What is the Central Dogma?
The central dogma of molecular biology describes the flow of genetic information:
Learning Goals:
- Describe semi-conservative DNA replication.
- Explain transcription from DNA to mRNA.
- Explain translation from mRNA to polypeptide.
- Use a codon table to determine amino acid sequences.
DNA Replication
| Step | Enzyme | Action |
|---|---|---|
| 1. Unwind | Helicase | Separates double helix |
| 2. Stabilize | SSB proteins | Prevent re-annealing |
| 3. Synthesize | DNA polymerase | Adds nucleotides 5'→3' |
| 4. Join | Ligase | Seals Okazaki fragments |
Semi-conservative: Each new DNA molecule has one old strand and one new strand.
Transcription (DNA → mRNA)
| Feature | Detail |
|---|---|
| Location | Nucleus |
| Enzyme | RNA polymerase |
| Template | Antisense (3'→5') DNA strand |
| Product | mRNA (5'→3') |
| Base pairing | A→U, T→A, G→C, C→G |
After transcription: mRNA processing (5' cap, 3' poly-A tail, intron splicing) → mature mRNA exits nucleus.
Translation (mRNA → Protein)
| Step | What happens |
|---|---|
| Initiation | Ribosome assembles on mRNA at start codon (AUG) |
| Elongation | tRNA brings amino acids; peptide bonds form |
| Termination | Stop codon (UAA/UAG/UGA) → ribosome releases polypeptide |
Codons: Each 3-nucleotide sequence on mRNA codes for one amino acid. 64 codons → 20 amino acids (redundancy = degeneracy).
Worked Examples
Example 1: DNA → mRNA → Amino Acid
DNA template: 3'–TAC GCA AAT–5' mRNA: 5'–AUG CGU UUA–3' Amino acids: Met – Arg – Leu
Example 2: Effect of a point mutation
If the third base in a codon changes (e.g., CGA → CGG), both code for Arg → silent mutation (no effect). But CGA → CCA = Arg → Pro → missense mutation.
Common Mistakes
- Confusing template and coding strand — mRNA is complementary to the template (antisense) strand, identical (except U for T) to the coding (sense) strand.
- Forgetting mRNA processing — In eukaryotes, pre-mRNA must be capped, polyadenylated, and spliced before translation.
- Using DNA bases in translation — Translation reads mRNA codons (with U), not DNA triplets (with T).
Exam Tips
- Always specify the direction (5'→3' for mRNA synthesis, 3'→5' for template reading).
- Know the start codon (AUG = Met) and stop codons (UAA, UAG, UGA).
- Mutations: silent (no AA change), missense (different AA), nonsense (premature stop), frameshift (insertion/deletion).
Related Topics
- Mendelian Genetics — Genes encode proteins that determine phenotype.
- Enzyme Kinetics — Proteins made by translation function as enzymes.