BPurines have a fused bicyclic ring system; pyrimidines have a single heterocyclic ring
CPurines are found only in DNA; pyrimidines are found only in RNA
DPurines carry three phosphate groups; pyrimidines carry one
The defining structural difference is the ring system of the nitrogenous base, not the sugar or phosphate. Purines (adenine and guanine) contain a pyrimidine ring fused to an imidazole ring — a bicyclic system. Pyrimidines (cytosine, thymine, uracil) contain only a single six-membered ring. Both purines and pyrimidines appear in DNA and RNA, and both can carry varying numbers of phosphate groups.
Question 2 True / False
A nucleoside and a nucleotide are the same thing — both consist of a nitrogenous base, a five-carbon sugar, and one or more phosphate groups.
TTrue
FFalse
Answer: False
A nucleoside consists of only a nitrogenous base covalently attached to a five-carbon sugar (ribose or deoxyribose) — no phosphate. A nucleotide adds one or more phosphate groups to the nucleoside (at the 5' carbon of the sugar). ATP, for example, is a nucleotide (adenosine triphosphate), while adenosine itself is the corresponding nucleoside. The phosphate group is the key distinguishing feature.
Question 3 Short Answer
Describe the naming logic for adenosine-based nucleotides (AMP, ADP, ATP) and explain what the 'mono,' 'di,' and 'tri' prefixes refer to.
Think about your answer, then reveal below.
Model answer: AMP (adenosine monophosphate), ADP (adenosine diphosphate), and ATP (adenosine triphosphate) all share the same nucleoside core — adenosine (adenine + ribose). The prefix indicates the number of phosphate groups attached at the 5' carbon: one for AMP, two for ADP, three for ATP. Each additional phosphate is linked by a high-energy anhydride bond, making the triphosphate form the primary energy currency of the cell.
The naming system applies consistently to all nucleotides: the nucleoside name (based on the base) plus mono/di/triphosphate. Guanosine gives GMP/GDP/GTP; cytidine gives CMP/CDP/CTP, and so on. Understanding this pattern lets you read biochemical shorthand fluently and recognize that the phosphate count determines both the name and the energy content of the molecule.