Question

Which one of the following statement is incorrect regarding the work of Erwin Chargaff?

• A. The base composition of DNA don't varies from one species to another.
• B. DNA specimens isolated from different tissues of the same species have the same base composition.
• C. The base composition of DNA in a given species does not change with an organism's age, nutritional state, or changing environment.
• D. The sum of the purine residues equals the sum of the pyrimidine residues; that is, A + G = T + C.

Hint:

Chargaff's rules can be summarized in two main points: 1) A=T and G=C, so Purines=Pyrimidines. 2) The exact percentage of these bases varies between different species. The incorrect statement violates the second point.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Chargaff's rules, derived from his experiments on DNA base composition, were essential for Watson and Crick's DNA double helix discovery. We must identify the statement that contradicts these rules.
Step 2: Detailed Explanation:
Let's examine each statement in light of Chargaff's findings:


(A) The base composition of DNA does not vary from one species to another. This statement is incorrect. Chargaff demonstrated that the proportions of A, G, C, and T bases do vary between species. For instance, the (A+T)/(G+C) ratio differs between humans and E. coli.

(B) DNA specimens from different tissues of the same species have the same base composition. This is correct. Chargaff established that a species' DNA base composition is consistent across all tissues.

(C) The base composition of DNA in a given species does not change with an organism's age, nutritional state, or changing environment. This is correct. Base composition is a stable characteristic of a species.

(D) The sum of the purine residues equals the sum of the pyrimidine residues; that is, A + G = T + C. This is correct. This is a key Chargaff rule, often expressed as A=T and G=C, implying A+G (purines) = T+C (pyrimidines).

Step 3: Final Answer:
Statement (A) is incorrect because it directly contradicts Chargaff's findings.