Answer questions about aspects of watson-crick model of dna structure
a. the directionality of complementary strands is antiparallel. explain what this means
b. base pairing is specific. how is this specificity determined - why don't just any nucleotide bases favorably interact?
c. specific numbers of non-covalent interactions hold together the two strands in the double helix. what are these interactions, and how many occur between each base pairing?
d. the phosphate backbone is able to interact with water on the outside of the molecule. why would this be a chemically favorable interaction?

a. The directionality of the complementary strand is antiparallel. The double-stranded DNA is antiparallel comprising two strands, which run alongside each other, however, point in reverse directions. In a double-stranded molecule of DNA, the 5 prime ends of one strand align with the 3 prime ends of the other strand, and vice versa.  

b. The mediation of base pairing is done by non-covalent hydrogen bonds. It is reversible, that is, the strands can separate and can come combined again without any chemical modification.  

The specificity of base pairing is illustrated by hydrogen bonds that is, the first strand produces hydrogen bonds only with a particular base in the second strand, and these two bases produce a base-pair.  

c. The interaction or association of non-covalent type, that is, hydrophobic interaction and hydrogen bonding. Collectively, they both are strong. In the process, one base associate with the other base on the complementary strand.  

d. The phosphate backbone possesses the tendency to associate with water on the outside of the molecule, and this is favorable chemically as both the strands are held together by hydrogen bond and hydrophobic interaction.  

The polar molecules of sugar can produce hydrogen bonds with the surrounding molecules of water. The negatively charged phosphate group associate with the positively charged ions. The nonpolar nitrogen bases are found within the molecule and associate favorably through stacking interactions.  

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: