Question

Fragile \(X\) syndrome is a common form of inherited mental retardation. The mutation in the disease allows the increase of a CGG repeat in a particular gene from a normal of about 30 repeats to \(200-1000\) repeats. This repeat is normally found in the \(5^{\prime}\) untranslated region of a gene for the protein FMR1. FMR1 might be involved in the translation of brain-specific mRNAs during brain development. The consequence of the very large number of \(C G G\) repeats in the DNA is extensive methylation of the entire promoter region of the FMR1 gene. Methylation of bases in DNA usually A. facilitates the binding of transcription factors to the DNA. B. makes a difference in activity only if it occurs in an enhancer region. C. prevents chromatin from unwinding. D. inactivates DNA for transcription. E. results in increased production of the product of whatever gene is methylated.

Short answer

Answer: Inactivates DNA for transcription.

Step-by-step solution

Understanding DNA methylation

First, let's understand what DNA methylation is. DNA methylation is a process that involves the addition of a methyl group to the Cytosine base in the DNA. This process plays a crucial role in controlling gene expression, and it can either activate or repress the transcription of specific genes.

Analyze the given options

Now that we have a basic understanding of DNA methylation, let's examine each option: A. Facilitates the binding of transcription factors to the DNA: Methylation can sometimes block the binding of transcription factors, which represses gene transcription. B. Makes a difference in activity only if it occurs in an enhancer region: This statement is incorrect because methylation can affect gene activity regardless of its location in the DNA. C. Prevents chromatin from unwinding: Methylation can lead to a more compact chromatin structure, which can prevent the chromatin from unwinding and make it less accessible to the transcription machinery. D. Inactivates DNA for transcription: Methylation can repress gene transcription by preventing the binding of transcription factors or making chromatin less accessible. E. Results in increased production of the product of whatever gene is methylated: Methylation can repress gene transcription, resulting in decreased expression of the gene's product.

Select the correct option

Based on the analysis of the given options, the correct answer is: D. Inactivates DNA for transcription. As the extensive methylation of the promoter region of the FMR1 gene leads to inactivation of DNA for transcription, which results in reduced expression of the gene's product.