Which of the following best explains why uracil is used in RNA instead of thymine?

Question

Question

This question was previously asked in

RRB NTPC Graduate Level CBT-I Official Paper (Held On: 06 Jun, 2025 Shift 2)

Answer 1

The correct answer is Uracil is energetically cheaper to synthesise.

Key Points

Uracil is used in RNA instead of thymine because it is energetically cheaper to synthesize, making it more efficient for cells to produce RNA.
Thymine contains an additional methyl group compared to uracil, which requires more energy for its synthesis.
RNA molecules are generally short-lived compared to DNA, and the energy-efficient synthesis of uracil suits RNA's transient nature.
In contrast, thymine is more stable and is used in DNA to protect it from degradation and mutations, ensuring long-term genetic storage.

Additional Information

Uracil and Thymine:
- Both uracil and thymine are pyrimidine bases, but thymine has an extra methyl group at the 5th carbon.
- The structural difference affects stability, with thymine being more resistant to hydrolysis than uracil.
Energetic Cost of Nucleotide Synthesis:
- Synthesizing thymine involves additional enzymatic steps and energy compared to uracil.
- This is why cells preferentially use uracil in RNA, where durability is less critical.
RNA vs. DNA Stability:
- RNA is a temporary molecule used in protein synthesis, making energy-efficient synthesis a priority.
- DNA, as the hereditary material, requires higher stability, achieved partly by using thymine instead of uracil.
Role of Uracil in RNA:
- Uracil pairs with adenine during transcription, forming stable but transient hydrogen bonds.
- Its presence is crucial for RNA's function in gene expression, including mRNA, tRNA, and rRNA.