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Pre AP Biology -GEN 1.1 Race to Discover DNA- MCQ Exam Style Questions -New Syllabus

Pre AP Biology -GEN 1.1 Race to Discover DNA- MCQ Exam Style Questions – New Syllabus 2025-2026

Pre AP Biology -GEN 1.1 Race to Discover DNA- MCQ Exam Style Questions – Pre AP Biology – per latest Pre AP Biology Syllabus.

Pre AP Biology – MCQ Exam Style Questions- All Topics

Question

Working on the Amazon River, a biologist isolated DNA from two unknown organisms, $P$ and $Q$. He discovered that the adenine content of $P$ was $15\%$ and the cytosine content of $Q$ was $42\%$. Which of these conclusions can be drawn?
a. The amount of adenine in $Q$ is $42\%$.
b. The amount of guanine in $P$ is $15\%$.
c. The amount of guanine and cytosine combined in $P$ is $70\%$.
d. The amount of thymine in $Q$ is $21\%$.
▶️ Answer/Explanation
Detailed solution

According to Chargaff’s rule, in double-stranded DNA, $A = T$ and $G = C$.
For organism $P$, if $A = 15\%$, then $T = 15\%$, totaling $30\%$ for $A + T$.
The remaining $70\%$ ($100\% – 30\%$) must be the combined amount of $G + C$.
Thus, $G = 35\%$ and $C = 35\%$ individually for organism $P$.
For organism $Q$, if $C = 42\%$, then $G = 42\%$, totaling $84\%$ for $G + C$.
The remaining $16\%$ is $A + T$, meaning $A = 8\%$ and $T = 8\%$.
Comparing these results, option c is the only mathematically correct conclusion.

Question

Which of the following statements describes an aspect of DNA structure?
a. Each DNA strand has a $3’$ $OH$ on one end and a $5’$ $OH$ on the other end.
b. Each strand of the double helix runs parallel to the other.
c. The binding of adenine to thymine is through three hydrogen bonds.
d. Bonds between components of the backbone (i.e., sugar–phosphate) are stronger than those between one strand and the other.
▶️ Answer/Explanation
Detailed solution

The correct option is d.
The sugar-phosphate backbone is held together by covalent phosphodiester bonds.
In contrast, the two strands are held together by weak hydrogen bonds between nitrogenous bases.
Covalent bonds require significantly more energy to break than hydrogen bonds.
Statement (a) is incorrect because the $5’$ end typically features a phosphate group, not a hydroxyl ($OH$) group.
Statement (b) is incorrect because DNA strands are antiparallel, running in opposite $5′ \to 3’$ directions.
Statement (c) is incorrect because Adenine and Thymine share two hydrogen bonds, while Cytosine and Guanine share three.

Question

The two “backbones” of a double-stranded $\text{DNA}$ molecule consist of
A. adenine bases and sugars.
B. phosphate groups and sugars.
C. adenine bases and thymine bases.
D. adenine bases and uracil bases.
▶️ Answer/Explanation
Detailed solution

The correct option is B.
$\text{DNA}$ is composed of nucleotides, each containing a sugar, a phosphate, and a base.
The structural “backbone” is formed by alternating deoxyribose sugars and phosphate groups.
These components are linked by strong phosphodiester bonds.
Nitrogenous bases (like adenine) point inward and are not part of the backbone itself.
Option $\text{D}$ is incorrect as uracil is found in $\text{RNA}$, not $\text{DNA}$.
The sugar-phosphate arrangement provides the stable framework for the double helix.

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