Explain why a base substitution is often less harmful than a base deletion or insertion.?

Answer 1

A base subsitution onto the DNA strand is likely to cause a change in the aminoacid sequence of the protein coded by the affected gene. However, this (one) point mutation could be silent (if the resulting codon codes for the same aminoacid) or could code for a similar aminoacid to the originally coded one, thus not affecting the structure of the protein itself.

Insertions or deletions, though, cause frame-shift mutations, which means that the whole sequence after the mutation will be affected. This generally results in the appearance of a STOP codon and early termination of the protein coded by the mutated gene.

In a couple of words: the second type of mutations cause changes to more aminoacids than the point mutations.

Answer 2

You have to think of this in the context of a sequence of codons.
Codons are triplets of nucleotides of which there are 64 possible combinations that can be built from the 4 bases, A, C, G and T. Each codon codes for a specific amino acid of which there are 20. Some amino acids have more than one possible codon that codes for them, so in the case of a mere nucleotide substitution, there is a chance that the codon of one amino acid is changed to another codon for the same amino acid. This is known as a "silent mutation."
Some amino acids are very similiar to each other, like serine and threonine, or aspartate and glutamate. If a codon is changed so that it results in a similiar amino acid being substituted for the first, it is considered a "conservative" change and is not likely to affect the function of the protein being coded for.

In a deletion or insertion, every single codon downstream is affected by such a change and this is referred to as a "frameshift" mutation.

Take a look at what happens when I leave out a letter.
Take a look tw hath appensw henI l eaveo uta l etter.

Chances are the shift will result in a premature termination of the protein. Not a good thing.

Answer 3

base substitution is less harmful than deletion or addition coz in the latter two the rest of the reading frame that follows is disturbed however in case of substitution only one codon that is one amino acid get changed or even there are rare chances that the substituition result in the codon which codes for the same amino acid according to the wobble hypothsesis.