Different cell, same DNA: the transcription factors present decide which genes get read.
A liver cell and a neuron hold identical DNA. The difference starts with which transcription factors each one contains. Those proteins decide which genes are read, so a cell type is really a pattern of switches.
Key terms
- Differentiation — a cell becoming a specialist, like a nerve cell, by switching on a set of genes. Its DNA doesn't change.
- Transcription factor — a protein that binds DNA and changes how much a gene is transcribed. Activators raise it; repressors lower it.
- Enhancer — control DNA, often far from its gene, where activators bind to boost that gene's transcription.
- Homeotic gene — a master switch. Its product is a transcription factor that tells a body region what to become.
- Inducible operon — off by default. An inducer, such as a lactose derivative, binds the repressor and pulls it off the operator.
- Repressible operon — on by default. The end product, such as tryptophan, binds the repressor so it can grip the operator.
How it works
- A cell receives a signal from its neighbors, or inherits molecules from the egg's cytoplasm. Either way, particular transcription factors become active in it.
- The factors bind control elements such as enhancers. Genes in one program can sit on different chromosomes yet share an element, so one factor switches them all on together.
- Some targets are transcription factors themselves. That makes a cascade: one master gene can switch on dozens of genes, enough to build a whole structure.
- Chromatin helps lock the choice in. Histone acetylation keeps active genes open. DNA methylation packs unused genes away without changing their sequence, and the pattern is copied at each division.
- The cell now makes the proteins that define its type: insulin in pancreatic beta cells, hemoglobin in developing red blood cells.
Why it matters
For most scenarios here, ask one question: which regulatory protein can reach and bind which DNA? Cell signaling often ends here, with a signal activating a transcription factor. A new sugar frees bacterial genes from their repressor within minutes. A homeotic gene switched on in the wrong segment builds the wrong structure there.
Exam traps
| Trap | Actually |
|---|
| Genes switched on together must sit side by side in an operon. | That's the bacterial way. Eukaryotic genes can be scattered across chromosomes; a shared control element lets one factor turn them all on. |
| Tryptophan sits on the operator and blocks polymerase. | Tryptophan binds the repressor, changing its shape so it can grip the operator. The pathway's own product is the off signal. |
| Lactose creates the enzyme, or makes the gene appear. | The gene was always there. A lactose derivative binds its repressor, the repressor lets go, and transcription starts within minutes. |
| A master gene makes the building material for its structure. | It makes a transcription factor. Only a switch controlling many genes can build a whole structure in the wrong place. |
Worked example
Researchers attach the control region of a liver-only gene (its enhancer and promoter) to a gene for a glowing protein, then inject the DNA into a fertilized mouse egg. In the adult mouse, every cell carries this construct, but only liver cells glow. What best explains the pattern?
- Every cell inherited the construct from the egg, so the difference isn't in the DNA. It's in regulation.
- The glow gene has only the liver control region, so it is transcribed only where activators bind that enhancer.
- Only liver cells glow, so only liver cells contain those activators.
Only liver cells have the transcription factors that bind the liver enhancer, so only they transcribe the glow gene.
An enhancer does nothing until its matching activator binds. The near miss is 'only liver cells received the construct' — but it went into the egg, so every cell has it. Another is 'the enhancer codes for a liver protein.' An enhancer codes for nothing; it's a landing site for activators.