Concept explainers
CASE STUDY | Crippled ribosomes
Diamond Blackfan anemia (DBA) is a rare, dominantly inherited syndrome characterized by bone marrow failure, birth defects, and a significant predisposition to cancer. Those affected with DBA usually develop anemia in the first year of life, have abnormal numbers of cell types in their bone marrow, and have an increased risk of developing leukemia and bone cancer. At the molecular level, DBA is caused by a mutation in any of 11 genes that encode ribosomal proteins. The common feature of all these mutations is the disruption of ribosome formation, ultimately affecting the stability or function of ribosomes.
Many questions about this disorder remain to be answered.
Given the central importance of ribosomes in maintaining life, how is it possible that individuals carrying mutations in ribosomal protein genes survive?
To review:
The reason for the survival ofindividuals with mutations in ribosomal protein gene, even though ribosomes are central to survival.
Introduction:
Mutations are changes in the genome of an organism and are permanent. When these mutations occur in the coding sequences, these have devastating effects as they can cause deadly diseases. However, if the mutations occur in one of the non-coding sequences, these are not manifested. Ribosomal proteins are proteins that constitute a ribosome along with ribosomal ribonucleic acid (rRNA). Ribosomes are crucial for protein synthesis as they are involved in the process of translation.
Explanation of Solution
The rRNA genes on the DNA (deoxyribonucleic acid) are repetitive fragments found clustered in many sites on the chromosome. These repetitive clusters consist of tandem repeats. Each of the clusters of repetitive DNA is separated by a spacer DNA sequence, which does not code for any protein.
Thus, one of the reasons why individuals with mutations in these genes could survive would be that the mutations occurred in one of the non-coding spacer DNA regions. Such mutations are called as missense mutations. Even if the mutations occurred in the coding region, because of the tandem repeats and repetitive clusters on many chromosomal sites, a single mutation on one of them will not affect the individual adversely to a point that makes it fatal.
Thus, it can be concluded that individuals with mutations in the ribosomal genes often survive because these genes are repetitive DNA sequences and are present at many chromosomal sites. The mutation in one of them will not affect the translation greatly in the sense that protein can beform in spite of mutation in either of them. A mutation can also occur in the non-coding DNA region and not affect the protein synthesis at all.
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Chapter 13 Solutions
Essentials of Genetics (9th Edition) - Standalone book
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