12. Which molecule does cytosine pair with in the complementary DNA strand?

A. thymine

B. adenine

C. alanine

D. acetylene

E. guanine

E guanine

Explanation:

Each nucleotide base can hydrogen-bond with a specific partner base in a process known as complementary base pairing: Cytosine forms three hydrogen bonds with guanine, and adenine forms two hydrogen bonds with thymine.

Answer: Guanine

Explanation:

The specific base pairings in DNA are Adenine to Thymine (A:T), and Cytosine to Guanine (C:G)

Hence, the double helix of DNA is held together by hydrogen bonds between pairs of bases as Guanine pair with Cytosine base with three hydrogen bonds while Adenine pair with Thymine with two hydrogen bonds.

Thus, cytosine binds to guanine on the complimentary DNA.