How does the information in a gene become a functional protein?
Explain how transcription produces mRNA from a gene and how translation uses that mRNA to build a polypeptide
Gene expression has two stages: transcription copies a gene into mRNA in the nucleus, and translation at the ribosome reads mRNA codons via tRNA to assemble a polypeptide.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
You need to describe both stages of protein synthesis and explain the roles of DNA, mRNA, RNA polymerase, ribosomes and tRNA. SACE wants the flow of information made explicit: DNA to mRNA to protein.
RNA versus DNA
Before the two stages, know how RNA differs from DNA:
- RNA is single-stranded; DNA is double-stranded.
- RNA uses the sugar ribose; DNA uses deoxyribose.
- RNA uses the base uracil (U) in place of thymine (T), so in RNA, A pairs with U.
Stage 1: Transcription
Transcription happens in the nucleus and copies a single gene into mRNA.
- Initiation. The enzyme RNA polymerase binds to the start (promoter) of the gene and unwinds the DNA, exposing the template strand.
- Elongation. RNA polymerase moves along the template strand and adds complementary RNA nucleotides. Where the template reads C, G, T, A, the mRNA gains G, C, A, U respectively (U replaces T).
- Termination. At the end of the gene, the completed mRNA strand is released and RNA polymerase detaches.
In eukaryotes the new transcript is processed: non-coding introns are removed and the coding exons are spliced together before the mature mRNA leaves the nucleus through a nuclear pore.
Stage 2: Translation
Translation happens at a ribosome in the cytoplasm (often on the rough endoplasmic reticulum) and decodes mRNA into a polypeptide.
- Initiation. The mRNA binds to a ribosome. The ribosome reads the mRNA in groups of three bases called codons, starting at the start codon AUG.
- Elongation. Each codon is matched by a tRNA molecule carrying a complementary three-base anticodon and a specific amino acid. tRNA anticodons base-pair with mRNA codons. As each tRNA arrives, the ribosome forms a peptide bond between adjacent amino acids, extending the growing chain.
- Termination. When the ribosome reaches a stop codon (UAA, UAG or UGA), no tRNA matches it, so the completed polypeptide is released.
The polypeptide then folds into a functional protein.
Putting the stages together
The whole pathway is summarised as the central idea of molecular biology: DNA to mRNA (transcription) to protein (translation). Transcription localises information copying to the nucleus where DNA is protected; translation lets that copy be read many times at ribosomes to make many protein molecules.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 SACE Stage 21 marksA gene product is synthesised in a cow (eukaryotic) cell. State the location in a cow cell where the first step in the synthesis of a gene product occurs.Show worked answer →
The nucleus. The first step in making a gene product is transcription, where mRNA is synthesised from the DNA template, and in eukaryotic cells this occurs inside the nucleus. The mark is for naming the nucleus.
2019 SACE Stage 22 marksThe first step in the synthesis of a gene product occurs in the nucleus of a eukaryotic cell. Explain why this process occurs in a different location in prokaryotic cells.Show worked answer →
For 2 marks, identify the structural difference and its consequence.
Prokaryotic cells have no nucleus (no nuclear membrane), so their DNA lies free in the cytoplasm.
Transcription must therefore occur in the cytoplasm, where translation also happens. This allows transcription and translation to be coupled (ribosomes can translate the mRNA while it is still being made), unlike in eukaryotes where the nuclear membrane separates the two steps.
2019 SACE Stage 23 marksExplain why, in eukaryotic cells, mRNA is processed before translation occurs.Show worked answer →
Three points cover the marks.
The primary transcript (pre-mRNA) contains both introns (non-coding) and exons (coding). Introns must be spliced out and the exons joined so that only the coding sequence is translated.
Processing adds a 5' cap and a poly-A tail, which protect the mRNA from degradation and help it leave the nucleus and bind the ribosome.
Without processing, ribosomes would translate the introns as well, producing a faulty, non-functional polypeptide. Processing therefore ensures the correct, functional protein is made.