Protein Synthesis and Gene Expression

From base sequence to protein shape

The order of bases in a gene is not arbitrary — it carries information. Specifically, the base sequence in a gene determines the amino-acid sequence used to build a particular protein. Different amino-acid sequences fold into proteins with different shapes, and a protein’s shape is what determines what it can actually do. That gives a required chain of reasoning:

DNA base sequence → amino-acid sequence → protein shape → protein function.

Notice where each term belongs: a base sequence is a property of DNA (and, in the next section, of mRNA); an amino-acid sequence is a property of a protein. Don’t describe a protein as having a “base sequence” — that mixes the two levels. Cambridge does not require the detailed structure of nucleotides for this objective, so the chain above is the depth an answer needs, not a fuller molecular explanation.

Why does shape matter so much? Because a cell’s proteins carry out its functions, and the required examples span very different jobs: enzymes, membrane carrier proteins, and receptors for neurotransmitters. If a base-sequence change alters the amino-acid sequence, the resulting shape can change, and a protein that depends on a precise shape — an enzyme’s active site, a receptor’s binding surface — can stop working properly. This is how DNA is able to control cell function without doing anything directly itself: it controls which proteins get made, and the proteins do the actual work.

How a protein is actually made

The gene coding for a protein stays exactly where it is — in the nucleus. It doesn’t travel out to where the protein is built. Instead, a working copy of its information is sent out:

1. Messenger RNA, or mRNA, is made as a copy of the gene, in the nucleus.
2. The mRNA moves from the nucleus into the cytoplasm.
3. It passes through, or associates with, a ribosome.
4. The ribosome assembles amino acids into a protein, in the order the mRNA specifies.

The sequence of bases in the mRNA is what determines the sequence in which amino acids are assembled, so the full information pathway required for this syllabus point is:

gene in DNA → mRNA → ribosome → amino-acid sequence → protein.

Keep the roles distinct in an answer: mRNA carries a copy of genetic information, it is not itself the protein; a ribosome assembles amino acids, it does not copy the gene. Cambridge explicitly does not require the detailed mechanisms of transcription or translation here — no codons, anticodons, tRNA or base-pairing rules are needed. State the sequence above and stop there.

Gene expression and why cells specialise

Most of the body cells in an organism contain the same genes. What differs between cell types is which of those genes are actually being used — a particular cell expresses many of the genes it needs while many other genes present in its nucleus are not expressed at all. A cell making a digestive enzyme and a cell making a neurotransmitter receptor can carry an almost identical set of genes while producing very different sets of proteins, simply because each is expressing a different subset.

The distinction worth holding onto for an exam answer is this: having a gene is not the same as expressing it. A cell can carry the genetic instructions for a protein it never actually makes. This is also the underlying reason unspecialised stem cells are able to give rise to differently specialised daughter cells despite starting from the same genetic information — differentiation is, at this level, largely a story about which genes get switched on.

What this sets up next

Protein synthesis explains what a cell does with its genetic information while it is alive and functioning. The next question this chapter asks is what happens to that genetic information when a cell divides — which is where mitosis and meiosis pick up the thread.