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

Klug - Concepts of Genetics 12th Edition

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Klug 12th Edition

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

문제 31c

15장, 문제 31c

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

검증된 단계별 안내

Step 1: Understand the nature of the mutation. A frameshift mutation caused by an insertion in Exon 7 means that nucleotides are added, shifting the reading frame of the mRNA from that point onward.

Step 2: Recall that the IL2RG gene codes for a protein of 375 amino acids. Since Exon 7 is near the end of the gene, the frameshift will alter the amino acid sequence starting from the mutation site and likely affect the C-terminal portion of the protein.

Step 3: Recognize that a frameshift often introduces premature stop codons downstream, which can truncate the protein, resulting in a shorter and usually nonfunctional protein product.

Step 4: Consider the functional consequences. Since the mutation affects the cytokine receptor component, the altered or truncated protein may fail to properly participate in immune system development, contributing to SCID pathology.

Step 5: Summarize that the insertion causing a frameshift in Exon 7 likely leads to a dysfunctional or truncated IL2RG protein, impairing its normal role in cytokine receptor signaling.

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Frameshift Mutation

A frameshift mutation occurs when nucleotides are inserted or deleted from the DNA sequence in numbers not divisible by three, altering the reading frame of the gene. This shift changes every amino acid downstream of the mutation, often resulting in a nonfunctional protein or premature stop codon. In the IL2RG gene, a frameshift in exon 7 would disrupt the normal protein sequence, likely impairing its function.

Gene Structure and Exons

Genes are composed of exons and introns; exons are coding regions that translate into protein sequences. The IL2RG gene has eight exons, each contributing specific amino acid sequences to the final protein. Mutations in exons, such as exon 7, directly affect the protein's amino acid sequence and structure, influencing its biological activity.

Protein Function in Immune System Development

The IL2RG gene encodes a common gamma chain protein essential for cytokine receptor function, which is critical for immune cell development and signaling. Mutations that alter this protein can disrupt cytokine receptor assembly or signaling, leading to severe combined immunodeficiency (SCID) by impairing immune system maturation and response.

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교과서 질문

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).

Deletion in Exon 2, in frame

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교과서 질문

Insertion in Exon 1, causing frameshift

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Deletion in Exon 2, causing frameshift

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Nonsense mutation in a coding region

Missense mutation

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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