1. Three nucleotide bases on mRNA (which are complementary to three nucleotide bases on DNA) are called codon. Each codon codes for the specific amino acid. So, for 1 amino acid in protein, there are 3 nucleotides on RNA (or DNA). That is why the coding sequence is three times as long as the protein sequence it codes for.
2. There are 20 different amino acids, but there are more than 20 combinations of three nucleotide bases on DNA. There are 64 different combinations of three nucleotide bases on DNA, which is about 3 times more than the number of amino acids. So, different combinations of three nucleotide bases code for one amino acid and this is the reason why the DNA sequence is so different when the protein sequence isn’t
