What are the steps of protein synthesis for GCSE Biology?
Steps of Protein Synthesis: Transcription and Translation
Protein synthesis; is the two-stage biological process consisting of transcription and translation, by which the genetic code stored in DNA is used to assemble amino acids into functional proteins. In eukaryotic cells, this process transfers genetic information from the nucleus to ribosomes in the cytoplasm.
1. Transcription (in the Nucleus):
- DNA Unwinding: An enzyme called RNA polymerase binds to a non-coding region in front of a gene and unzips the two strands of the DNA double helix.
- mRNA Formation: RNA polymerase reads the template DNA strand and builds a complementary single strand of messenger RNA (mRNA) using free RNA nucleotides. In RNA, uracil (\text{U}) pairs with adenine (\text{A}), while cytosine (\text{C}) pairs with guanine (\text{G}).
- Nuclear Exit: The completed mRNA strand detaches from the DNA, passes out of the nucleus through a nuclear pore, and moves into the cytoplasm.
2. Translation (at the Ribosome):
- Ribosome Binding: The mRNA molecule attaches to a ribosome, which acts as the site of protein assembly.
- Codon Reading: The ribosome reads the bases on the mRNA in groups of three, known as codons (triplets).
- tRNA Delivery: Carrier molecules called transfer RNA (tRNA) bring specific amino acids to the ribosome. Each tRNA molecule has an anticodon complementary to a specific codon on the mRNA strand.
- Peptide Bond Formation: The ribosome joins adjacent amino acids together with peptide bonds to form a growing polypeptide chain.
- Protein Folding: Once a stop codon is reached, the polypeptide chain detaches and folds into a unique three-dimensional shape to form a specific protein, such as an enzyme, antibody, or hormone.
EXAM INSIGHT / PITFALL: In GCSE exams, remember that RNA does not contain thymine (\text{T}). When writing complementary mRNA sequences, adenine (\text{A}) on the DNA template always pairs with uracil (\text{U}) on the mRNA.