Question

Which nitrogen base is not present in RNA?

Answer 1

Thymine (T) and Uracil (U) in DNA and RNA:

Thymine (T) is a nitrogenous base that is present in DNA, but it is not found in RNA. Instead, RNA contains uracil (U), which is chemically similar to thymine but differs by the absence of a methyl group at the carbon-5 position. Both thymine and uracil are pyrimidine bases, meaning they each have a six-membered ring structure composed of carbon and nitrogen atoms. However, they play distinct roles in DNA and RNA.

Key Differences between Thymine and Uracil:
- Thymine is found only in DNA, while uracil is found only in RNA.
- Thymine contains a methyl group (-CH3) attached to the carbon-5 position of the pyrimidine ring, whereas uracil lacks this methyl group.

Base Pairing in DNA and RNA:
In DNA, thymine pairs with adenine (A) through two hydrogen bonds, following the base pairing rule that A pairs with T. This pairing is crucial for the formation of the double-stranded helical structure of DNA.
In RNA, however, uracil pairs with adenine (A) in a similar manner, but since RNA is typically single-stranded, this base pairing is part of the transcription process where RNA is synthesized based on the DNA template. Although both thymine and uracil pair with adenine, their presence in DNA and RNA, respectively, helps distinguish the two nucleic acids.

Summary:
- Thymine is exclusive to DNA, while uracil is exclusive to RNA.
- Both thymine and uracil are pyrimidine bases and are involved in base pairing with adenine (A) in DNA and RNA respectively.
- Thymine’s methyl group distinguishes it from uracil, which helps identify RNA in biological processes.