Gene Expression: Transcription and Translation GCSE Biology

What is a gene?

A gene is a section of DNA that codes for a specific protein. The protein's identity — and therefore its shape and function — is determined by the sequence of bases in the gene (A, T, C, G). Because proteins control every structural and chemical process in the cell, the DNA base sequence is ultimately the instruction manual for the entire organism.

The human genome contains approximately 20,000–25,000 protein-coding genes, located on 46 chromosomes inside the nucleus. Each cell contains the full genome, but different cells express different genes — which is how a liver cell and a neuron can have the same DNA but completely different structures and functions.

What is the genetic code?

The DNA base sequence is read in groups of three bases called codons (or base triplets). Each codon specifies one amino acid. There are 4 bases, giving 4³ = 64 possible codons, but only 20 amino acids — so most amino acids are coded for by more than one codon (the code is degenerate). Three codons are stop signals that end the protein chain.

What happens during transcription?

Transcription produces a single-stranded copy of the gene as messenger RNA (mRNA):

1. The enzyme RNA polymerase binds to the gene's promoter region and unzips the DNA double helix at that gene.
2. Free RNA nucleotides align opposite the template strand of DNA by complementary base pairing: A pairs with U (not T — mRNA uses uracil in place of thymine), C with G.
3. RNA polymerase joins the nucleotides together, synthesising the mRNA strand.
4. The mRNA peels away from the DNA; the DNA double helix closes.
5. The mRNA leaves the nucleus through a nuclear pore and travels to the cytoplasm.

Key difference from DNA replication: transcription copies only one gene (not the whole chromosome), uses RNA nucleotides (not DNA), and the product is single-stranded mRNA.

What happens during translation?

Translation builds the protein at a ribosome:

1. The mRNA binds to a ribosome in the cytoplasm.
2. Transfer RNA (tRNA) molecules bring amino acids to the ribosome. Each tRNA has an anticodon (three bases) that is complementary to one mRNA codon.
3. The ribosome reads the mRNA in triplets (codons). The tRNA with the matching anticodon docks, bringing its amino acid alongside the previous one.
4. A peptide bond forms between adjacent amino acids.
5. The ribosome moves along the mRNA (translocation), reading the next codon.
6. This continues until a stop codon is reached. The completed polypeptide chain is released.

The chain of amino acids folds into a specific 3-D shape — the protein's structure — which determines its function.

How do transcription and translation connect to protein function?

A mutation (change in the DNA base sequence) can alter the mRNA codon, which may insert a different amino acid into the polypeptide. If the amino acid change alters the protein's shape — particularly at the active site of an enzyme — the protein may no longer work correctly.

What is the difference between a structural protein and an enzyme?

Both are produced by gene expression, but they have different roles:

All are made by the same transcription–translation process; their function depends entirely on their amino acid sequence and folded shape.

Frequently asked questions

Why does mRNA use uracil instead of thymine?

Thymine in DNA contains a methyl group that makes it stable over the long timescales a chromosome must persist. mRNA is a temporary molecule — it is made, used and degraded within minutes or hours. Uracil is chemically simpler (no methyl group) and less energetically costly to produce. Using uracil in mRNA rather than thymine may also help the cell distinguish mRNA from DNA fragments, which always contain thymine; this assists quality-control mechanisms.

How can the same DNA in every cell produce such different cell types?

All body cells contain an identical copy of the entire genome. What differs between cell types is which genes are switched on or off — a process called gene regulation. Specialised regulatory proteins bind to specific regions of DNA, activating some genes and silencing others. A liver cell expresses genes for liver-specific enzymes; a muscle cell expresses genes for contractile proteins. This cell-type-specific gene expression is established during embryonic development and maintained throughout life.

What happens when a mutation changes one base in a gene?

A single base change is a point mutation. If the codon change still codes for the same amino acid (because the genetic code is degenerate), there may be no effect — this is a silent mutation. If the codon change produces a different amino acid, it is a missense mutation: the protein may still function, function poorly, or not at all. If the change produces a stop codon, it is a nonsense mutation: the protein is cut short and almost certainly non-functional. For example, the mutation that causes sickle-cell anaemia is a single base change in the haemoglobin gene.

Why do cells produce proteins constantly?

Proteins are functional molecules, not permanent structures. Enzymes are used up (not in the reaction, but they can be denatured over time or targeted for degradation). Hormones like insulin are secreted and broken down rapidly. Structural proteins such as collagen are replaced as they wear. Cells regulate protein levels by controlling both how much mRNA is transcribed and how quickly each mRNA and protein is degraded. This continuous production allows cells to respond to changing conditions — producing more of an enzyme when its substrate is abundant, or less when it is not.

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