Thymine vs Uracil: DNA Stability — NEET Biology
📚 Practice ConceptIn fact, the presence of thymine at the place of uracil also confers additional stability to DNA. (Detailed discussion about this requires understanding of the process of repair in DNA, and you will study these processes in higher classes.)
Which of the following statements correctly identify reasons contributing to DNA being chemically more stable than RNA? S1: The absence of a 2'-hydroxyl group in the deoxyribose sugar of DNA makes its phosphodiester backbone less susceptible to hydrolysis and thus more stable. S2: The presence of thymine (5-methyl uracil) in DNA instead of uracil confers additional chemical stability, as thymine is less prone to deamination reactions than uracil. S3: DNA's double-stranded helical structure provides a physical shield for the bases, making them less accessible to chemical mutagens and enzymatic degradation. S4: The complementary base pairing within DNA's double helix allows for efficient repair mechanisms, contributing to its overall genetic stability by correcting mutations. S5: While DNA is generally more stable, the occasional presence of uracil in DNA (through deamination of cytosine) can be easily repaired due to its single-stranded nature.
View solution & NCERT explanation
Correct answer: B — S1, S2, S3 and S4 are correct
The core concept tested is the reasons for DNA's chemical stability compared to RNA. Let's evaluate each statement: S1: The absence of a reactive 2'-OH group in DNA's deoxyribose sugar makes it less prone to hydrolysis, enhancing its chemical stability. This statement is correct. S2: The presence of thymine (5-methyl uracil) in DNA, instead of uracil, confers additional stability. This is a known chemical difference between DNA and RNA that enhances DNA's stability. This statement is correct. S3: The double-stranded helical structure of DNA does indeed protect the bases from external chemical damage and degradation, contributing to its stability. This statement is correct. S4: DNA's complementary strands allow for repair mechanisms, as one strand can serve as a template to correct errors in the other, thereby maintaining genetic stability. This statement is correct. S5: DNA is *double-stranded*, not single-stranded. While deamination of cytosine to uracil does occur and is repaired, the repair mechanism relies on the double-stranded nature of DNA to identify and correct the mismatch using the complementary strand as a template. Therefore, this statement is NOT correct. Thus, statements S1, S2, S3, and S4 are correct. Incorrect options: A. Excludes S4 which is correct. C. Includes S5 which is incorrect and excludes S1 which is correct. D. Includes S5 which is incorrect and excludes S2 which is correct. The final answer is B. Recap: DNA's superior stability stems from the lack of a 2'-OH group, the presence of thymine, its double-stranded structure, and the availability of repair mechanisms that leverage complementarity.
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