## Correct Answer: C. 22% This question tests **Chargaff's rules**, the fundamental principle governing DNA base composition. Chargaff's rules state: (1) the amount of adenine (A) equals thymine (T), and (2) the amount of guanine (G) equals cytosine (C). Therefore, A = T and G = C, and A + T + G + C = 100%. Given: Thymine (T) = 28%, therefore Adenine (A) = 28% (by Chargaff's rule 1). Combined A + T = 28% + 28% = 56%. Since all four bases must sum to 100%: G + C = 100% − 56% = 44%. By Chargaff's rule 2, G = C, so: 2C = 44% **C = 22%** This principle applies universally to double-stranded DNA across all organisms—from bacteria to humans. The complementary base pairing (A-T via 2 hydrogen bonds, G-C via 3 hydrogen bonds) on opposite strands ensures this stoichiometric relationship. Understanding Chargaff's rules is essential for solving problems on DNA composition, GC content (which varies by species and affects DNA stability), and molecular evolution studies used in Indian epidemiological research. ## Why the other options are wrong **A. 28%** — This is wrong because it confuses cytosine with thymine. While T = 28% by the given data, cytosine is paired with guanine, not thymine. A student selecting this has misapplied Chargaff's rule or assumed all bases are equal—a common trap when students forget that only A=T and G=C, not all four bases being identical. **B. 44%** — This is wrong because 44% is the combined percentage of G + C (the remaining bases after accounting for A + T = 56%). A student selecting this has calculated the sum of guanine and cytosine together but failed to divide by 2 to isolate cytosine alone. This represents a calculation error in applying Chargaff's rule 2. **D. 36%** — This is wrong because it has no logical basis in Chargaff's rules. A student selecting this may have attempted arbitrary arithmetic (e.g., 100% − 28% − 36% = 36%, or other miscalculations) without understanding the complementary base pairing principle. It is a distractor for those who guess or apply incorrect formulas. ## High-Yield Facts - **Chargaff's rule 1**: In double-stranded DNA, A = T (adenine equals thymine in percentage). - **Chargaff's rule 2**: In double-stranded DNA, G = C (guanine equals cytosine in percentage). - **Base composition formula**: A + T + G + C = 100%; if T = 28%, then A = 28%, and G + C = 44%, so C = 22%. - **GC content** varies by organism (e.g., *Mycobacterium tuberculosis* ~65% GC, human ~41% GC) and affects DNA thermal stability and antibiotic resistance. - **Chargaff's rules apply only to double-stranded DNA**; single-stranded DNA or RNA do not follow these ratios. ## Mnemonics **A=T, G=C (Pair Rule)** In double-stranded DNA: **A**denine pairs with **T**hymine (2 H-bonds), **G**uanine pairs with **C**ytosine (3 H-bonds). Equal amounts on opposite strands → equal percentages in the whole molecule. **50-50 Rule for Purines and Pyrimidines** Purines (A+G) = Pyrimidines (T+C) = 50% each. If you know T, you know A; then G+C fills the rest, and G=C splits it equally. ## NBE Trap NBE pairs thymine percentage directly with cytosine percentage (option A: 28%) to catch students who confuse complementary base pairing rules or assume all bases occur in equal amounts. Option B (44%) traps those who calculate G+C correctly but forget to divide by 2. ## Clinical Pearl GC content is clinically relevant in India: *Mycobacterium tuberculosis* (causative agent of TB, a major public health burden) has ~65% GC content, which correlates with its resistance to many antibiotics and its ability to survive in harsh intracellular environments. Understanding base composition helps predict organism characteristics and drug susceptibility patterns. _Reference: Lehninger Principles of Biochemistry (Nelson & Cox), Chapter 8; or Harper's Illustrated Biochemistry, Chapter 36 (DNA Structure and Replication)_
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