Why is the genetic code based on a three bas sequence and not one, two or four?

Answer 1

That is how it is. It isn't a man-made thing so asking 'why' is really pointless.

Answer 2

Asking the question of "why" takes the science out of the answer. For that you can rely on the other answers.

The explanation using the number of amino acids as a reason approaches the question the wrong way. The bases in DNA would also determine the overall number of amino acids possible, so that is a rather weak explanation. As well, the formation of the amino acid comes last. I'll come back to this in a moment.

In science, we can do experiments and formulate hypotheses and the like, and in the end, postulate the "how".

So, how is it that the genetic code relies on a three base sequence? I believe the answer lies in thinking of the process of gene expression, which is one of the coding functions of all of our cells. The very earliest life forms were likely RNA based organisms, instead of DNA. We know of RNA molecules that have both enzymatic and coding functions. The transfer RNA molecule that is involved in gene expression, happens to bind three bases in sequence on one side of the tRNA and an amino acid on the other side. As this passes through/along the ribosome, a protein (gene product) is created. So, at a molecular level, the three base pairs are needed because of what the tRNA is sized to bind to. Also important size-wise is the likelihood of it binding on the ribosome, either in the E,A, or P sites. A larger tRNA probably wouldn't bind as efficiently. Ribosomes were likely present evolutionarily well before proteins became the "money-shot", so to speak.

All of this said, going back to the 20 amino acids thing, is that throughout the evolution of biological entities, there has likely always been a shuffling here and there of what we know today. It could be that there were organisms along the way that did code by means of 1,2 or 4 base sequences, or that used five bases instead of four, and utilized many more amino acids. It could be that too many amino acids makes life less simple, an over-riding factior in biology. Too few, and we can't survive. Yet, out of the twenty aa's, we only need to intake around half of those, and our cells make the rest by modifying the ones we intake. Is that a sign that earlier life existed on fewer amino acids? Another clue is the number of different transfer RNA molecules our cells possess. Mammals have 100 to 110 different tRNA's, yet from the Code, we only need about 60. Bacteria have 60. Why do mammals have almost twice as many?

Or should I say, how is it that mammals have 110 different tRNA's?

Answer 3

well, it is the standard code already that was established by James Watson and Francis Crick.

what matters most in the DNA sequence is that Nucleotides pair in a specific way called the Base-Pair Rule.Adenine pairs to Thymine while Guainine pairs to Cytosine. The rungs of the ladder can occur in any order (as long as the base-pair rule is followed). failure to do so will lead to mutation

Answer 4

Nobody knows for sure. Three bases are required to code for the twenty amino acids plus a stop code, and nature prefers economy, so four, being one more than necessary, would be evolutionally inferior.

Answer 5

rhsaunders is correct.
1 is not enough (as with 4 bases, this would only allow 4 amino acids)
2 is not enough (as this would allow encoding of 4^2 = 16 amino acids); and
4 is too many (able to encode 4^4 = 256 amino acids),
... to encode the 20 standard amino acids

Answer 6

I suspect the answer is - because.

It is what it is.

If you dig any deeper into questions like that, you are going to run up against th God hypothesis - and while it may be the only explanation, there will be a lot of people who won't like it.

So - just because.