Which base is present in DNA but replaced by uracil in RNA?

Which base is present in DNA but replaced by uracil in RNA?

• A. Thymine
• B. Uracil
• C. Cytosine
• D. Guanine

Answer: (A)

Thymine is the base used in DNA, while RNA swaps it out for uracil. The presence of thymine in DNA helps maintain genetic stability: thymine has a methyl group that distinguishes it from uracil, so the cellular repair systems can tell when a cytosine has deaminated to uracil and fix it correctly. If DNA used uracil like RNA does, those repair mechanisms would often mistake ordinary uracil for damage, making mutations harder to detect. RNA, on the other hand, uses uracil (and a ribose sugar) because it’s suitable for its typically short-lived, transient roles and involves less metabolic cost.

Thymine is the base used in DNA, while RNA swaps it out for uracil. The presence of thymine in DNA helps maintain genetic stability: thymine has a methyl group that distinguishes it from uracil, so the cellular repair systems can tell when a cytosine has deaminated to uracil and fix it correctly. If DNA used uracil like RNA does, those repair mechanisms would often mistake ordinary uracil for damage, making mutations harder to detect. RNA, on the other hand, uses uracil (and a ribose sugar) because it’s suitable for its typically short-lived, transient roles and involves less metabolic cost.