Why is DNA composed by 4 bases, not more or less than?

Answer 1

Since nucleotides pair-up in a specific manner A-T and G-G, you always need an even number of nucleotides.
With 3 nucleotides making up a codon, and 4 nucleotides available, this is a good number for coding for 20 amino acids, plus the STOP codons: there are 4^3 (=64) possible codons with this combination.
If there were only 2 nucleotides, it would only code for 2^3 codons (=8), so you would need a codon length of at least 5 to code for all 20 possible amino acids, plus the STOP codons (2^5 = 32), and this would dramatically reduce the efficiency of the genetic code - almost halving the amount of information per length it could carry.
Having 6 nucleotides in DNA would entail having two more synthetic pathways for the production of those nucleotides - so this is also inefficient.

Basically, 4 nucleotides in 3-nucleotide-long codons is the "best" combination.

Answer 2

The way our body works is that there are about 20 amino acids. These are the building blocks of proteins, which are ultimately the machinery that builds our cells and other structures and maintains them. Obviously, amino acids are a different "language" from DNA, as one is a protein and the other is genetic material.

So, in order to "translate" from DNA into amino acids, the cells came up with a system. A consecutive number of DNA bases would translate into one specific amino acid. So how many bases would be enough?

Firstly, consider that if DNA needs to be replicated to be able to be passed on down to the next generation, it has to be in the double helical structure that Watson and Crick made famous. Which means that each strand of the DNA must be complementary to the other strand. Which is to say that ultimately, the number of DNA bases must be even. For example, 2, 4, or 6.

Somehow evolution determined that 3 consecutive bases would translate into 1 amino acid. Less than that and the cell would need more DNA bases to effectively cover all the possible amino acids. This would make the genes way to huge and bulky, as well as increasing the risk for mutations. More than that and there would be a lot of redundancy. Meaning, the number of possible combinations that the DNA can produce far outnumbers the number of possible amino acids.

So, with this triplet system, how many bases are required to code for 20 amino acids? From mathematics, it can be shown that b^3 is the formula to apply, where b is the number of bases available for DNA and the 3 represents the 3 consecutive bases. Solving that you'd get b = 4 (since b was determined to be even numbers). b=2 would not cover all the amino acids, and b > 4 would give too much redundancy.

Yup, that's my long take on this. Hope it wasn't too hard to follow through the deluge of information. =p

Answer 3

I guess 4 bases is the minimum you need to make enormous combinations of genetic code. There are twenty-something amino acids, with three nucleotides per amino acid you really don't need more than 4x4x4 = 64 possible codes to cover them all. Nature tends to use no more than needed (unlike man).

Answer 4

It works. Why not something else? You can hypothesize, but it's hard to know - maybe that is the most efficient number; or maybe that's the first number that worked well and there's no strong reason to change it. Or maybe that's the number that our alien creators decided on because they have only four fingers. etc., etc.

Answer 5

Because it just happens to be that way? You can find another base in RNA if you really want.

Answer 6

those are all the bases that are known.

so DNA can only use what exist.