AP Biology · Unit 6 · Topic 6.6
Every cell in your body has the same genes. Compare what four cell types are actually expressing and you can see where the difference really lives.
Learning objective 6.6.A Explain how the same genome can give rise to cells with different structures and functions.
Each of these cells carries an identical list of genes. Pick one and watch which of them it is actually using — and, more importantly, which it is not.
| Gene | What it makes |
|---|
These genes are present, intact and inherited in every one of the three cells. A red blood cell precursor carries the instructions for building a lens crystallin and never builds one. Nothing was thrown away during differentiation — it was switched off.
Switch a gene on in the sample and see how many of the three cells still match. The point is to find out how quickly you can narrow it down — and which genes never narrow it down at all.
| Cell type | Expresses all of them? | Its job |
|---|
| Cell type | Genes needed to identify it | Which ones would do |
|---|
Every pair overlaps by exactly the housekeeping set, and by nothing else. That is not a coincidence of the example — it is what makes these three recognizably different cell types rather than three variations on one.
A signal is released at the left-hand end of a row of identical cells and falls off with distance. Every cell has the same genome and the same thresholds. Move the top threshold and watch what happens to the pattern.
Read it downwards and every step is something you have already met: a signal reaching a receptor, a transcription factor being activated, a promoter being bound, a gene being transcribed. Development is not a separate subject with its own mechanisms; it is these mechanisms applied to cells in different places.
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