iGCSE Biology (0610)-4.1 Biological molecules - iGCSE Style Questions Paper 2

Question

In a DNA molecule, bonds between pairs of bases hold two strands together. High temperatures can break these bonds to form two single strands of DNA.

The base pair C and G is held together more strongly than the base pair A and T.

Two DNA molecules containing the same number of bases were studied.

In DNA molecule $1$, $20\%$ of bases are T.
In DNA molecule $2$, $20\%$ of bases are C.

Which DNA molecule would require the lowest temperature to break into single strands?

(A) DNA molecule $1$, because there are more A bases present.
(B) DNA molecule $1$, because there are more G bases present.
(C) DNA molecule $2$, because there are more A bases present.
(D) DNA molecule $2$, because there are more G bases present.

▶️ Answer/Explanation

To find the molecule that breaks at the lowest temperature, we need the one with the fewest G-C pairs, as G-C pairs have three hydrogen bonds while A-T pairs only have two. In DNA molecule $1$, if T is $20\%$, then A must also be $20\%$ (total $40\%$ A-T), leaving $60\%$ for G-C ($30\%$ each). In DNA molecule $2$, if C is $20\%$, then G must also be $20\%$ (total $40\%$ G-C), leaving $60\%$ for A-T ($30\%$ each). Since molecule $2$ has a higher percentage of A-T pairs ($60\%$ vs $40\%$) and a lower percentage of G-C pairs, it is held together more weakly. Furthermore, if G-C is only $40\%$ in molecule $2$, then A must be $30\%$, which is more than the $20\%$ A bases in molecule $1$.
✅ Answer: (C)

Question

A section of a chromosome contains $4500$ bases. There are $1450$ $G$ bases.

How many $A$ bases are there?

(A) $800$
(B) $1450$
(C) $1600$
(D) $2900$

▶️ Answer/Explanation

According to Chargaff’s rules, in a double-stranded DNA molecule, the amount of Guanine ($G$) equals Cytosine ($C$), and Adenine ($A$) equals Thymine ($T$). Given there are $1450$ $G$ bases, there must also be $1450$ $C$ bases. Together, $G + C = 1450 + 1450 = 2900$. Subtracting this from the total gives $4500 – 2900 = 1600$ bases for the $A + T$ pairs. Since $A$ and $T$ are present in equal amounts, we divide by $2$: $1600 \div 2 = 800$. Therefore, there are $800$ Adenine bases.

✅ Answer: (A)

Question

Which chemical element is found in proteins but not in carbohydrates?

(A) carbon
(B) hydrogen
(C) nitrogen
(D) oxygen

▶️ Answer/Explanation

Carbohydrates are organic compounds consisting primarily of carbon, hydrogen, and oxygen, typically in a $2:1$ ratio of hydrogen to oxygen. Proteins, however, are made of amino acids which contain an amino group ($-NH_{2}$). This means all proteins must contain nitrogen in addition to carbon, hydrogen, and oxygen. While some proteins also contain sulfur, nitrogen is the defining element that distinguishes them from carbohydrates in this list. Carbon, hydrogen, and oxygen are common to both macromolecules, making nitrogen the correct distinguishing factor.

✅ Answer: (C)

iGCSE Biology (0610)-4.1 Biological molecules – iGCSE Style Questions Paper 2

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