Question

You encounter an extraterrestrial organism that makes proteins consisting of a combination of 100 types of amino acids. You also discover that it only uses a combination of 2 possible nucleotides (P and W) in its DNA and RNA. If other aspects of this creature’s biology are more like those of terrestrial organisms, what would be the minimal length of codons needed to code for all 100 types of amino acids?

Question 7 options:

	100 nucleotides
	8 nucleotides
	50 nucleotides
	7 nucleotides
	6 nucleotides

Answer 1

Let us look at our original genetic code, wherein we have 4 nucleotides and 20 amino acids. if we took that each codon was coded by only a single nucleotide, it could code for only 4 distinct amino acids. If the codons were of 2 nucleotide length, there could be 4* 4 = 16 codons which still cannot code distinctly for 20 amino acids. the next option, codons of 3 nucleotide length can have 4^3 or 64 combinations, leading to a number higher than 20 and so is able to code for all the amino acids with each amino acid being coded by distinct codons, even though some amino acids are coded by multiple codons.

In the same way, here we have 2 nucleotides and 100 amino acids. so we need combinations of these 2 nucleotides in at least 100 distinct ways. 2⁶is 64 which is less than 100. However, if the codons were 7 nucleotides in length, we have 2^7 or 128 possible codons, meaning that we can easily code for all the 100 amino acids as well as the stop codon.

So, the codon should be at least 7 nucleotides in length.