In the dna double helix, how are the two strands of dna held together?

a. hydrogen bonds between sugar groups and phosphate groups hold the two strands of dna together.
b. chemical bonds between base pairs hold the two strands of dna together.
c. hydrogen bonds between the base pairs hold the two strands of dna together.
d. chemical bonds between sugar groups and phosphate groups hold the two strands of dna together.

The answer is C. Hydrogen bonds between the base pairs hold the two strands of DNA together.

Explanation:

Watson and Crick proposed the double helical model of DNA. This model says that DNA exists as a double helix. It has two polynucleotide strands. Each strand consists of a sequence of nucleotides linked together by phosphodiester bonds. The backbone is formed of sugar and phosphate. The nitrogen bases are projected more or less perpendicular to the sugar phosphate backbone but face inside. The base pairing is specific. The two polynucleotide strands are held together in their helical structure by hydrogen bonding between bases in opposing strands. Adenine and thymine form two hydrogen bonds. Guanine and cytosine form three hydrogen bonds.

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