In DNA, A pairs with T and G pairs with C. Which of the following statements is incorrect in this respect ?

Question

Accepted Answer

The correct option is 2: A+T = G+C

This question tests your understanding of the foundational chemical and structural rules governing double-stranded DNA, primarily derived from Erwin Chargaff's rules and the complementary base pairing discovered by Watson and Crick. The correct option to identify is the statement that is *incorrect* based on these facts. The core base pairing rule is that Adenine (A) always forms two hydrogen bonds with Thymine (T), and Guanine (G) always forms three hydrogen bonds with Cytosine (C). This obligatory pairing leads to the crucial finding that in any double-stranded DNA molecule, the total molar amount of A must equal the total molar amount of T (A=T), and the total molar amount of G must equal the total molar amount of C (G=C). Based on these equalities, the statement *A+T = G+C* is generally incorrect. While the total number of purine bases (A+G) must always equal the total number of pyrimidine bases (T+C) in the entire molecule, the proportions of the two complementary pairs (A/T versus G/C) vary widely across different species. For instance, a human cell's DNA has a significantly higher proportion of A-T base pairs compared to many bacteria, meaning the concentration of (A+T) is typically much larger than the concentration of (G+C). Therefore, since A+T does not necessarily equal G+C, this statement is the incorrect one and is the correct answer to the question.

The incorrect option is 1: A/T ratio is constant

This statement is correct based on the rule of complementary base pairing. Since every Adenine (A) on one strand is obligatorily paired with a Thymine (T) on the complementary strand, the total number or concentration of A bases in the entire double helix must precisely equal the total number or concentration of T bases (A=T). Mathematically, this means that the ratio of A to T is always 1 (A/T = 1). Because 1 is a constant, this statement is factually true for all double-stranded DNA molecules.

The incorrect option is 3: G is always equal to C

This statement is correct and is a direct consequence of Chargaff's rules, which are themselves a reflection of the precise nature of complementary base pairing. Guanine (G) pairs exclusively with Cytosine (C) in a double-stranded DNA helix, forming three hydrogen bonds. Due to this pairing requirement, the total concentration or count of G bases must equal the total concentration or count of C bases (G=C). This equality holds true for all double-stranded DNA and is fundamental to its structure.

The incorrect option is 4: The sequence of nucleotides on one strand of DNA can be used to determine the sequence on the complementary strand.

This statement is correct; it describes the very definition of complementarity, which is central to DNA's structure and function. Because base pairing is specific (A with T, G with C), the sequence of one strand dictates the sequence of its partner strand. For example, if a segment of one strand is G-A-T, the complementary strand must read C-T-A. This predictable relationship allows the cell to accurately replicate its genetic material and is the basis of the genetic code's stability.