Nitrogenous bases are categorized into two groups based on their ring structure: purines and pyrimidines. This classification is essential for understanding how bases pair in the DNA double helix.
Purines (Adenine and Guanine) consist of a double-ring structure. They are larger molecules than pyrimidines and are found in both DNA and RNA.
Pyrimidines (Cytosine, Thymine, and Uracil) consist of a single-ring structure. While Cytosine is found in both, Thymine is exclusive to DNA, and Uracil is exclusive to RNA.
A phosphodiester bond is a strong covalent bond that links two adjacent nucleotides. It forms between the phosphate group attached to the 5' carbon of one nucleotide and the hydroxyl group on the 3' carbon of the next nucleotide.
This linkage is created through a condensation reaction, which results in the release of a water molecule (). Because the bond involves one phosphate group forming two ester linkages (one to each sugar), it is termed a 'phospho-di-ester' bond.
The continuous chain of alternating sugar and phosphate groups is known as the sugar-phosphate backbone. This backbone is highly stable and protects the nitrogenous bases that project inward from the chain.
| Feature | DNA Nucleotide | RNA Nucleotide |
|---|---|---|
| Pentose Sugar | Deoxyribose ( at 2') | Ribose ( at 2') |
| Pyrimidine Bases | Cytosine, Thymine | Cytosine, Uracil |
| Purine Bases | Adenine, Guanine | Adenine, Guanine |
| Polymer Form | Usually double-stranded | Usually single-stranded |
Identify the Bond: In diagrams, always look for the phosphate group bridging two sugars. The phosphodiester bond is the entire linkage, not just a single line between atoms.
Carbon Numbering: Remember that the base always attaches to the 1' carbon and the phosphate to the 5' carbon. The bond to the next nucleotide always occurs at the 3' carbon.
Bond Confusion: A common mistake is confusing phosphodiester bonds (covalent bonds within the backbone) with hydrogen bonds (weak attractions between bases). Phosphodiester bonds hold the strand together, while hydrogen bonds hold two strands to each other.
Reaction Types: Always specify that bond formation is a condensation reaction (releasing water) and bond breakage is hydrolysis (requiring water). Examiners often look for these specific terms.
