Talmudic Question #27

 

Do all genes have some homology with others? If all are derived from the genome of LUCA (Last Universal Com­mon Ancestor), the answer is easy. But, to start an ar­gument, what if there were extraneously made pieces of genetic material lying around waiting to be picked up by LUCA's descendents? (Courtesy of Enrique Merino and Ana Gutierrez)

 
 

 
 

Comments

[Bill Martin]

Dear Enrique and Ana,

that's an interesting question, the answer is an unabiguous yes, because of the formulation. Let's ask "...with all others?" And let's make it even more concrete, leaving genes from outer space out of the issue for a minute. Let's consider the origin of genes. IF we assume that the origin of genes from the elements is possible (which we are assuming here, I suppose, even for genes from outer space, unless they somehow did not arise from the elements, but we ARE in outer space, so what, exactly, is the difference anyway...). Ahem. IF we assume that the origin of genes from the elements is possible, then does the origin of something that is almost a gene but not yet a gene happen only one single time -- one molecule, from which all gene molecules descend? That is where your question leads, I think. Refusing to entertain the issue concerning the definition of a gene, I would muse that if there were molecules in a spot somewhere that could give rise to one molecule of what one would call gene, then that the same process would be giving give rise to lots of different molecules of gene, if for no other reason than molecules tend to occur in fairly large numbers when the conditions for them to occur are right. Otherwise we are all descended from one single pretty small molecule, which I have trouble finding at all likely.

After all, if there was only enough gene-like organized C, N, O, H, P (and a dash of S in those pesky modified tRNA bases) to make one single little bitty gene to begin with, then we have a pretty severe problem making more of it for round two to get to luca or anywhere else. That leads us to lots of molecules, more than one at the non-gene-to-gene threshold at least. And if bits of those different first gene like molecules made some contribution via their however-replicated descandants into what became a luca, then not all genes are homologous (similar by virture of common specific molecular origin in this question). So it would seem that even if all genes can be traced to luca, the answer is still not "easy", because it depends upon how the genes in luca came to be, even if luca never stooped so low as to pocket a used gene from outer space. So small things in large numbers considered, the most satisfying answer appears to be a comforting no.

Its like with cells: Genes always arise from preexisting genes, except at the origin of genes from the elements, where the problem sprouts new facets.