Question

Which of the following alterations of the codons ATTGCC is most serious?

• A. ATCGCC
• B. ATTGCA
• C. ATTCCCGCC
• D. ATTTGCC

Answer 1

The correct option is D ATTTGCC

In option A and B, the given sequence is changed from ATTGCC to ATCGCC and ATTGCA respectively by replacement of one base with other; exchange of one nitrogenous base for other base is called substitution. It changes only single codon and thereby replaces single amino acid with other; entire reading frame is not affected. In option C, the given sequence is changed from ATTGCC to ATTCCCGCC by addition of three bases and thus one amino acid but there is no effect on rest of the reading frame. In option D, the given sequence is changed from ATTGCC to ATTTGCC by addition of “T” at 4th place. The gene mutations that alter the base sequence of whole genetic frame from the point of the mutation are called frame shift mutations. Here, the reading frame of base sequence is shifted forward due to insertion of one nucleotide base which in turn changes the succeeding codon sequence from ATT- GCC to ATT- TGC- C and cause the most serious alternation among the given options. Hence, option D is correct answer.