**Core Concept:** DNA and RNA are nucleic acids essential for genetic information storage and cellular processes. They contain nucleotides with a sugar-phosphate backbone and a nitrogenous base. Two types of nitrogenous bases are present: purines and pyrimidines. Purines include adenine (A) and guanine (G). Pyrimidines consist of thymine (T) and cytosine (C) in DNA, and uracil (U) in RNA.

**Why the Correct Answer is Right:** The correct answer is adenine (A) because it is a pyrimidine base found in both DNA (paired with thymine) and RNA (paired with uracil). Adenine plays a crucial role in genetic replication, transcription, and translation processes in cells.

**Why Each Wrong Option is Incorrect:**

A. Thymine (T): Thymine is a purine base, not a pyrimidine. It is paired with adenine in DNA.
B. Cytosine (C): Cytosine is a pyrimidine base, but it is paired with guanine in DNA, not adenine.
C. Uracil (U): Uracil is another pyrimidine base, found in RNA, not adenine. It is paired with adenine in DNA, which is incorrect.
D. Gوانين: گوانين (Guanine) is a purine base, not a pyrimidine. It is paired with adenine in RNA, not adenine in DNA.

**Clinical Pearl:** Understanding the differences between purines and pyrimidines is essential for understanding genetic processes, as they represent the fundamental building blocks of DNA and RNA nucleotides. Any confusion between these bases can lead to errors in genetic information processing and cellular functions.