Protein Synthesis: Transcription and Translation
The Central Dogma: DNA → RNA → Protein
Transcription
Transcription produces an mRNA copy of a gene in the nucleus:
- RNA polymerase binds the promoter sequence upstream of the gene
- It unwinds DNA and reads the template strand 3'→5'
- Builds the mRNA strand 5'→3', using ribonucleotides (A, U, G, C — note Uracil replaces Thymine)
- Terminates at a terminator sequence
The resulting pre-mRNA undergoes processing in eukaryotes:
- 5' cap added (protects mRNA, aids ribosome binding)
- Poly-A tail added at 3' end (stability)
- Introns (non-coding) spliced out; exons joined → mature mRNA exported to cytoplasm
Translation
Translation occurs at ribosomes in the cytoplasm. The mRNA is read in triplets called codons. Each codon specifies one amino acid (or start/stop signal). Key steps:
- Initiation: small ribosomal subunit binds mRNA at start codon (AUG = methionine)
- Elongation: tRNA anticodons match codons; peptide bond forms between amino acids (peptidyl transferase activity of the large subunit)
- Termination: stop codon (UAA, UAG, UGA) is reached; release factor frees the polypeptide
The genetic code is degenerate (multiple codons → same amino acid), nearly universal across all life.
Codon Chart Basics
AUG = Start (Met). Stop codons: UAA, UAG, UGA. The genetic code has 64 codons for just 20 amino acids — hence redundancy (degeneracy). Wobble at the 3rd codon position allows some mismatching.
Transcription vs Translation
| Feature | Transcription | Translation |
|---|---|---|
| Location (eukaryote) | Nucleus | Cytoplasm (ribosomes) |
| Template | DNA template strand | mRNA |
| Product | mRNA | Polypeptide (protein) |
| Enzyme | RNA polymerase | Ribosomes + tRNA + aminoacyl-tRNA synthetases |
| Monomer units | Ribonucleotides | Amino acids |
| Direction | 5'→3' synthesis | 5'→3' reading of mRNA |
Which RNA carries amino acids to the ribosome during translation?
tRNA has an anticodon loop that base-pairs with the mRNA codon, and it carries the corresponding amino acid at its 3' end.
Correct answer: tRNA (transfer RNA)
What is the start codon and which amino acid does it encode?
AUG is the universal start codon, encoding methionine. Translation always begins at AUG.
Correct answer: AUG — Methionine
Explain the difference between an intron and an exon, and what happens to each during mRNA processing.
Only exons contribute to the final protein product.
Correct answer: Exons are coding sequences that are retained in the mature mRNA and translated into protein. Introns are non-coding intervening sequences that are removed (spliced out) during pre-mRNA processing in the nucleus before the mRNA is exported.