Which of the following mutations would be most likely to have a harmful effect on an organism?
a. a deletion of three nucleotides near the middle of a gene
b. a single nucleotide deletion in the middle of an intron
c. a single nucleotide deletion near the end of the coding sequence
d. a single nucleotide insertion downstream of, and close to, the start of the coding sequence

d. a single nucleotide insertion downstream of, and close to, the start of the coding sequence.

Explanation:

A single nucleotide insertion downstream and close to the start of coding sequence will produce the most harmful effect among all these given options because it will cause the entire reading frame to shift in the beginning itself leading to insertion of wrong amino acids in the entire polypeptide chain, hence the entire protein will have extremely different amino acids than the original protein was supposed to have and such protein will be dysfunctional.

In option (a), it is given that the three nucleotides near the middle of the gene will be deleted. This scenario will produce comparatively less harmful effect because in this case only one amino acid which is coded by this deleted triplet codon will be absent in the polypeptide.

In option (b), a single nucleotide deletion has occurred that too in an intron. It will produce no harmful effect in case of eukaryotes because introns are ultimately removed before translation so the amino acids in the polypeptide will remain the same. In case of prokaryotes such deletion may cause some harmful effect in case of mRNA only because introns in prokaryotic mRNA are not removed. But, if the mRNA is for prokaryotic tRNA and rRNA then there will be no harmful effect because they undergo processing in which introns are removed.

In option (c) also there will be very less harmful effect because deletion is occurring near the end of coding sequence which will lead to insertion of wrong amino acid in the polypeptide at the end of polypeptide chain that means very less abnormal amino acids will be present so effect will be less harmful.

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i am positive it is c. condensing but i may be wrong