Question

A.  Suppose that the sequence below was the open reading frame of a gene. It’s not the entire mRNA molecule, just the protein coding sequence and in this case the first three bases is the start codon. The sequence is given as DNA sense strand because DNA is where mutations happen.  

5’-ATGTCTAAACTGCGTGAGTAA-3’

i.  Give the sequence of the polypeptide it encodes. Make sure to mark the carboxyl and amino terminal ends.

ii.   Give a mutant version of the second codon that has a single base substitution that is a synonymous mutation.

iii.   Give a mutant version of the fourth codon that has a single base substitution that is a missense mutation.

iv. Write a mutant version of the entire sequence that is an example of an in frame insertion. Indicate what part of the sequence is different.

Answer 1

Ans.

Given sense DNA strand-

5'-ATGTCTAAACTGCGTGAGTAA-3'

RNA POLYMERASE always transcribe Antisense strand means 3' ---> 5' .

Hence Antisense DNA Will be-

3'-TACAGATTTGACGCACTCATT-5'

mRNA-

5'-AUGUCUAAACUGCGUGAGUAA-3'

polypeptide-

NH2-met-ser-lys-leu-arg-glu-cooH.

ii. Synonymous mutation in second codon.

Antisense mutant DNA-

3'-TACTGATTTGACGCACTCATT-5'

Transversion mutation because A (purine) is replaced by T (pyrimidine)

mRNA-

5'- AUGACUAAACUGCGUGAGUAA-3'

polypeptide-

NH2-met-thr-lys-leu-arg-glu-cooH.

iii. Fourth codon missense mutation-

Antisense mutant DNA in fourth codon-

3'-TACAGATTTGCCGCACTCATT-5'

Transversion mutation because A is replaced by C (pyrimidine)

mRNA-

5'-AUGUCUAAACGGCGUGAGUAA-3'

polypeptide-

NH2-met-ser-lys-arg-arg-glu-cooH.

iv. Insertion-

Insertion of C in between second and 3rd codon then all remaining reading frame in right side will be changed as-

Mutated antisense DNA strand-

3'-TACAGAC​​TTTGACGCACTCATT-5'

mRNA-

5'-AUGUCUGAAACUGCGUGAGUAA-3'

polypeptide-

NH2-met-ser-glu-thr-ala-cooH.

Because 6th codon is converted in stop codon.