Why don’t different organisms have nucleic acid genomes containing different bases and sugar?

Question

All organisms contain genetic material. Be it DNA or RNA, both of them have a fixed pentose group (deoxyribose and ribose). Also they contain the same types of nitrogenous bases, Adenine, Thymine(or Uracil), Guanine, Cytosine. What makes ribose and these notrogenous bases so good that among all the molecules in our pre-historic chemical pool, these selective compounds made our genetic material.

Why don't different organisms have different bases/sugars? I understand that we all have a common evolutionary ancestor so the the genetic material will be similar. But mutation is possible, so why is that there are no nucleic acids found naturally that are made of different bases or sugars? Just for example, if we had a hexose like glucose for the sugar instead of pentose..

Answer 1

Everyone is giving you a hard time for this question. But it's a great question! Good for you for asking it.

The answer is that we really don't know why nucleic acids are composed of ribose, as opposed to any other sugars. We don't even know why life on earth only uses L-ribose instead of D-ribose. Or if instead of being carbon-based, we don't know if extraterrestrial life could be silicon-based.

Similarly, two D and L forms of amino acids also exist. (Right handed vs left handed mirror-image molecules that otherwise act the same.) Organisms on earth only use the L-form! We have no idea why. If a meteorite landed on earth that contained D-amino acids, that would really throw scientists for a loop.

(*this link might be of interest to you: https://en.wikipedia.org/wiki/Murchison_meteorite#Organic_compounds)

I'm going to give you a word of advice, and I really hope you get to read this. You are an "out of the box" thinker and the average people will get stumped when you ask questions like this. Ignore them.

If everyone asked grey, imaginative questions instead of following the black-and-white rules they learned, society wouldn't function. But individuals like you help advance society. We need both types of people equally. You happen to be in the latter category. People in the first category will have a difficult time with the questions you ask. Accept and respect them, but continue on and keep asking your questions anyway.

Eventually, you will find people who think like you do. I suggest finding others interested in astrobiology, which is a growing field and that needs people like you in it. Maybe biochemistry or evolutionary physiology too.

Answer 2

You are incorrect in your assertion that:

…there are NO nucleic acids found naturally that are made of different bases or sugars

Different bases are found in the anti-codon loop of transfer RNA, in the cap of eukaryotic messenger RNA and in ribosomal RNA, which also contains modified sugars.

You might say that this is pedantic or not to the point, as these variants do not occur in the genetic material. However at least it demonstrates that Nature has ‘played’ with this and decided only to use these fancy Lego bricks for special projects.

I dislike questions of the sort you pose, because any answer tends to be of the form “The structure we have is favoured because it can do this, this and this, which nothing else can do as well.” And, of course, we really don’t know that nothing else can do it — we just assume so because things are the way they are.

With that disclaimer, one might suggest that Mother Nature has shown a willingness to improve on things by moving from RNA genomes to ‘better’ DNA genomes with a different sugar and one different base. (My own explanation of why this is ‘better’ can be found in answer to Why deoxyribose for DNA and ribose for RNA?.) So although we can’t be sure that another system would not work as well, the fact Mother Nature has stuck with DNA suggests that she has found nothing that works sufficiently better to move to that. (In academic circles this circular argument is expressed in terms of evolutionary advantage.)

Answer 3

It sounds like you are confused about how Darwinian evolution "works". For example, as we survey the current life forms in this biosphere there is absolutely no evidence that these species (or their molecules, which you refer to) are optimized in any way, shape, or form. All we can state with any certainty is that at some time in the past, during one, or more, selective events, all the species that used ribose, deoxyribose, and the five common bases (A, C, G, T, and U) for their genetic material, survived to reproduce, and contribute progeny to the next generation.

If that putative selective event was different, then perhaps all the pentose-based species would have died off, and we would all have hexose-based genetic material.

We don't know how many different genetic material recipes were tried out before this one "won".

Evolution can only work on, or select, or select against, the current batch of organisms. It is not an optimization algorithm, it is a filter that lets variants through.

It may well be that our genetic material, for example, replicates faster than the other candidates. That could be a selective advantage, but it doesn't have to be.

So what are you really asking here (since your original question contains incorrect assumptions)?

Answer 4

A small extra note on your question: you are wrongly using the term 'mutation'. Mutation is a process that can work on genes (or chromosomes). As the definition of mutation states: "The changing of the structure of a gene, resulting in a variant form which may be transmitted to subsequent generations, caused by the alteration of single base units in DNA, or the deletion, insertion, or rearrangement of larger sections of genes or chromosomes."

So mutation works on genes (and their order, sequence, ...), with genes as the 'smallest unit'. It does not work on the molecular structure of genes. If changes in the molecular structure of the pyrimidines/purines are made, these are chemical reactions resulting in other molecules. Which is a completely different proces than 'mutation'.

Answer 5

We all have a common ancestor. It had a set of genetic mechanisms that worked, and was overall better at surviving than whatever else was available at the time.

Its advantage may have been its DNA/RNA, or it may have been something else and the DNA/RNA just came along for the ride.

At this point you have a planet full of of moderately successful organisms that are all genetically related. If any completely new organisms appeared, they would be out-competed before they got off the ground.

Now, you can imagine an organism getting a mutation changing the way the genetic code is read. What happens then? That organism would be unable to read its own genome, and it would die.

What would be needed for a successful switch of code? Let say that we want to use Neoadenine instead of Adenine.

First, the cell needs to produce Neoadenine instead of Adenine.
Second, it needs to produce transfer-RNA that correctly transcribes codons containing Neoadenine.
Third, it needs to change all the existing genes to the new code.

Not only this, but it needs to do all this while not being eaten by its unmutated cousins.

So far, this has happened only once, when either DNA became RNA or the other way around.

The odds of it happening again are small, outside a laboratory dish.