WAEC Biology 2012 Papers

Concept:DNA strands are joined by specific complementary base pairing.Explanation:DNA contains four nitrogenous bases: $\text{adenine}$, $\text{guanine}$, $\text{cytosine}$, and $\text{thymine}$.By complementary base pairing, $\text{adenine}$ always pairs with $\text{thymine}$, and $\text{guanine}$ always pairs with $\text{cytosine}$.Option A is wrong because the two strands of the double helix are held together by hydrogen bonds between the bases, not by covalent bonds.Option B is wrong because a DNA nucleotide contains $\text{deoxyribose}$ sugar, not $\text{ribose}$; $\text{ribose}$ is found in $\text{RNA}$.Option D is wrong because $\text{adenine}$ is opposite $\text{thymine}$, not $\text{cytosine}$.Therefore, the only true statement is that $\text{guanine}$ is opposite $\text{cytosine}$.Answer:C. $\text{Guanine}$ is the opposite of $\text{cytosine}$