Question

weak hydrogen bonds between the nitrogenous bases hold the two strands of a dna molecule together. but hydrogen bonds can only form between certain nitrogenous bases. this is called complementary base pairing. in any dna molecule, adenine pairs only with thymine, and guanine pairs only with cytosine. complete the paragraph to describe the model. the dna model above shows nucleotides. according to the rules of complementary base pairing, cytosine always pairs with , and always pairs with adenine.

Explanation:

1. To find the number of nucleotides: Count the nitrogenous bases. In the diagram, we can see 8 nitrogenous bases (A, T, C, G, T, A, G, C), so there are 8 nucleotides (each nucleotide has a nitrogenous base, sugar, and phosphate).
2. Complementary base pairing: From the text, "guanine pairs only with cytosine" and "adenine pairs only with thymine". So cytosine pairs with guanine, and thymine pairs with adenine.

Answer:

The DNA model above shows $\boldsymbol{8}$ nucleotides. According to the rules of complementary base pairing, cytosine always pairs with $\boldsymbol{guanine}$, and $\boldsymbol{thymine}$ always pairs with adenine.