What is a mutation ?

A mutation is a permanent change in the DNA sequence of a gene. Mutations in a gene's DNA sequence can alter the amino acid sequence of the protein encoded by the gene. How does this happen? Like words in a sentence, the DNA sequence of each gene determines the amino acid sequence for the protein it encodes. The DNA sequence is interpreted in groups of three nucleotide bases, called codons. Each codon specifies a single amino acid in a protein.

Mutate a sentence!

We can think about the DNA sequence of a gene as a sentence made up entirely of three-letter words. In the sequence, each three-letter word is a codon, specifying a single amino acid in a protein. Have a look at this sentence:

Thesunwashotbuttheoldmandidnotgethishat.

If you were to split this sentence into individual three-letter words, you would probably read it like this:

The sun was hot but the old man did not get his hat.

This sentence represents a gene. Each letter corresponds to a nucleotide base, and each word represents a codon. What if you shifted the three-letter "reading frame?" You would end up with

T hes unw ash otb utt heo ldm and idn otg eth ish at.

Or

Th esu nwa sho tbu tth eol dma ndi dno tge thi sha t.

As you can see, only one of these three "reading frames" translates into an understandable sentence. In the same way, only one three-letter reading frame within a gene codes for the correct protein.
Now, going back to the original sentence:

Thesunwashotbuttheoldmandidnotgethishat.

See how you can mutate the reading frame of this sentence by inserting or deleting letters within the sentence.
It's easy to make mutations that create "nonsense" sentences. Can you make mutations that maintain or change the meaning of the sentence without creating such nonsense?