a) the missence mutation in the mtAlaRS gene may occur at two positions. the one in which leucine is substituted by arginine at 155th position. leucine is a non-polar/ hydrophobic amino acid while arginine is a polar amino acid with basic charge. therefore, the folding pattern of protein related to the sequence of amino acids and thus its function will be altered. in mitochondria, tRNA (transfer ribonucleic acid) aminoacylation will get affected by this mutation. similarly, at position 592, arginine is substituted by trytophan. tryptophan is an amino acid with an aromatic ring which means the molecular weight is high compared to the arginine. thus, it also alters the structure and folding pattern of protein leading to malfunctioning. in mitochondria, diacylation in the case when tRNA is charged with wrong amino acid will not take place, leading to the incorporation of wrong amino acids in the polypeptide chain.
b) the second mutation i.e., the substitution of arginine by tryptophan is more severe. because, the result of it would be no proofreading system on the addition of wrong amino acids, and completely different (maybe lethal) protein may get synthesized.
7th EditionDavid L Nelson, Michael M. Cox 6th EditionDavid L Nelson, Michael M. Cox