What is the difference between purines and pyrimidines, and how do they pair in DNA and RNA?

Purines and pyrimidines are nitrogenous bases found in DNA and RNA. They differ in their chemical structure and pairing rules.

Purines are larger nitrogenous bases consisting of two carbon-nitrogen rings fused together. Adenine (A) and guanine (G) are purines found in both DNA and RNA. Pyrimidines are smaller nitrogenous bases consisting of a single carbon-nitrogen ring. Cytosine (C) and thymine (T) are pyrimidines found in DNA, while uracil (U) replaces thymine in RNA.

In DNA, purines always pair with pyrimidines through hydrogen bonding. Adenine pairs with thymine (two hydrogen bonds) and guanine pairs with cytosine (three hydrogen bonds). This base pairing is essential for DNA replication and transcription.

In RNA, purines and pyrimidines also pair through hydrogen bonding, but with some differences. Adenine still pairs with uracil (two hydrogen bonds) and guanine still pairs with cytosine (three hydrogen bonds). However, RNA can form complex secondary structures due to the presence of single-stranded regions, allowing for additional base pairing interactions.

Understanding the differences between purines and pyrimidines and their pairing rules is crucial for understanding the structure and function of DNA and RNA.