How is it possible that over 20 million living organisms on the planet are made up of only 5 types of nucleotide based arranged in long chains?

Answer/Explanation:

The DNA in all living organisms is made up of 4 bases, adenine, thymine, guanine, cytosine. The RNA replaces thymine with uracil, making 5 types of nucleotide. The number of nucleotide pairs in a genome can range from half a million up to 100,000 million - meaning there are an exponential number of combinations of these 4 bases.

Imagine an organism exists that has only 2 nucleotides (this is over 200,000x smaller than even the smallest bacterial genome). If we allow any nucleotide at each of the 2 positions, then we have 4x4 (4²) or 16 possible combinations of sequences. For a nucleotide length of 4, the total number of possible combinations are 4⁴ or 256.

Since we are dealing with many millions of nucleotides, there are essentially infinite combinations of nucleotides, giving rise to the variation that produces over 20 million organisms on the planet.