A professor of mine wrote that: $\frac{A+T}{G+C}$ is constant in double stranded DNA of different origin. However

The Chargaff rules state that due to base pairing in the DNA it holds that :

$A=T$ and $G=C$ and from that follows that $A+G=T+C$ in all DNA molecules

and thus: $\frac{A+G}{C+T}$ should be constant, no?

Who is in the wrong?


closed as unclear what you're asking by canadianer, David, WYSIWYG, kmm, Bryan Krause May 15 at 20:26

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  • $\begingroup$ Was your professor talking about Chargaff rules or about GC content? Why didn't you ask the professor to clarify? $\endgroup$ – Bryan Krause Apr 30 at 18:43
  • $\begingroup$ Constant in what sense? For all organisms? Please edit your question and make sure you recall correctly and completely what your professor said. And please, no ‘rules’ in any argument — just invoke what is known about the structure of DNA. $\endgroup$ – David Apr 30 at 19:05
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    $\begingroup$ It states verbatim: for double stranded DNA of different origins it holds that : $\frac{A+T}{G+C}$ , the ratio is wrong, and the one I gave in my answer is correct, right? $\endgroup$ – schokakola Apr 30 at 19:08

Context is important.

A/T == 1 and G/C == 1 (approximately, to within a measurement error) are Chargaff's rules for double-stranded DNA, occurring because A pairs with T and G pairs with C.

(A+T)/(G+C), or (G+C)/(A+T+G+C), are completely different ratios unrelated to Chargaff's rules. These are roughly constant across individuals for a given species and can be used to distinguish different species' genomes in an approximate manner.

I am not your professor, but it is possible in writing what they wrote, the professor was referring to whole-genome GC content within members of a species, in which case the professor is correct. If the professor was discussing Chargaff's rules, then the professor was wrong, but probably just made a mistake in writing: these things happen.

  • $\begingroup$ He states (verbatim): with double stranded DNA of different origin , the ratio $\frac{A+T}{G+C}$ is constant. Your answer doesn't really answer or help with anything , and just creates more confusion. $\endgroup$ – schokakola Apr 30 at 18:57
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    $\begingroup$ @schokakola I prefer to not just assume your professor is an idiot: in my experience, assuming people are idiots is counterproductive when other explanations exist. Thinking through multiple possible meanings is a good way to test your knowledge, so that was the intent of my answer. I've clarified my answer to make clear that if your professor wrote that ratio referring to Chargaff's rules, it is a mistake. $\endgroup$ – Bryan Krause Apr 30 at 18:59
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    $\begingroup$ @schokakola Can you clarify what exactly is still unclear to you compared to what you already know? "Your professor is right" or "your professor is wrong" is not an appropriate answer for Biology.SE, answers need to explain the concept and underlying biology. $\endgroup$ – Bryan Krause Apr 30 at 19:01
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    $\begingroup$ Let me remind you of the Code of Conduct - I am being very patient with you because you are addressing me, but if you were addressing anyone else I would not have such patience. See also biology.meta.stackexchange.com/questions/3377/… $\endgroup$ – Bryan Krause Apr 30 at 19:04
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    $\begingroup$ @schokakola The answer to your question depends on the context. I don't want you to go to your professor and tell them they are wrong if they are actually correct but you misunderstood. Therefore, my answer gives two explanations: a case where they could be wrong (if they were in fact talking about Chargaff's rules), and a case where they could be right (if they were talking about something else). Now you'd have to go to your notes and find what the context was so that you know which one is the case. The only one here who is knowledgeable about what was said in your class is you. $\endgroup$ – Bryan Krause Apr 30 at 19:08

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