DNA and RNA differ in their sugar, nitrogenous bases, number of strands and biological roles. In IB Biology A1.2, DNA is generally described as double-stranded and containing deoxyribose and thymine, whereas RNA is generally single-stranded and contains ribose and uracil.
Key Differences
Both DNA and RNA are nucleic acids made from nucleotide monomers. Each nucleotide contains a phosphate group, a pentose sugar and a nitrogenous base, but the following features distinguish the two molecules.
| Feature | DNA | RNA |
|---|---|---|
| Full name | Deoxyribonucleic acid | Ribonucleic acid |
| Pentose sugar | Deoxyribose | Ribose |
| Bases | Adenine, thymine, cytosine and guanine | Adenine, uracil, cytosine and guanine |
| Strands | Usually two antiparallel strands forming a double helix | Usually one strand |
| Main role | Stores genetic information | Allows genetic information to be expressed, including through protein synthesis |
In DNA, complementary base pairing occurs between adenine and thymine and between cytosine and guanine. The two strands are held together by hydrogen bonds between these complementary bases.
In RNA, uracil replaces thymine. During transcription, an RNA molecule is synthesized using a DNA template: adenine on the DNA template pairs with uracil in RNA. Messenger RNA then carries the base sequence used during translation, while transfer RNA and ribosomal RNA also contribute to polypeptide synthesis.
A common misconception is that RNA is simply a temporary, single-stranded copy of DNA. Although messenger RNA performs this role, other forms of RNA have different structures and functions.
Exam Technique
This content is required at both SL and HL. For a command term such as distinguish, compare the same feature directly: “DNA contains deoxyribose, whereas RNA contains ribose.” Avoid listing unrelated facts, and give both sides of each difference to secure the mark.