Final answer:
The nitrogenous base that is typically modified upon transcription of tRNA gene to form a rare type of base is uracil. Uracil, a pyrimidine found only in RNA, can be enzymatically altered to produce modified bases such as pseudouridine, crucial for tRNA function and stability.
Step-by-step explanation:
The nitrogenous base that is frequently modified enzymatically to become a rare type of nitrogenous base upon transcription of tRNA gene is uracil. During the maturation process of tRNA, uracil can undergo enzymatic modifications to form bases such as pseudouridine and dihydrouridine, among others. These modifications are important for tRNA function and stability.Nitrogenous bases are categorized as either purines or pyrimidines. Adenine (A) and guanine (G) are purines, each possessing a double carbon-nitrogen ring structure. In contrast, cytosine (C), thymine (T, found only in DNA), and uracil (U, found only in RNA) are pyrimidines, characterized by a single carbon-nitrogen ring. These base pairings play a crucial role in the structure and function of nucleic acids.