A muscle cell and a nerve cell in the same body carry the same genome. Yet one is packed with contractile proteins and the other with ion channels. Gene regulation in eukaryotes explains how: each cell type keeps a different set of genes switched on.
Key idea
Cell types differ because they express different sets of genes, controlled by different combinations of transcription factors. Nearly every cell keeps the whole genome; it is the pattern of expression, held in place by epigenetic marks, that makes a cell specialized.
- Combinatorial control: a gene's enhancers may need several activators at once. With a few dozen transcription factors in different combinations, cells can set up many distinct patterns.
- Master regulators: some transcription factors switch on a whole program. MyoD, for example, turns on many muscle genes, including more MyoD.
- Evidence the genes are still there: a single cell from a carrot root, grown in a dish, can grow into a whole carrot plant. Many plants regrow from cuttings of a stem or a leaf.
Why it works
Why does the pattern last? Specialization is reinforced by feedback: a master regulator often activates its own gene, so once on, it stays on. Epigenetic marks then lock it in and are copied at each division. Signals from neighboring cells, as in cell signaling, decide which program starts in the first place.
Worked examples
Worked example: Which genes are on?
Gene X needs activators A and B. Gene Y needs A and C. Gene Z needs B and D. A cell makes activators A and B only. Which genes are expressed?
- X: needs A and B; both present. On.
- Y: needs C; absent. Off.
- Z: needs D; absent. Off.
Answer: Only gene X.
Worked example: Same genes, different proteins
A liver cell makes albumin; a skin cell does not. Does the skin cell have the albumin gene?
- Nearly all cells keep the full genome.
- The skin cell lacks the transcription factors (and open chromatin) needed to read the albumin gene.
Answer: Yes. It has the gene but does not express it.
Common mistake
Thinking specialized cells have thrown away the genes they do not use. Nearly all body cells keep the whole genome (mature red blood cells in mammals, which lose their nucleus, are a famous exception). A check: if a plant can regrow from one specialized cell, that cell must still hold every gene.