DNA (deoxyribonucleic acid) is the molecule that carries genetic information in all living organisms. The genetic information is stored in the sequence of bases along the DNA strand — this sequence ultimately encodes the order of amino acids in proteins (via transcription and translation). Its structure is precisely suited to its function: the double helix is chemically stable, protecting the base sequence from damage, yet the two strands can be separated to allow replication and transcription.
Tip
When asked what carries genetic information, always say the sequence of bases — not the DNA molecule itself. Examiners expect you to be specific: it is the order of the bases (e.g. A, T, G, C…) that encodes the information, not the sugar-phosphate backbone or the bases in isolation. A common mistake is writing “DNA contains genetic information” without explaining that it’s the base sequence that actually encodes it.
Nucleotide Structure
What you need to know (based on the AQA specification)
What you need to know (based on the AQA specification)
Nucleotides are the monomers of nucleic acids. Each nucleotide consists of a pentose sugar, a nitrogenous base and a phosphate group.
Each DNA nucleotide is made of three components:
- Deoxyribose — a 5-carbon (pentose) sugar (note: deoxy means one fewer oxygen than ribose)
- Phosphate group — links nucleotides together in the backbone
- Nitrogenous base — one of four bases: Adenine (A), Thymine (T), Cytosine (C) or Guanine (G)
Nucleotides are joined together by phosphodiester bonds, formed in a condensation reaction between the phosphate of one nucleotide and the sugar of the next, creating the sugar-phosphate backbone.
DNA Double Helix
What you need to know (based on the AQA specification)
What you need to know (based on the AQA specification)
DNA is a double helix in which two antiparallel polynucleotide strands are joined by hydrogen bonds between complementary base pairs.
DNA consists of two polynucleotide strands coiled into a double helix:
- The two strands run antiparallel — one runs 5’→3’, the other 3’→5’
- The strands are held together by hydrogen bonds between complementary bases on opposite strands
- The sugar-phosphate backbones face outward; the bases face inward
Complementary Base Pairing
Bases pair in a specific way — this is known as complementary base pairing:
| Base | Pairs with | Hydrogen bonds |
|---|---|---|
| Adenine (A) | Thymine (T) | 2 |
| Cytosine (C) | Guanine (G) | 3 |
Tip
C–G pairs have 3 hydrogen bonds; A–T pairs have 2. A higher proportion of C–G pairs means a more stable DNA molecule (harder to separate the strands). This is relevant to DNA denaturation and replication questions.
Why Antiparallel?
The two strands run in opposite directions (antiparallel) because the phosphodiester bonds in each strand are oriented differently. This is essential for DNA replication — the two template strands are read in opposite directions by DNA polymerase.
DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, C, G | A, U, C, G |
| Strands | Double-stranded | Single-stranded |
| Stability | More stable | Less stable |
Tip
RNA uses uracil (U) instead of thymine (T). Both pair with adenine, but uracil lacks the methyl group found on thymine.
Exam Question Practice
Describe the structure of a DNA molecule.
(4 marks)Hint
Think about the components of a nucleotide, how the two strands are held together, and the orientation of the strands.
Mark Scheme
- DNA is made of nucleotides linked by phosphodiester bonds — these covalent bonds form the sugar-phosphate backbone of each strand (1 mark)
- Each nucleotide contains three components: deoxyribose (a pentose sugar), a phosphate group and a nitrogenous base (A, T, C or G) (1 mark)
- The two strands are held together by hydrogen bonds between complementary base pairs: adenine pairs with thymine (2 hydrogen bonds) and cytosine pairs with guanine (3 hydrogen bonds) (1 mark)
- The two strands run antiparallel (one 5’→3’, the other 3’→5’) and are coiled into a double helix (1 mark)
Tips from examiner reports
- Common error: saying ‘covalent bonds’ instead of hydrogen bonds between bases.
- Many students forget ‘antiparallel’ — this is often a mark point.
- Deoxyribose must be named, not just ‘pentose sugar’.