A type of bacteria is discovered in which the genetic materi

A type of bacteria is discovered in which the genetic material is a single, circular molecule of double-stranded DNA in which some instances of one of the bases (either A, T, C, or G) is modified by the addition of a methyl (–CH3) group. In analyzing the DNA of this organism, the individual strands of the circular DNA molecule are first separated and then the base composition of each strand determined. One strand consists of 22% A, 26% T, 18% C, 30% G, and 4% M; the other strand consists of 26% A, 22% T, 25% C, 22% G, and 5% M, where “M” represents the chemically modified bases. Which base in double-stranded DNA does M pair with?

adenine

b. thymine

c. cytosine

d. guanine

Solution

d. guanine

In DNA methylation, the methylation mostly occurs at cytosine. Cytosine pairs with guanine.