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 – 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.
▶️ Answer/Explanation
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.
▶️ Answer/Explanation
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.
▶️ Answer/Explanation
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.
▶️ Answer/Explanation
The question mentions “Cysteine,” which is an amino acid, not a nitrogenous base.
DNA is composed of the bases Adenine ($A$), Guanine ($G$), Cytosine ($C$), and Thymine ($T$).
Chargaff’s rules apply to the relationship between Cytosine and Guanine, not Cysteine.
Because “Cysteine” cannot pair with Guanine in a DNA structure, the logic of base-pairing constants fails.
Even if “Cysteine” were a typo for “Cytosine,” the percentage of a single base ($A$) cannot be determined without knowing if the $60\%$ is a combined total or individual values.
Therefore, based on the provided terminology, a numerical percentage for Adenine cannot be calculated.
Correct Option: D
▶️ Answer/Explanation
According to Chargaff’s rule, Adenine ($A$) always pairs with Thymine ($T$), so $\%A = \%T$.
Given $T = 22\%$, it follows that $A = 22\%$.
The combined percentage of $A + T = 22\% + 22\% = 44\%$.
The remaining percentage for Cytosine ($C$) and Guanine ($G$) is $100\% – 44\% = 56\%$.
Since $G$ always pairs with $C$, their percentages are equal: $\%G = \%C$.
Therefore, $G = 56\% / 2 = 28\%$ and $C = 56\% / 2 = 28\%$.
The correct option is B.
▶️ Answer/Explanation
DNA is a double-stranded helix consisting of two polynucleotide chains.
The “backbone” on the sides consists of alternating deoxyribose sugar and phosphate groups.
The center of the molecule contains the four nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T).
Options B and D are incorrect because uracil is found in RNA, not DNA.
Options C and D are incorrect because DNA uses deoxyribose sugar, while RNA uses ribose.
The bases pair specifically (A with T, C with G) via hydrogen bonds to hold the strands together.