This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure of DNA, Subject name: Biochemistry
If a 4 nucleotides 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: Ans. d. 256 (Ref: Harper 28/e p353)If there would have been two nucleotides in each codon, the chance of their varied combination will be 42, i.e. 16, and three nucleotide in each codon means there will occur 43, i.e. 64 codons are possible. According to the above calculation, if there occurs 4 nucleotides in a codon, the chance of their varied combination will be 44, i.e. 256.Number of Nucleotides in CodonNumber of Coded Amino Acids242= 16343= 64444= 256