WAEC 2012 Biology · Question 49

In the structure of DNA, which of the following statements is true?

Options

A.The double helix are held together by covalent bonds
B.Nucleotide is made up of ribose, phosphate and an organic nitrogen compound
C.Guanine is the opposite of cytosine
D.Adenine is the opposite of cytosine

Answer: C. Guanine is the opposite of cytosine

Explanation written with AI and checked against the answer key

Step 1: Analyze the question We need to pick the one true statement about DNA structure.

Step 2: Key concepts

DNA is a double helix made of two polynucleotide strands.
The two strands are held together by hydrogen bonds between complementary bases.
Base pairing rule: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).
A DNA nucleotide = deoxyribose sugar + phosphate + nitrogenous base.

Step 3: Evaluate each option

A. Wrong. The strands are joined by hydrogen bonds, not covalent bonds. (Covalent bonds link the sugar and phosphate within each strand's backbone.)
B. Wrong. DNA contains deoxyribose, not ribose. Ribose is in RNA.
C. Correct. Guanine sits opposite cytosine in the double helix, forming three hydrogen bonds. "Opposite" here means complementary base pairing.
D. Wrong. Adenine pairs with thymine, not cytosine.

Step 4: Answer

Tip: Remember "Apples in the Tree, Cars in the Garage" (A-T, C-G). Also remember that hydrogen bonds link the strands (weak, easily separated during replication), while covalent bonds make up the backbone. In WASSCE, watch for "ribose" (RNA) vs. "deoxyribose" (DNA) traps.

AI can make mistakes. Report anything that looks wrong.