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You are watching: How many nitrogen bases make up a codon

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How have the right to just four nitrogenous bases--adenine, cytosine, guanine, and also uracil--possibly password for every 20 amino acids?

Once scientists figured out that messenger RNA (mRNA) servedas a copy of every gene"s DNA and specified the sequence of amino acids inproteins, they immediately had many more questions about the process of proteinformation. Specifically, these researchers knew that proteins space made from 20different amino acids. Moreover, they additionally knew the there were only fournucleotides in mRNA: adenine (A), cytosine (C), guanine (G), and also uracil (U). Buthow exactly could these 4 nucleotides password for all 20 amino acids? Theanswer to this question turned the end to be much easier than one could expect.


Right away, researcher knew that the hereditary code to be more complicated than one nucleotide per amino acid. After ~ all, if this to be the case, a person"s DNA can only password for four different amino acids. In fact, even two nucleotides per amino mountain (i.e., a doublet code) might not account for 20 amino acids, due to the fact that such a code gives only 16 permutations (four bases at each of 2 positions = 4 × 4 = 16 amino acids).




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Early researchers researching the hereditary code had to determine if the mRNA encoding amino acids was non-overlapping. Was it every sequential set of 3 nucleotides encoding one amino acid? Or was it overlapping, v each three-nucleotide code start on sequential solitary nucleotides?
Thus, beforehand researchers quickly determined that the smallest mix of As, Cs, Gs, and Us that might encode every 20 amino mountain in RNA would be a triplet (three-base) code. A triplet combination, or codon, would enable for 64 feasible combinations (four bases at each of three positions = 4 × 4 × 4 = 64). However, with just 20 amino acids, a triplet password would also suggest redundancy–in other words, an ext than one codon could correspond to the same amino acid, or there might even it is in "spare" or unused codons. If such "spare" codons to be present, what was their purpose? go they serve to "break up" the code, much like commas in a sentence? Furthermore, how might a three-nucleotide code be "read" through the protein-forming machinery of the ribosome? to be it one overlapping or non-overlapping password (Figure 1)? was it a consistent code, or were there "commas" (spare nucleotides) in between codons that offered as signals for the following amino acid (Table 1)? These inquiries were answer by way of number of elegant experiments.