How is it that a code consisting of only four letters as in dna can specify all the different parts of an organism and account for all the diversity of organisms on this planet

The genetic code can be defined as a set of rules used to encode information from genetic material (DNA or RNA ) into proteins. There are sequences of nucleotide triplets, -codons which specify which amino acid will be added next during translation (protein synthesis). Genetic code consists of 64 codons and this means that some amino acids are specified by more than one codon.

Genetic code has a role to determine amino acid sequence of protein (primary structure of protein). Location and the function of all those synthesized proteins is determinated by regulatory genomic regions (gene regulatory codes).

Proteins perform a wide range of functions within organisms such as catalysing metabolic reactions, DNA replication, stimuli response, structure role, transport role. So, all the diversity of organisms on this planet is due to protein variety.