Why can't dna, rna, and proteins be used to compare only dead organisms?

I am not too sure, but I believe it is because DNA is actually apart of living organisms along with Protein. So it can't only be compared with dead organisms but they can be living.

Hope this helps!

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.