The big idea: Epigenesis is how a multicellular organism develops its different cell types: cells with the same DNA develop different patterns of gene expression and so differentiate.
The points to remember
- Every cell made from the zygote by mitosis has the same genome.
- During development, signals switch on different genes in different cells.
- Each cell's pattern of gene expression decides what it differentiates into.
- The base sequence of the DNA is not changed by epigenetic changes.
- So the cells' phenotype changes, but their genotype does not.
Remember it as: One genotype, many phenotypes.
Real example: liver and nerve cells. A liver cell and a nerve cell from the same person carry exactly the same DNA. They became so different during development because each switched on a different set of genes.
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Phenotype changes, genotype does not: Epigenetic changes alter which genes are expressed, not the genes themselves.
| Muscle cell | Nerve cell | |
|---|---|---|
| Genotype (base sequence) | The same | The same |
| Genes expressed | Muscle proteins such as actin and myosin | Nerve proteins such as ion channels |
| Phenotype | Long, contracts | Long branches, carries impulses |
The pattern is kept: When a differentiated cell divides by mitosis, its daughter cells keep the same pattern of gene expression, so a liver cell gives rise to more liver cells.
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How it is asked: Short written answers on how cells with the same DNA become different, or why daughter cells keep their parent's identity; 1A items on what differentiation is.
IB-style questionExplain[3 marks]
Explain how cells with identical DNA can develop into different cell types.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.