Final answer:
Thymine is the nucleotide base found exclusively in DNA and does not appear in RNA, where it is replaced by uracil. In DNA, adenine pairs with thymine, and guanine pairs with cytosine, following the rule that purines pair with pyrimidines.
Step-by-step explanation:
The nucleotide base found only in DNA, not in RNA, is thymine (B). In DNA, the four nitrogen bases are adenine (A), guanine (G), cytosine (C), and thymine (T). However, in RNA, the base thymine is absent and is replaced by uracil (U), while the other three bases—adenine, guanine, and cytosine—are present in both types of nucleic acids. This distinction is crucial for the processes of transcription and replication of genetic information.
DNA and RNA differ in the pyrimidine bases they contain. DNA includes the pyrimidine base thymine, whereas RNA contains uracil. Remembering that 'cut the pyramid' to classify pyrimidines can be helpful, where 'c' stands for cytosine, 'u' for uracil, and 't' for thymine, representing the pyrimidine bases in nucleic acids.
Base pairing in DNA is also specific; adenine (A) always pairs with thymine (T) in DNA, through two hydrogen bonds, and guanine (G) pairs with cytosine (C) through three hydrogen bonds—reflecting the principle that purines always pair with pyrimidines.