Final answer:
Double-stranded DNA is primarily held together by hydrogen bonds between complementary nitrogenous bases and stabilized by covalent bonds in the sugar-phosphate backbone.
Step-by-step explanation:
The structure of double-stranded DNA is held together by specific kinds of bonds between the two complementary strands. The correct answer to the types of bonds that hold one complementary strand of DNA to the other is hydrogen bonds. These bonds form between the complementary nitrogenous bases on the interior of the DNA double helix, specifically between adenine and thymine, which form two hydrogen bonds, and between guanine and cytosine, which form three hydrogen bonds. In addition to hydrogen bonds, the DNA double helix is stabilized by covalent bonds within the sugar-phosphate backbone of each strand, where the phosphate group of one nucleotide bonds to the sugar of the next nucleotide. The two strands of DNA run antiparallel to each other, meaning they are oriented in opposite directions.