Who proposed the polynucleic model, stating that dna and rna were composed of nucleotides?

Polynucleic model of  DNA and RNA was proposed by Phoebus Aaron Levene in 1909.

Explanation:

In early 1900's, Phoebus Aaron Levene worked on the composition of nucleic acid and after analysis found that DNA molecule was made of smaller molecules called nucleotides made of pentose sugar (deoxyribose), phosphates (PO⁴) and nitrogenous bases (adenine, thymine, guanine and cytosine) and called them backbone of the DNA.

Levene also showed the linked order of nucleotides -phosphate-sugar-base to form units as well as the differences between the pentose sugar of DNA and RNA.