Final answer:
Thymine (T) is the nitrogenous base that is not found in mRNA; instead, RNA contains uracil (U) in place of thymine, while adenine (A), guanine (G), and cytosine (C) are present in both DNA and RNA.
Step-by-step explanation:
The nitrogenous base that is not found in mRNA is thymine (T). In DNA, the four nitrogenous bases are adenine (A), guanine (G), cytosine (C), and thymine (T). However, in RNA, thymine is replaced by uracil (U). Therefore, while DNA contains thymine, RNA contains uracil in its place. This is one of the key differences between DNA and RNA structures. The purpose of this substitution is related to the different structural and functional roles of these molecules in the cell.
Adenine and guanine, which are purines, are found in both DNA and RNA. Cytosine is a pyrimidine also common to both. However, the pyrimidine base thymine is exclusive to DNA, which is complemented by uracil in RNA. Understanding these bases is critical, as they form the genetic code that is translated into proteins.
Therefore, when considering the components of mRNA, adenine (A), guanine (G), and cytosine (C) are present, but thymine (T) is not, confirming that it is the correct answer to the question posed.