Erwin Chargaff’s discovery that DNA contains equimolar amounts of guanine and cytosine and also equimolar amounts of adenine and thymine has come to be known as Chargaff’s rule:
G = C and A = T
(a) Does Chargaff’s rule imply that equal amounts of guanine and adenine are present in DNA? That is, does G = A?
(b) Does Chargaff’s rule imply that the sum of the purine residues equals the sum of the pyrimidine residues? That is, does A + G = C + T?
(c) Does Chargaff’s rule apply only to double-stranded DNA, or would it also apply to each individual strand if the double helical strand were separated into its two complementary strands?

Verified step by step guidance

1. Step 1: Understand Chargaff's rule. Chargaff's rule states that in double-stranded DNA, the amount of guanine (G) equals the amount of cytosine (C), and the amount of adenine (A) equals the amount of thymine (T). This is due to the base-pairing nature of DNA, where G pairs with C and A pairs with T.
2. Step 2: Address part (a). Chargaff's rule does not imply that G = A. The rule only states that G = C and A = T. The amounts of purines (A and G) and pyrimidines (C and T) can vary between different DNA molecules, so G and A are not necessarily equal.
3. Step 3: Address part (b). Chargaff's rule does imply that the sum of purine residues equals the sum of pyrimidine residues in double-stranded DNA. This is because purines (A and G) pair with pyrimidines (T and C), so the total number of purines (A + G) equals the total number of pyrimidines (C + T).
4. Step 4: Address part (c). Chargaff's rule applies specifically to double-stranded DNA because it is based on the complementary base-pairing between the two strands. If the double helix is separated into its two individual strands, the rule no longer applies to each strand individually, as the base composition of a single strand is not constrained by complementary pairing.
5. Step 5: Summarize the implications. Chargaff's rule is a fundamental principle of double-stranded DNA structure, reflecting the complementary nature of base pairing. It does not imply equality between guanine and adenine, but it does ensure that purines and pyrimidines are balanced in double-stranded DNA. The rule does not apply to single strands of DNA.

Key Concepts

Chargaff's Rule

Chargaff's Rule states that in DNA, the amount of guanine (G) is equal to the amount of cytosine (C), and the amount of adenine (A) is equal to the amount of thymine (T). This principle is fundamental in understanding the base pairing in DNA, where G pairs with C and A pairs with T, ensuring the stability of the double helix structure.

Purines and Pyrimidines

DNA consists of two types of nitrogenous bases: purines (adenine and guanine) and pyrimidines (cytosine and thymine). Chargaff's Rule highlights the relationship between these bases, indicating that the total number of purines (A + G) equals the total number of pyrimidines (C + T) in a DNA molecule, which is crucial for maintaining the DNA structure.

Single-Stranded vs. Double-Stranded DNA

Chargaff's Rule primarily applies to double-stranded DNA, where complementary base pairing occurs. However, when considering individual strands, the rule does not hold in the same way, as each strand may have varying amounts of purines and pyrimidines. Understanding this distinction is essential for analyzing the composition of DNA strands in different contexts.