Epigenetic Tags Are Chemical Modifications That Determine Which Genes Are Active or Silent
- Imagine a library where some books are locked away, while others are open for reading.
- The books represent genes, and the locks are epigenetic tags.
- Unlike changes to the DNA sequence, these tags can be influenced by the environment and sometimes passed on to future generations.
- Two primary mechanisms regulate these tags: DNA methylation and histone modification.
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.
Epigenetic tags act as "instructions" that determine whether specific genes are turned on or off.
Key Mechanisms of Epigenetic Regulation
1. DNA Methylation
- DNA methylation involves the addition of a methyl group ($CH_3$) to cytosine bases in DNA, typically occurring in CpG islands near promoter regions.
- This process silences genes by:
- Blocking Transcription Factors and RNA Polymerase: Methylation prevents these molecules from binding to the promoter, halting transcription of the downstream gene.
- Recruiting Repressor Proteins: Methylated DNA attracts proteins that compact chromatin, making it inaccessible for transcription.
Tumor suppressor genes are often silenced by hypermethylation in cancer cells.
2. Histone Modifications
- Chemical changes (e.g., acetylation, methylation) to histone proteins.
- Modify how tightly DNA is wrapped around histones, affecting gene accessibility.
- Acetylation: Loosens chromatin to activate transcription.
- Methylation: Can activate or repress transcription depending on the site.
- Again, think of histone methylation as a dimmer switch.
- Depending on where the methyl group is added, it can either brighten (activate) or dim (repress) gene expression.
In embryonic stem cells, activating methylation marks keep pluripotency genes active, while repressive marks silence differentiation genes until they are needed.
Epigenetic Tags Are Inherited During Mitosis and Meiosis
1. Inheritance During Mitosis
- When cells divide, their epigenetic patterns are passed to daughter cells.
