del libro di testo

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).

Missense mutation

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Domanda del libro di testo

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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Domanda del libro di testo

Skin cancer carries a lifetime risk nearly equal to that of all other cancers combined. Following is a graph [modified from K. H. Kraemer (1997). Proc. Natl. Acad. Sci. (USA) 94:11 14] depicting the age of onset of skin cancers in patients with or without XP, where the cumulative percentage of skin cancer is plotted against age. The non-XP curve is based on 29,757 cancers surveyed by the National Cancer Institute, and the curve representing those with XP is based on 63 skin cancers from the Xeroderma Pigmentosum Registry.

Explain why individuals with XP show such an early age of onset.

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