Epigenetic Tags Are Heritable, So Patterns of Gene Expression Can Be Passed On
Genes are like books in a library, and epigenetic tags decide which are open to read and which are locked away.
These tags change gene expression without altering the DNA sequence.
The key point of this topic is that these tags are heritable, so a pattern of gene expression can be copied to new cells and, in limited cases, to offspring.
Definition
Epigenetic tag
An epigenetic tag is a chemical modification to DNA or histone proteins that regulates gene expression without altering the underlying DNA sequence.
Note
Two tag types set gene expression: DNA methylation of cytosine bases tends to silence genes.
Histone modifications such as acetylation loosen or tighten chromatin to activate or repress genes.
The detailed mechanism of these tags belongs to D2.2.6, so here the focus is how they are inherited.
Mitosis Copies Epigenetic Tags So Specialised Cells Keep Their Identity
When a cell divides by mitosis, its epigenetic tags are copied to both daughter cells.
Methylation patterns are maintained by maintenance methyltransferases, which re-add methyl groups to the new DNA strand after replication.
This lets specialised cells such as muscle or nerve cells keep their identity through many divisions.
Example
Skin cells keep making keratin because inherited tags keep the keratin genes switched on.
Keratin is the protein that forms hair, nails, and the outer layer of skin.
Meiosis Erases Most Tags, But Some Cross Generations
Most epigenetic tags are erased during the formation of sperm and eggs, resetting the genome for the next generation.
A small number of tags survive this reset, allowing transgenerational epigenetic inheritance.
This is why some environmentally driven traits can appear in offspring even though the DNA sequence is unchanged.
Hint
Epigenetic tags let organisms adjust gene expression to the environment without changing DNA.
This flexibility is called phenotypic plasticity.
Why Transgenerational Inheritance Is Limited
Epigenetic changes are often reversible, so their effects may not last.
Only the few tags that escape erasure during gamete formation can be passed on.
Which tags persist is still poorly understood, making this a difficult field to study.
Heritable Epigenetic Tags Have Uses in Medicine and Agriculture
Medicine: epigenetic therapies target abnormal methylation and histone marks to treat diseases such as cancer.
Agriculture: understanding inherited tags helps breed crops that are more resilient to drought and salinity.
Active recall
How are epigenetic tags copied to daughter cells during mitosis?
What is the role of maintenance methyltransferases after DNA replication?
What happens to most epigenetic tags during the formation of gametes?
Define transgenerational epigenetic inheritance.
Give one reason transgenerational epigenetic inheritance is limited.
