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Protein synthesis transfers sequence information through different molecular forms. Follow the template strand, RNA processing and tRNA interactions without confusing their roles.
What you'll be able to do
During transcription, a region of DNA becomes accessible and RNA polymerase uses one DNA strand as a template. Complementary RNA nucleotides pair with exposed bases and are joined into an RNA chain. RNA contains uracil rather than thymine, so a template adenine is paired with uracil in the transcript.
In the syllabus prokaryotic model, transcription directly produces mRNA. In eukaryotes, a protein-coding transcript initially forms pre-mRNA, which is processed including removal of introns and joining of exons. The mature mRNA can leave the nucleus through pores. DNA itself does not have to travel to the cytoplasmic ribosome.
A ribosome associates with mRNA and reads successive codons. Each appropriate tRNA carries a particular amino acid and has an anticodon complementary to a codon. Positioning tRNAs allows peptide-bond formation between neighbouring amino acids, extending a polypeptide in the encoded order.
Energy is required in the overall translation process, including loading amino acids onto appropriate tRNAs and other steps. The tRNA does not manufacture an amino acid from the codon; it delivers an amino acid matched by the cellular charging system. When translation terminates, the chain is released and can fold or undergo further processing.
If a question supplies a codon table, establish whether it lists mRNA codons and whether the provided DNA is the coding or template strand. A coding strand has the same base sequence as the corresponding RNA apart from T/U, while a template must be complemented and its orientation considered.
A genome includes the organism’s genetic information, including non-coding sequence; a proteome describes its protein complement or the range expressed in a defined cell and condition. Different specialised cells can express different proteins despite having broadly the same nuclear DNA. AQA does not require memorising which particular codon specifies each amino acid: practise using provided data.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
Common mistakes to avoid
Key takeaways
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.