Its structure was evidenced by Photo 51, an X-ray captured in 1952
I believe the answer is:
dna
(Other definitions for dna that I've seen before include "Deoxyribonucleic acid (abbreviation)" , "Briefly, substance in chromosomes used by police to identify criminals" , "Substance that stores genetic information - often used by police" , "Substance in chromosomes that stores genetic information, used by police" , "Substance in chromosomes that stores genetic data" .)