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If an organism on Earth has got DNA, would it always have C,T,G,A?

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closed as off-topic by David, James, theforestecologist, fileunderwater, kmm Mar 4 '18 at 22:06

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  • $\begingroup$ I have left school 16 years ago and this question is not homework related. I had no idea how to approach googling this question. I like SE a lot and the answer I got here is great. $\endgroup$ – J. Doe Mar 5 '18 at 4:30
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If an organism on Earth has got DNA, would it always have C,T,G,A?

Yes! All three domains of life (Archaea, Bacteria and Eukaryota) as well as all DNA viruses use these four same nucleotides.

Genetic code variations

There are however slight variations in the genetic code. In other words, there are variation in the meaning of a sequence of nucleotides and the corresponding protein sequence. See Alternative_genetic_codes.

RNA based life

There are also viruses (which you may and may not want to consider alive) that use RNA (and hence U instead of T) instead of DNA. See RNA virus.

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Yes, there are no variations on this. There are 3 domains of life; Archaea, Bacteria and Eukaryota. All use these identical nucleotides. All use all four of them, and this is one of many lines of evidence for the single origin of life on Earth.

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No, you can check out non-canonical bases on Wikipedia. I believe that most organisms, including E. coli and Homo sapiens have base modifications. In E. coli, the base modifications help them identify phage DNA and destroy it using restriction enzymes. In humans, many base modifications exist. A prominent modification, methylcytosine, can influence the state of chromatin and hence the expression of genes. Wikipedia mentions more obscure cases, e.g. a phage that has 2,6-diaminopurines instead of adenines.

I think that one should clarify the nomenclature in each context. I would envisage two cases:

  • a simplified case where base modifications are not considered, here methylcytosine = cytosine, methyladenine = adenine
  • a stricter case where modifications are considered, here methylC does not equal cytosine

Our simplified view of the reality of nucleotidic bases can be justified and explained by the limitations of current sequencing methods. It is impossible, as far as I know, for Illumina sequencers to differentiate modified from unmodified bases. Even sequencers like PacBio's Sequel or Oxford Nanopore's MinION that can detect modified bases can do so only at very high coverages and with some errors.

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  • $\begingroup$ Methylation does not make nucleotides not "ACTG". I don't believe that was the question. AGCTs are in everything. $\endgroup$ – Karl Kjer Mar 4 '18 at 13:21
  • $\begingroup$ @KarlKjer, well I don't know the exact definition of 'ACTG'. That's a bit vague and it would be clearer to say 'adenine, cytosine, guanine, thymine' bases. $\endgroup$ – charlesdarwin Mar 5 '18 at 10:30

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