What types of mutations are the most harmful

Frameshift mutations

Explanation:

Frameshift mutations are generally much more serious and often more deadly than point mutations. Even though only a single nitrogen base is affected, as with point mutations, in this instance, the single base is either completely deleted or an extra one is inserted into the middle of the DNA sequence.

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the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.