A nucleotide is made of three components: a phosphate group, a five-carbon pentose sugar, and a nitrogenous base. This structure is required at both IB Biology SL and HL in subtopic A1.2, Nucleic acids.
The Reasoning and Structure
The pentose sugar forms the central part of a nucleotide. The phosphate group is bonded to the sugar, and the nitrogenous base is also bonded to the sugar. Therefore, the base is not bonded directly to the phosphate.
The type of sugar and the possible nitrogenous bases depend on whether the nucleotide forms DNA or RNA:
| Feature | DNA nucleotide | RNA nucleotide |
|---|---|---|
| Pentose sugar | Deoxyribose | Ribose |
| Possible bases | Adenine, thymine, cytosine or guanine | Adenine, uracil, cytosine or guanine |
| Number of components | Three | Three |
A nucleotide is the monomer from which nucleic acids are built. In DNA, complementary bases pair through hydrogen bonds: adenine pairs with thymine and cytosine pairs with guanine. In RNA, uracil replaces thymine, so adenine can pair with uracil.
Nucleotides join to form polynucleotides. Covalent sugar/phosphate bonding produces the continuous sugarphosphate backbone of DNA or RNA. The sequence of bases attached to this backbone stores genetic information.
A common misconception is that a nucleotide is made only of a sugar and a base. A sugar joined to a base is a nucleoside; adding a phosphate group makes it a nucleotide. Another misconception is that DNA contains uracil: DNA contains thymine, whereas RNA contains uracil.
Exam Technique
For an IB diagram, use a circle for the phosphate group, a pentagon for the pentose sugar and a rectangle for the nitrogenous base. Connect both the phosphate and the base to the sugar, and label all three components. If asked to “state” the components, list them directly; if asked to “distinguish” DNA and RNA nucleotides, compare their sugars and bases using both molecules.
