Mixing up histone acetylation and DNA methylation
- The idea
- Predict
- See it
- Why
- Check
The idea
“Acetylating histones packs DNA tighter, and methylating DNA turns a gene on.”
Two small tags, often taught together, are easy to swap. And adding one to a histone sounds like it should tighten its grip.
The short version
The mistake: Acetylating histones packs DNA tighter, and methylating DNA turns a gene on.
What's actually true: Histone acetylation loosens chromatin and helps turn genes on. DNA methylation, usually at a promoter, turns them off.
- DNA is negative, and histone tails are rich in positive lysine. The tails cling to the DNA and pack it tightly, so its genes are hard to reach.
- Acetylation cancels those positive charges. The tails loosen, and genes there are easier to transcribe.
- DNA methylation puts methyl groups on cytosines, often in a promoter. Proteins that bind them call in enzymes that pack the chromatin and usually silence the gene.
- Neither tag changes the base sequence. They change which genes get read.
On the exam: Acetyl on histones opens chromatin and turns genes on. Methyl on promoter DNA turns them off.