Why are exogenous purines and pyramidines not incorporated into the cell's DNA/ RNA?

Question

Food contains both purine and pyramidine bases. But, from the books and teachers, we learned that the cell adops the denovo synthesis or salvage pathway.
But my question is:
Why are exogenous purines and pyramidines not incorporated into the cell's DNA/ RNA?

My postgraduate student posed this question. I went through couple of biochemistry and molecular biology books. And I also googled. I need an answer to satisfy student.

Answer 1

Organisms differ in their ability to synthesize nitrogenous bases. Some parasites (for example, Tetrahymena pyriformis, T. cruzi, and S. mansoni) require purines and pyrimidines through diet because they are not capable of de novo synthesis. Even more complex metazoans can take up the building blocks of nucleic acids if they are present. Otherwise, assays using tritiated thymidine or chemotherapies based on nucleoside analogues would not work.

Obligate parasites may be able to survive solely by using host resources but other organisms would still need to be able to synthesize nitrogenous bases as needed. For a complex organisms to supply cells distant from the digestive tract with these bases would require the development of a system to take up and redistribute these nutrients. There could be complications with signaling because a purine could cross react with an ATP receptor.

Answer 2

I am not a student of biology generally nor specifically, but, my understanding is that no part of DNA/RNA is incorporated and genetics all starts from what is there to be seen and its genesis theory is unsubstantiated theory in absence of immediate process sensing observation but inter-observational comparison for DNA complex magnitude classification.

Did you mean to write 'the cell adops the denovo synthesis' or 'the cell adopts the denovo synthesis'? DNA/RNA simply does not live up to our standards for immutable perfection. Perhaps the DNA reopens after formation closure? But that does not seem wise to me unless that is what stem cells need for their differentiation.

Answer 3

Yep, wondered why myself as all living creatures have their individual signature as DNA/RNA and food is a living thing... maybe you have bypassed a classifications of the subjects... but all in all, it is under the DNA/RNA.