Module 2: Protein Synthesis
These free OCR A Level Biology Protein Synthesis revision notes have been written for specification point 2.1.3(g.)
Protein synthesis
Protein synthesis is when a ribosome uses genetic code (from transcription) to make a polypeptide chain from amino acids (during translation).
The table below outlines transcription and translation:
| Process | Location | Purpose |
|---|---|---|
| Transcription |
Nucleus (in eukaryotes) Cytoplasm (in prokaryotes) | Synthesise a complementary mRNA copy of a gene |
| Translation | Cytoplasm (on ribosomes) | Read mRNA and assemble amino acids into a polypeptide chain |
Transcription
- The DNA double helix unwinds and the hydrogen bonds between bases break.
- RNA polymerase binds to the DNA and adds free complementary RNA nucleotides to the DNA bases (A–U, C–G); these are held in place by temporary hydrogen bonds.
- Phosphodiester bonds form between RNA nucleotides, producing a strand of pre-mRNA that is a copy of the coding strand.
In eukaryotes, this pre-mRNA undergoes splicing:
- Introns (non-coding regions) are removed.
- Exons (coding regions) are joined to form mature mRNA.
In eukaryotes, mRNA must leave the nucleus via a nuclear pore and travel to the cytoplasm for translation.
Translation
- mRNA binds to the ribosome.
- tRNA brings specific amino acids.
- The anticodon on tRNA pairs with codon on mRNA via temporary hydrogen bonds.
- Ribosomes hold tRNA in place (with temporary hydrogen bonds).
- Ribosome catalyses the formation of peptide bonds between amino acids using energy from ATP.
- The ribosome moves along mRNA until a stop codon is reached (and then detaches from the polypeptide).