The Big Picture
DNA → RNA → Protein🍽️ The Central Dogma of Molecular Biology
The central dogma describes the flow of genetic information in a cell: DNA is transcribed into RNA, and RNA is translated into protein. DNA contains the instructions; proteins do the actual work — as enzymes, structural molecules, hormones, and transporters. Every protein your body makes starts as a sequence of bases in your DNA.
Think of it as a restaurant: the DNA recipe book (never leaves the office/nucleus) → mRNA waiter takes the order to the kitchen → ribosome kitchen reads the order → tRNA delivery workers bring ingredients (amino acids) → the completed protein dish is served.
| Stage | Where | What Happens | Key Molecule |
|---|---|---|---|
| Transcription | Nucleus | DNA is used as a template to make a complementary mRNA strand | RNA polymerase, mRNA |
| RNA Processing | Nucleus | Introns (non-coding) are removed; exons (coding) are spliced together; cap and tail added | Spliceosomes, mature mRNA |
| Translation | Ribosome (cytoplasm / rough ER) | mRNA sequence is read codon by codon; tRNA delivers amino acids; polypeptide chain is built | Ribosome, tRNA, mRNA |
| Post-translational modification | ER, Golgi | Polypeptide folds into its 3D shape; may be modified, cleaved, or combined | Finished protein |
Transcription
Writing the Order Ticket📋 DNA → mRNA: Copying One Gene
Transcription is the process of making an mRNA copy of a gene. It occurs in the nucleus. RNA polymerase reads the DNA template strand and builds a complementary RNA strand. Unlike DNA replication, only ONE strand of DNA is used as the template (the template/antisense strand), and only ONE gene is transcribed at a time. The result is a pre-mRNA molecule that is then processed before leaving the nucleus.
Steps of Transcription
DNA template strand: 3'-TACGCAATTGCG-5'
Step 1: RNA polymerase reads the template 3'→5'
Step 2: Add complementary RNA bases (A↔U, T in DNA → A in RNA; A in DNA → U in RNA; G↔C)
T→A, A→U, C→G, G→C, C→G, A→U, A→U, T→A, T→A, G→C, C→G, G→C
mRNA produced: 5'-AUGCGUUACGC-3'
Notice: AUG at the start — this is the start codon! This mRNA is ready for translation.
Translation
Reading the Order, Building the Dish🍳 mRNA → Protein: Assembling the Dish
Translation is the process of reading the mRNA sequence (codon by codon) and assembling the corresponding chain of amino acids — the polypeptide. It occurs at the ribosome in the cytoplasm. The ribosome has three sites (E, P, A) and moves along the mRNA one codon at a time. tRNA molecules bring the correct amino acids, and peptide bonds link them together to build the growing chain.
Understanding Codons
Every three consecutive bases on mRNA = one codon = one amino acid instruction. The ribosome reads from left (5') to right (3'), starting at the AUG start codon.
Each codon is read by a matching tRNA anticodon. The amino acids they carry are joined by peptide bonds.
Steps of Translation
🏭 Ribosome Sites (A, P, E)
- A site (Aminoacyl) — incoming tRNA with new amino acid enters here
- P site (Peptidyl) — tRNA carrying the growing polypeptide chain sits here
- E site (Exit) — empty tRNA leaves the ribosome here
- Ribosome moves A→P→E direction as translation proceeds
🔗 The Peptide Bond
- Forms between the carboxyl (-COOH) group of one amino acid and the amino (-NH₂) group of the next
- Catalysed by the peptidyl transferase activity of the large ribosomal subunit (rRNA)
- A condensation reaction — water is released
- Each bond formed extends the polypeptide by one amino acid
Mutations
Errors in the Recipe⚠️ When the Recipe Goes Wrong
A mutation is a permanent change in the DNA base sequence. Mutations can affect the protein produced — changing its amino acid sequence and potentially its structure and function. Gene mutations affect a single gene; chromosomal mutations affect whole chromosomes. Whether a mutation matters depends on what type it is, where in the gene it occurs, and what protein it affects.
| Mutation Type | What Changes | Effect on Protein | Example |
|---|---|---|---|
| Substitution (point mutation) | One base replaced by another | May change one amino acid (missense), create a stop codon (nonsense), or have no effect (silent — degenerate code) | Sickle cell anaemia: A→T changes one codon → different amino acid in haemoglobin |
| Insertion | One or more bases added | Frameshift — all codons downstream are shifted → usually completely different protein, often non-functional | Adding one base shifts the entire reading frame from that point |
| Deletion | One or more bases removed | Frameshift (if not multiples of 3) — all downstream codons shifted → usually non-functional protein | Removing one base disrupts reading frame of entire downstream sequence |
| Inversion | Segment of DNA reversed | Varies — may disrupt one or many codons | ATGCCG → ATGGCC (segment CGC reversed to GCC) |
| Silent mutation | Base substitution but same amino acid coded | No effect on protein — due to degeneracy of the genetic code | CGA → CGU both code for arginine |
Original mRNA: AUG | UUU | GGA | CCU | UAA
Met — Phe — Gly — Pro — STOP
After insertion of one base (C after position 3):
AUG | CUU | UGG | ACC | UUA | A...
Met — Leu — Trp — Thr — Leu — ... (completely different protein, stop codon lost)
🧬 The Mutation
- Single base substitution: A → T in the haemoglobin beta gene
- This changes the codon GAG (glutamic acid) → GUG (valine)
- One amino acid change in the 147-amino acid beta chain
- But this single change is enough to completely alter protein behaviour
💔 The Consequences
- Mutant haemoglobin (HbS) stacks into rigid fibres when oxygen is low
- Red blood cells become sickle-shaped — rigid and sticky
- Block capillaries → pain crises; damage to organs
- Sickle cells are fragile and break down — causing anaemia
- Heterozygous carriers have some protection against malaria — an evolutionary trade-off
🎯 Kitchen Inspection
Eight questions on protein synthesis.