If a double-stranded DNA sample were composed of 10% thymine, what would be the percentage of guanine? a. 10%b. 20%c. 40%d. 80%

c. 40%

Explanation:

DNA is composed of smaller subunits called nucleotides.

Structure of a nucleotide contains a 5-carbon sugar (Deoxyribose), phosphate group and one of the four nitrogenous bases.

Adenine and Thymine are nitrogenous bases found in DNA. Other nitrogenous bases are cytosine, guanine and uracil.

Adenine, Thymine, Guanine and Cytosine are found in DNA whereas in RNA, Uracil is present instead of Thymine.

DNA is a double stranded structure.

In DNA, adenine of one strand always pairs with thymine of the other strand (complementary). Similarly, guanine of one strand pairs with cytosine of complementary strand. Adenine pairs with Thymine via 2 hydrogen bonds whereas cytosine pairs with guanine via 3 hydrogen bonds. This hydrogen bonding maintains the double helix structure of DNA.

It is implied that for every thymine present in DNA there will be adenine on the complementary stand with which it will bind via 2 hydrogen bonds.

So if a double stranded DNA sample contains 10% Thymine, then it will also contain 10% adenine. This makes the combined contribution of adenine and thymine to be 20%. The left over 80% of DNA will contain equal amounts of cytosine and guanine as each guanine binds with cytosine via 3 hydrogen bonds. So if we divide 80% into 2 equal parts then it is 40%. Hence, there is 40% guanine, 40% cytosine, 10% adenine and 10% thymine.