1. a. Analysis of the DNA of the Phenylalanine hydroxylase (PAH) gene in a patie
ID: 98582 • Letter: 1
Question
1. a. Analysis of the DNA of the Phenylalanine hydroxylase (PAH) gene in a patient with phenylketonuria revealed a mutation in the protein coding region whose predicted effect would be to replace the amino acid aspartic acid, with histidine. Nevertheless, no mutant protein could be found in the patient’s cells. Briefly explain how this could happen.
b. How is it possible for a gene with a mutation in the coding region to encode a polypeptide with the same amino acid sequence as the non-mutant gene?
Explanation / Answer
1a.This may happen either because of mutation occurring in non coding regions such as introns or correction of mutaion by post translational modification. Introns are those regions in DNA which are called as junk DNAs as they have no role in the expression of proteins.Perhaps the mutation occurred in this region and thus the protein produced was mutation free.Similarly post transcriptional modification might have reverted the mutated change which has caused the protein to be expressed normally.Sites that often undergo post-translational modification are those that have a functional group that can serve as a nucleophile in the reaction:for eg.amine group of histidine and carboxylates of aspartic acid easily serve the target for post transcriptional modification.
b.This may happen due to degeneracy of genetic code.This means that there are many instances in which different codons specify the same amino acid. For eg. For example, six different codons indicate the amino acid leucine: UUA, UUG, CUA, CUG, CUC, and CUU. When any one of these codons turns up in the message, the cellular protein-building machinery inserts a leucine into the growing amino acid chain.Thus if there is a mutaion which changed the codon from UUA to UUG ,it will still code for leucine.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.