The following is a list of mutations that have been discover

The following is a list of mutations that have been discovered in a gene that has 48 exons and encodes a very large protein of 2042 amino acids. For each mutation, indicate whether you would expect to see a detectable change in the size or the amount of mature mRNA and/or a detectable change in the size or the amount of the protein product (assume that changes in size or amount must be greater than 1% of normal values to allow detection). Do you think this change is likely to affect gene function? Why? (may not be one right answer) Val421Lys (changes amino acid 421 from valine into lysine) AAG1021AAA (changes codon 1021 from AAG to AAA) AAG576UAG One base-pair insertion at codon 721 Deletion of 6 bases in exon 15 Mutation of the first nucleotide in the eighteenth intron of the gene, causing exon 18 to be spliced to exon 20, thus skipping exon 19 Deletion of the poly-A addition site G to A substitution in the 5\' UTR Insertion of 1000 base pairs in the sixth intron (does not alter splicing)

Solution

a) Valine is a hydrophobic side chain amino acid and lysine is a positively charged amino acid. Valine side chain is very non reactive and rarely involved in protein function but it sometimes pay role in substrate recognition.Lysine side chain is positively charged and forms hydrogen bonds and salt bridge structures with oppositely charged residues. So a single nucleotide substitution from valine to lysine can lead change in gene function.But no detectable change in size of mRNA or protein will be observed .

b)AAG and AAA codons code for the same amino acid Lysine so a change from AAG to AAA will not change the gene function nor the size or amount of mRNA and protein

c)Change from AAG to UAG (UAG is a stop codon) will terminate translation prematurely and lead to formation of a truncated protein . Hence change in gene function is evident as well as observable change in the size of protein .But size of mRNA will be the same.

d)One base pair insertion at 721 will lead to change in frame of the sequence leading to frame shift mutation . Hence functionality of the protein will be lost and sometimes change in frame of a sequence can lead to stop codon insertion in between sequences and hence premature termination of translation can occur. Single base insertion will not cause any detectable change in size of mRNA . Protein size change change can occur if a stop codon is inserted. But if there is no stop codon insertion then detectable changes in protein size will not occur.

e) Deletion of 6 bases in exon 15 i.e. 2 codons can lead to change in protein function if the amino acids it codes for is vital for protein function .But detectable changes in size of mRNA and protein will not occur.

f)Mutation in the first nucleotide in 18th intron of the gene causing exon 18 to be spliced to exon 20 skipping exon 19 will cause observable change in protein size as well as size of mRNA as a complete exon is skipped during the process.

g)Poly A tail protects the mRNA molecule from enzymatic degradation in the cytoplasm and aids in transcription termination,export of the mRNA from the nucleus, and translation. Hence change in gene function is evident and protein translation may not even occur in its absence. Observable change in mRNA size will occur.

h)Within the 5\' UTR is a sequence that is recognized by the ribosome which allows the ribosome to bind and initiate translation. So if the substitution is critical ribosome binding will not occur hence translation will not be initiated. mRNA size or protein size will not be affected.

i) Insertion in the intronic regions doesnot affect any gene function as introns are not a part of mRNA . Hence it will not be translated either . Size of mRNA or protein will not be affected as the insertion is in the intronic regions ( non coding regions )