This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
If a 4 nucleotide sequence code for an amino acid instead of 3 then theoretically how many unique amino acids could be coded by such a system? 
Select and explain. 
A. 16, B. 64, C. 128, D. 256
D: 256
Answer: D: 256 
Explanation: There are four types of nucleotides (A, T, G, C).
Normally the codons are made up of 3 nucleotides. Thus there are 43 = 64 possible codon combinations.
If the codons are made up of 4 nucleotides, then there would be 44 = 256 possible codon combinations.
If the codons are made up of 2 nucleotides, there would be 42 = 16 possible codon combinations.