From DNA to Protein: Transcription and Translation
Part of DNA Genome — GCSE Biology
This deep dive covers From DNA to Protein: Transcription and Translation within DNA Genome for GCSE Biology. DNA structure, function, and the human genome It is section 4 of 13 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 4 of 13
Practice
25 questions
Recall
25 flashcards
From DNA to Protein: Transcription and Translation
DNA carries instructions for making proteins, and proteins control almost everything in the cell — enzymes speed up reactions, structural proteins build tissues, hormones carry messages. But DNA never leaves the nucleus. So how does the cell use the instructions?
DNA is the master recipe book that stays locked in the library (the nucleus). You can't take the original book out. Instead, you photocopy the recipe you need — that photocopy is mRNA. You carry it to the kitchen (the ribosome) and cook the dish (the protein). The original book is never damaged because it never leaves the library.
Step 1 — Transcription (in the nucleus)
Transcription is the process of copying one gene from DNA into a molecule of mRNA (messenger RNA).
- The section of DNA containing the gene unzips — the hydrogen bonds between complementary base pairs break, separating the two strands.
- One strand acts as a template. Free RNA nucleotides line up alongside it using complementary base pairing rules.
- A new strand of mRNA is assembled as a complementary copy of the template strand. Note: mRNA uses uracil (U) instead of thymine (T), so where there is an A in the template DNA, the mRNA has a U.
- The mRNA strand is single-stranded. It leaves the nucleus through nuclear pores and travels to a ribosome in the cytoplasm.
Step 2 — Translation (at the ribosome)
Translation is the process of reading the mRNA code to build a chain of amino acids.
- The mRNA attaches to a ribosome in the cytoplasm.
- Every 3 bases on the mRNA form a codon, and each codon codes for one specific amino acid.
- Molecules of tRNA (transfer RNA) each carry a specific amino acid. The tRNA anticodon matches the complementary mRNA codon, bringing the correct amino acid to the ribosome.
- The ribosome joins amino acids together with peptide bonds as it moves along the mRNA, one codon at a time.
- The growing chain of amino acids eventually folds into a specific 3D shape — this shape determines what the protein does.
The Key Chain to Remember
DNA base sequence → mRNA base sequence → amino acid sequence → protein shape → protein function
A different base sequence produces a different amino acid sequence, which produces a different protein shape, which means different function. This is how the same 4-letter DNA alphabet can produce thousands of different proteins.
Quick Check: Explain how a change in one DNA base could result in a non-functional enzyme. (4 marks)
A change in one DNA base (a mutation) alters the base sequence of the gene. During transcription, this altered sequence is copied into mRNA, changing one codon. During translation, the changed codon codes for a different amino acid. The amino acid sequence of the enzyme is therefore different, so the enzyme folds into a different 3D shape. The active site of the enzyme is no longer the correct shape for its substrate to bind, so the enzyme cannot catalyse the reaction — it is non-functional.