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).