Nucleic acids and the structure of DNA
Nucleic acids are chains of nucleotides (base + sugar + phosphate). DNA is a double helix of two antiparallel strands held by base pairing.
Building blocks of a nucleotide
| Component | In DNA | In RNA |
|---|---|---|
| Purine bases (two rings) | Adenine (A), guanine (G) | Adenine (A), guanine (G) |
| Pyrimidine bases (one ring) | Cytosine (C), thymine (T) | Cytosine (C), uracil (U) |
| Pentose (5-carbon) sugar | 2-Deoxyribose | Ribose |
| Phosphate | Links sugars by 3′,5′-phosphodiester bonds | Same |
Nucleoside vs nucleotide
Nucleotides also work on their own: ATP is the energy currency, cAMP is a second messenger, and NAD⁺, FAD and coenzyme A contain an adenine nucleotide.
Watson–Crick double helix (B-DNA)
- Two strands coil around one axis as a right-handed double helix.
- Antiparallel: one strand runs 5′→3′, its partner runs 3′→5′.
- The sugar–phosphate backbone is outside; the flat bases are stacked inside.
- Complementary pairing: A pairs with T by 2 hydrogen bonds; G pairs with C by 3.
- Chargaff's rule: A = T and G = C, so purines equal pyrimidines (A + G = T + C).
- About 10 base pairs per turn (newer books give 10.5). One turn is 3.4 nm long, bases are 0.34 nm apart, and the helix is 2 nm wide.
- Major and minor grooves run along the surface; proteins read the bases through them.
DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Pyrimidine bases | Cytosine, thymine | Cytosine, uracil |
| Strands | Double-stranded helix | Usually single-stranded; may fold on itself |
| Chargaff's rule | Followed | Not followed |
| Where found | Mainly nucleus (some in mitochondria) | Made in nucleus; works mostly in cytoplasm |
| Stability | Stable; resists alkali | Less stable; broken down by alkali (2′-OH group) |
| Forms / types | Mainly B-DNA (also A and Z forms) | mRNA, tRNA, rRNA and small RNAs |
| Main job | Stores genetic information; copies itself (replication); template for RNA (transcription) | Uses that information to make proteins |
Folate (with help from vitamin B12) is needed to make thymine and purine nucleotides for new DNA, so a lack of either causes megaloblastic anaemia. Purines break down to uric acid; people with gout limit purine-rich foods such as organ meats. The coenzymes NAD⁺ and FAD are nucleotides built from niacin and riboflavin.
2 marks: “State Chargaff's rule.” In double-stranded DNA, A = T and G = C, so total purines equal total pyrimidines.
5 marks: “Distinguish between DNA and RNA.” Tabulate sugar, bases, strands, Chargaff's rule, location, stability, types and function.
10 marks: “Describe the structure of DNA (Watson–Crick model).” Draw the labelled double helix, list its features with numbers, explain base pairing and Chargaff's rule, then functions.
Purines are "Pure As Gold" (adenine, guanine). Pyrimidines: "CUT the Py" (cytosine, uracil, thymine).