The question of exactly how DNA arose (and how many times) is not yet resolved; There are a number of hypotheses that have supporting evidence, and that have not yet been shown to be incorrect, all of which involve simpler chemical precursors. None of them include a 'who', much less a God, as there is exactly zero evidence for such a thing, and to postulate one would raise far more questions than it would answer.
Our best hypotheses for the origin of DNA is that it evolved from RNA precursors:
The first step in the emergence of DNA has been most likely the formation of U-DNA (DNA containing uracil), since ribonucleotide reductases produce dUTP (or dUDP) from UTP (or UDP) and not dTTP from TTP (the latter does not exist in the cell). Some modern viruses indeed have a U-DNA genome, possibly reflecting this first transition step between the RNA and DNA worlds. The selection of the letter T occurred probably in a second step, dTTP being produced in modern cells by the modification of dUMP into dTMP by thymidylate synthases (followed by phosphorylation). Interestingly, the same kinase can phosphorylate both dUMP and dTMP. In modern cells, dUMP is produced from dUTP by dUTPases, or from dCMP by dCMP deaminases. This is another indication that T-DNA originated after U-DNA. In ancient U-DNA cells, dUMP might have been also produced by degradation of U-DNA.
The origin of DNA also required the appearance of enzymes able to incorporate dNTPs using first RNA templates (reverse transcriptases) and later on DNA templates (DNA polymerases). In all living organisms (cells and viruses), all these enzymes work in the 5' to 3' direction. This directionality is dictated by the cellular metabolism that produces only dNTP 5' triphosphates and no 3' triphosphates. Indeed, both purine and pyrimidine biosyntheses are built up on ribose 5 monophosphate as a common precursor. The sense of DNA synthesis itself is therefore a relic of the RNA world metabolism. Modern DNA polymerases of the A and B families, reverse transcriptases, cellular RNA polymerases and viral replicative RNA polymerases are structurally related and thus probably homologous (for references, see a recent review on viral RNA-dependent RNA polymerases.) This suggests that reverse transcriptase and DNA polymerases of the A and B families originated from an ancestral RNA polymerase that has also descendants among viral-like RNA replicases.
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The emergence of DNA is therefore a result of progressively more simple precursor molecules, via simple chemistry. DNA comes from RNA, which comes from simple Phosphates and simple sugars, such as Ribose. Phosphate is just Phosphorous and Oxygen, and will form spontaneously if these two elements are present; similarly Ribose can form spontaneously where water and carbon are present, particularly in reducing conditions where energy is available, but carbon is relatively scarce. Both Phosphate and a variety of simple monosaccharides, including Ribose, have been observed in interstellar dust clouds - they form by atoms simply sticking together. The atoms themselves are generated by nuclear fusion in the cores of stars, from Hydrogen. So the origin of DNA is:
Hydrogen
-stars-> Carbon, Phosphorous, Nitrogen and Oxygen
Phosphorous, Oxygen and Hydrogen
-simple chemistry-> Phosphates and simple sugars; Amino acids
Phosphates and simple sugars
-simple chemistry-> RNA
RNA and Amino Acids
-RNA catalysed chemistry-> Proteins
RNA
-Protein catalysed chemistry-> DNA
DNA
-Protein catalysed chemistry-> all of modern biochemistry
Each step starts with something simpler, and leads to something a little more complex; And so, from very simple beginnings, we can explain the existence of the complexity we see today.
The alternative hypothesis - that complex systems can only derive from systems that are more complex than themselves - runs into a logical dead-end. If humans are so complex that only a hugely complex and powerful God could have made them, then God requires an even more complex and powerful creator; which in turn requires an even more complex and powerful creator, and so on forever. The only way to break this death-spiral of logic is to assume that at some point, something just exists without cause. There is exactly zero reason to imagine that this 'uncaused cause' would be highly complex. I can stomach the idea of a few simple particles spontaneously arising from nothing, or having simply existed eternally, far more easily than the idea that something sufficiently complex as to have intelligence could do so. And all the evidence points towards simple origins. We know that the universe gets simpler, the further back in time we look. A handful of quarks, plus a few billion years, leads to everything we observe today without any need for intelligent intervention of any kind, prior to the (relatively recent) spontaneous evolution of intelligence.
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