Which type of mutation would be more harmful to an organism? When one of the nucleotides are missing...?

Question

or when one has been replaced by another nucleotide? Could you please explain?

Answer 1

When one of the nucleotides is missing. It's called a frameshift mutation and will alter how the cell reads DNA/mRNA to make the encoded protein. After the gene is transcribed everything up to the point of the deletion of a nucleotide will be fine when the mRNA is translated into protein, but at the point of the deletion and beyond the missing nucleotide will shift the reading frame by one. Remember how the ribosome reads the mRNA to discern the approprate amino acid sequence, whereby one amino acid is represented by a 3 nucleotide sequence=triplet with some overlap that we call codon's. There are ~61 codons and ~20 amino acids. The code is degenerate, it has a wobble position at the end because of the larger number of codons than amino acids ( The reason is outside the scope of this question), so that it's possible that CGU/CGG/CGC/CGA all can mean the same thing, in this case arginine. But imagine a situation like that described above, where you have CGUGGCA.... which means arginine+glycine..... If we get a deletion at the first position we no longer read it by CGU-GGC+A...., but rather GUG+GCA which means Valine+Alanine. You can see how we have completely changed the amino acid composition, and this will go on for the rest of the protein so we get something completely different in all aspects than the original. Cheers.

Answer 2

Missing. Nucleotides go in groupes of threes, so switching out one only screws with one group. Example

ABC DEF GHI JKL > ABM DEF GHI JKL, only one group has changed.

However, if you remove one: from the original, like

ABD EFG HIJ KL

Every single group gets screwed up instead of one.