Who proposed that the structure of dna is a double helix with two polynucleotide chains running in opposite directions and held together by hydrogen bonding between pairs of nitrogenous bases?

James Watson and Francis Crick

Explanation:

James Watson and Francis Crick with few other scientists proposed that DNA is made up of a double polynucleotide chains that are coil/twisted around each other to forming a helical structure. The two DNA chains runs anti-parallel in opposite direction to each other.

Each strand of the DNA is made up of long chain of nucleotides. Each nucleotide contains a sugars (D-ribose), a phosphate group and a nitrogen base forming the backbone of each DNA strand.

They are both DNA strands are held together by hydrogen bond between pairs of nitrogenous bases.

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.

bradykinin is a physiologically and pharmacologically active peptide of the kinin group of proteins, consisting of nine amino acids.