How does a deletion of a base in the DNA base sequence lead to a mutated protein?

It is important to note that bases on the DNA (and mRNA) are read in separate sets of 3s along a DNA/(mRNA) stand (this is called a triplet code in DNA, or the codon in mRNA). So, if a base is deleted, the whole sequence undergoes a frame shift in which the base after the deleted one is read with the original group (E.g. ATTGCAC --> ATGCAC). This means that the triplet code (the set of 3 bases) is different. Note above how both triplet codes have been altered by this. This will happen to ALL the sequences following the deletion, so all those triplet codes will be altered and incorrect.

As the codon on the mRNA is transcribed from this altered DNA sequence, it will also have a different codon. The codon on the mRNA is translated into a polypeptide, with each codon of 3 bases coding to 1 specific amino acid. Hence, if the codon is wrong, the amino acid is wrong, so the polypeptide is wrong.

Polypeptides are long chains of amino acids strung together, and are folded into specific 3D shapes (the tertiary structure) to form a protein. They do this by forming hydrogen, ionic and/or disulphate bonds with the specific amino acids in the chain. If these amino acids aren’t there because a different one has been coded for by the mRNA due to the deletion in the DNA, then these bonds cannot form. Hence, the 3D shape will not be formed so the protein is not formed correctly, hence mutating it.