DNA · replication · transcription · code · translation · lac operon · HGP · fingerprinting
DNA (deoxyribonucleic acid) is a long polymer of deoxyribonucleotides. Its length is usually expressed as the number of nucleotides (bp).
| Organism | Genome size |
|---|---|
| φX174 (bacteriophage) | 5386 nucleotides |
| Bacteriophage lambda (λ) | 48,502 bp |
| Escherichia coli | 4.6 × 10⁶ bp |
| Human (haploid) | 3.3 × 10⁹ bp |
A nucleotide has three parts:
| Purines (double ring) | Pyrimidines (single ring) |
|---|---|
| Adenine (A), Guanine (G) | Cytosine (C), Thymine (T in DNA) |
| In RNA, Uracil (U) replaces Thymine. | |
Built on Chargaff's rules (A=T, G=C; so purines = pyrimidines) and the X-ray diffraction data of Maurice Wilkins & Rosalind Franklin.
| Feature | Value / Description |
|---|---|
| Strands | Two, coiled in a right-handed helix |
| Orientation | Antiparallel (one 5′→3′, the other 3′→5′) |
| Base pairing | A=T (2 H-bonds), G≡C (3 H-bonds) — complementary |
| Pitch (one turn) | 3.4 nm containing ~10 base pairs |
| Rise per base pair | 0.34 nm (3.4 Å) |
| Diameter | Uniform 2 nm (purine always pairs a pyrimidine) |
A human diploid cell's DNA is ~2.2 metres long, yet fits inside a tiny nucleus through tight packaging. (Distance between two consecutive base pairs = 0.34 nm.)
| Euchromatin | Heterochromatin |
|---|---|
| Loosely packed, lightly stained | Densely packed, darkly stained |
| Transcriptionally active | Transcriptionally inactive |
Using Streptococcus pneumoniae: S strain (smooth, capsulated, virulent) killed mice; R strain (rough, no capsule, avirurent) did not. Heat-killed S alone was harmless, but heat-killed S + live R killed the mice, and live S bacteria were recovered — R had been transformed by some 'principle' from S.
They purified biochemicals from heat-killed S and showed the transforming principle is DNA: DNase abolished transformation, but protease and RNase did not.
³⁵S (protein) stayed outside; ³²P (DNA) entered the bacteria, proving DNA is the genetic material.
Evidence suggests RNA was the first genetic material. RNA could act as a genetic material and a catalyst (ribozyme). Being reactive, it was unstable, so evolution favoured the chemically more stable DNA for information storage, with RNA taking over many functional/catalytic roles (rRNA, tRNA).
Watson & Crick proposed semiconservative replication: each old strand serves as a template, so each daughter DNA has one parental and one new strand.
| Leading strand | Lagging strand |
|---|---|
| Synthesised continuously | Synthesised discontinuously as Okazaki fragments |
| — | Fragments joined by DNA ligase |
Transcription = copying genetic information from one template strand of DNA into RNA. Only a segment of DNA and only one strand is copied.
| Bacteria | Eukaryotes |
|---|---|
| Single RNA polymerase | Three RNA polymerases |
| σ (sigma) factor for initiation; ρ (rho) factor for termination | RNA Pol I → rRNA; Pol II → hnRNA (mRNA precursor); Pol III → tRNA, 5S rRNA, snRNA |
George Gamow proposed a triplet code. Nirenberg & Khorana (with Ochoa's enzyme, polynucleotide phosphorylase) deciphered it.
| Property | Meaning |
|---|---|
| Triplet | Three bases = one codon = one amino acid |
| Degenerate | Most amino acids have more than one codon |
| Unambiguous & specific | One codon codes for only one amino acid |
| Nearly universal | Same codons across most organisms (e.g. UUU = Phe everywhere) |
| Non-overlapping & commaless | Read continuously, base by base, without gaps |
Postulated by Crick: tRNA has an anticodon loop complementary to the codon and an amino-acid acceptor end. It has a clover-leaf secondary structure; an initiator tRNA exists, but there is no tRNA for stop codons.
Translation = polymerising amino acids in the order dictated by mRNA, joined by peptide bonds.
In prokaryotes, gene expression is regulated mainly at transcription initiation. The classic model is the lac operon (Jacob & Monod).
| Gene | Product / role |
|---|---|
| i | Repressor protein |
| z | β-galactosidase (breaks lactose → galactose + glucose) |
| y | Permease (lactose entry) |
| a | Transacetylase |
| No lactose (OFF) | Lactose present (ON) |
|---|---|
| Repressor binds the operator → RNA polymerase blocked → no transcription. | Lactose (inducer) binds repressor → repressor inactivated → RNA polymerase transcribes z, y, a → enzymes made. |
An international 'megaproject' (1990–2003) to sequence the ~3 × 10⁹ bp human genome, coordinated by the US DOE and NIH (with the Wellcome Trust and others). It gave rise to bioinformatics.
| Expressed Sequence Tags (ESTs) | Sequence Annotation |
|---|---|
| Focus on the expressed genes only | Sequence the whole genome, then assign functions to regions |
Developed by Alec Jeffreys. It compares highly variable regions of DNA between individuals.