Explain how transcription produces mRNA from a gene and how translation uses that mRNA to build a polypeptide

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.

1. Initiation. The enzyme RNA polymerase binds to the start (promoter) of the gene and unwinds the DNA, exposing the template strand.
2. 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).
3. 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.

1. 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.
2. 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.
3. 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.

The roles to keep straight

Exam mark schemes reward naming the right molecule for the right job, so fix these:

• DNA stores the gene; only the template strand is read during transcription.
• RNA polymerase unwinds the DNA and builds mRNA from free RNA nucleotides.
• mRNA carries the copied message from nucleus to ribosome; its codons are read $5'$ to $3'$.
• Ribosome holds the mRNA and tRNAs in position and catalyses peptide-bond formation.
• tRNA carries a specific amino acid and an anticodon that base-pairs with the matching codon.

Prokaryotes versus eukaryotes

A frequent contrast: in prokaryotes there is no nuclear membrane, so transcription and translation occur together in the cytoplasm and can be coupled - a ribosome can begin translating an mRNA before transcription of that mRNA is finished. In eukaryotes the nuclear envelope separates the two stages in space and time: transcription and mRNA processing happen in the nucleus, and only the mature mRNA is exported to the cytoplasm for translation. Prokaryotic mRNA also lacks introns, so no splicing is needed. Being able to explain this difference, and link it to the absence of a nucleus, is a recurring SACE question.

Why one gene can make many proteins

A single mRNA can be translated by many ribosomes at once (a polyribosome), and the gene can be transcribed repeatedly, so a cell can produce large quantities of a protein quickly when needed. This amplification - one gene, many mRNA copies, each read many times - is part of why gene regulation (controlling when transcription happens) is such a powerful control point, linking this dot point directly to gene regulation.