Question

the two strands of a dna double helix are held together by __ that form between pairs of nitrogenous bases. options: hydrogen bonds, s - s bonds, ionic bonds, covalent bonds, hydrophilic interactions. part d a nucleotide is composed of a(n) __. options: phosphate group, a nitrogen - containing base, and a five - carbon sugar; sulfhydryl group, a nitrogen - containing base, and a five - carbon sugar; amino group, a nitrogen - containing base, and a five - carbon sugar; phosphate group, a nitrogen - containing base, and a hydrocarbon; glycerol, a nitrogen - containing base, and a five - carbon sugar.

Explanation:

1. In DNA structure, the two strands of the double - helix are held together by hydrogen bonds between complementary nitrogenous bases (A - T and G - C). S - S bonds are not involved in this connection. Ionic bonds and covalent bonds do not hold the two strands together in this context, and hydrophilic interactions are not the main force holding the DNA strands.
2. A nucleotide is composed of a phosphate group, a nitrogen - containing base, and a five - carbon sugar. A sulfhydryl group, amino group, hydrocarbon, or glycerol are not components of a nucleotide in the context of DNA structure.

Answer:

1. A. hydrogen bonds
2. A. phosphate group, a nitrogen - containing base, and a five - carbon sugar