Structure of DNA: MDCAT Biology notes
Structure of DNA MDCAT notes: nucleotides, purines and pyrimidines, Watson-Crick double helix, base pairing, genes and Meselson-Stahl replication.
DNA as the genetic material
DNA stores biological information in discrete units called genes. A gene is a length of DNA with a particular function, usually coding for a polypeptide or an RNA. The gene is the basic unit of biological information and determines a specific character. The position of a gene on a chromosome is its locus.
Building blocks
The subunit of DNA is the nucleotide. Each nucleotide is made of three parts: deoxyribose sugar, a phosphate and a nitrogenous base. Nucleotides join through phosphodiester bonds (C–O–P–O–C). These are ester linkages that form the sugar-phosphate backbone.
| Purines (double ring) | Pyrimidines (single ring) |
|---|---|
| Adenine, Guanine | Cytosine, Thymine, Uracil |
| In both DNA and RNA | Thymine only in DNA; uracil only in RNA |
DNA contains A, G, C and T. Uracil is not present in DNA.
Watson and Crick double helix (1953)
The model was based on Chargaff's rule, which says A = T and G = C, and on X-ray diffraction data from Rosalind Franklin and Maurice Wilkins.
- Two polynucleotide strands coil around each other as a right-handed double helix. The sugar-phosphate backbones are on the outside and the bases are inside.
- The strands are antiparallel: one runs 5′→3′ and the other 3′→5′.
- Base pairing: A pairs with T by 2 hydrogen bonds, and G pairs with C by 3 hydrogen bonds. GC-rich DNA is more stable because it has more hydrogen bonds.
- A purine always pairs with a pyrimidine. This keeps the helix at a constant diameter of 2 nm.
- One complete turn is 3.4 nm (34 Å) long and holds 10 base pairs, which are 0.34 nm apart.
Complementary pairing means each strand fixes the sequence of the other. This allows DNA to act as a blueprint during replication.
Replication
Three models were proposed:
| Model | Daughter molecules |
|---|---|
| Conservative | The parent double helix stays intact; one daughter is completely new |
| Semi-conservative (correct) | Each daughter has one parental strand and one new strand |
| Dispersive | Parental DNA is scattered, so every strand is a mixture of old and new pieces |
Meselson and Stahl experiment
E. coli were grown in heavy $^{15}\mathrm{N}$ and then moved to light $^{14}\mathrm{N}$ medium. DNA was separated by density.
- After 1 generation: 100% hybrid ($^{15}\mathrm{N}$/$^{14}\mathrm{N}$).
- After 2 generations: 50% hybrid, 50% light.
- This result fits only the semi-conservative model.
Enzymes
- DNA polymerase III adds nucleotides only in the 5′ → 3′ direction.
- DNA ligase joins DNA fragments, such as the Okazaki fragments on the lagging strand.
Key formulas
- $\%A = \%T$, $\%G = \%C$, and $\%A + \%G = 50\%$.
- Number of turns = base pairs ÷ 10. Length = turns × 3.4 nm.
- Worked example: if a DNA sample has 30% adenine, then T = 30%, and G = C = 20%.
Common MDCAT traps
- Cytosine pairs with guanine, never with adenine.
- Purines are A and G only. Cytosine is a pyrimidine.
- One turn is 3.4 nm, not 3.4 Å. The spacing between base pairs is 0.34 nm, which is 3.4 Å.
- mRNA and DNA share the sugar-phosphate backbone, but not ribose, thymine or the double helix.
- After two rounds in $^{14}\mathrm{N}$, no heavy duplex remains.
Quick revision
- G≡C has 3 hydrogen bonds; A=T has 2.
- Purine–pyrimidine pairing keeps the 2 nm diameter.
- Replication is semi-conservative.
- Ligase joins fragments; polymerase works 5′→3′.
- Uracil is absent from DNA.