Thread: Nucleotides ----> Amino Acid

Natural Selection naturally selected for 3 nucleotides to code for one Amino Acid (or a 'signal-stop' translation).

It's curious, because there are 64 different nucleotide triplets, and only 20 Amino Acids, meaning many of the AA's are coded by more than one triplet.

Why shouldn't Nature have naturally selected for 2 nucleotides to be the coding principle, or 4? Or whatever? Is it random?

From what I remember from 1st year biochem many years ago it is a trade off between allowing some redundancy so closely related sequences of nucleotides can code for the same amino acid to reduce the effect of mutations in one position and the energy cost involved in making longer DNA chains if there are more nucleotides in each codon. This isn't my field though, someone who knows more biology may correct me but that's my understanding of it.

Originally Posted by TheLayman

Natural Selection naturally selected for 3 nucleotides to code for one Amino Acid (or a 'signal-stop' translation).

It's curious, because there are 64 different nucleotide triplets, and only 20 Amino Acids, meaning many of the AA's are coded by more than one triplet.

Why shouldn't Nature have naturally selected for 2 nucleotides to be the coding principle, or 4? Or whatever? Is it random?

Some genes code many proteins, depending on from which way it is read (4 possible ways of each gene to be read, not counting introns/exons). It is just as random as the rest of evolution and progress is.

So the multiple instances of ways to code for a certain protein, it takes the amino-acids that are more numerous in proteins to have more possible codes. To prevent a frameshift of totally absorbing all sulphate, or tryptophane in a cell, and thus killing it.

For short it could be interpreted as a safeguard to help mutations and differentiating of different proteins, with fewer chance of proteins being unusable. Sometimes evolution needs to bridge a wider gap, for which a certain mutation has a slight disadvantage, but the next mutation has a big advantage.

So i agree with PhDemon on some parts, but it has nothing to do with costs involved with energy costs for making longer DNA chains. I couldn't conceive a scenario of which this would happen, but please correct me if i am wrong about this as well.

It could be I've misremembered the "energy cost" part. I definitely remember being taught about the redundancy and limiting the effect of mutations in one nucleotide on protein structure but it was a long time ago, the details are hazy.

Originally Posted by TheLayman

Why shouldn't Nature have naturally selected for 2 nucleotides to be the coding principle...

When people first began to think about the genetic code it was predicted to be a triplet code in advance (notably by cosmologist George Gamow). A duplet code can't discriminate between 20 amino acids - there are only 16 possible combinations (4 X 4). It was realised that a triplet code would give 64 options, more than enough to do the job.

Some ideas on the evolution of the code suggest that the current code evolved from a duplet code; other ideas suggest it evolved from a quadruplet code. In addition to the reasons mentioned above as to why the code is triplet I'd throw in to the mix the suggestion that it is an historical accident. Or perhaps there are stereochemical reasons? I don't think the question has been resolved.