Epigenesis Is How Genetically Identical Cells Develop Into Different Specialised Cell Types
A multicellular organism develops from a single, undifferentiated cell into an organised system of specialised cells, tissues, and organs.
This gradual building of pattern and complexity during development is called epigenesis.
Every cell in the organism carries the same genome, so differences between cell types cannot come from different DNA sequences.
Instead, cells become specialised because each cell type switches on a different pattern of gene expression.
Differentiation: The Same Genome, Read Differently in Each Cell Type
Definition
Epigenesis
Epigenesis refers to the development of patterns of differentiation in cells, enabling them to perform specific functions.
During development, cells become specialised through differentiation.
Differentiation is guided by epigenetic tags that ensure only the genes needed for a cell's role are expressed.
These tags are chemical marks added to DNA or to the histone proteins that package it, and they change how the genetic code is read, not the code itself.
The two main types of tag are methylation of DNA and modification of histones, whose detailed mechanisms are covered in D2.2.6.
Example
In muscle cells, genes for muscle proteins like actin and myosin are expressed.
In those same muscle cells, genes for neuron-specific proteins are silenced.
A Library of Genes
Analogy
Every cell holds the same full library of books (genes), but each cell type only opens the books it needs for its role.
Epigenetic tags decide which books are left open and which are locked away.
Differentiation Is Stable and Passed On Through Mitosis
Once a cell has differentiated, its pattern of gene expression is stable and usually kept for the life of the cell.
When the cell divides by mitosis, its epigenetic tags are copied to both daughter cells.
This is why a muscle cell divides to make more muscle cells, keeping the whole cell lineage specialised in the same way.
Note
Epigenesis changes a cell's phenotype (which proteins it makes) without changing its DNA base sequence.
So all the differentiated cells in one organism share an identical genome but express it differently.
Environment Can Shape Epigenetic Patterns During Development
Epigenetic tags can respond to environmental cues, so conditions during development can influence which genes a cell expresses.
Most tags are erased when sperm and eggs form, but some persist and can be passed to the next generation.
Example
In the Dutch Hunger Winter study, children of mothers who went through famine in pregnancy showed altered glucose metabolism, likely from epigenetic inheritance.
Epigenetics and How We Think About Inheritance
Theory of Knowledge
How does epigenetic inheritance challenge the traditional view that only DNA sequence is passed between generations?
Does it suggest that a parent's experiences can shape future generations?
Active recall
What is epigenesis?
How can cells with an identical genome become different specialised cell types?
Why does epigenesis change a cell's phenotype but not its DNA base sequence?
How is a differentiated cell's pattern of gene expression kept when it divides by mitosis?
