Histone acetylation is the addition of acetyl groups (-COCH3) to histone proteins. It is an epigenetic modification that usually switches genes on.Why histones hold DNA tightly
Histones have 'tails' that stick out from the nucleosome. These tails contain many amino acids called lysine, which carry a positive charge. DNA is negatively charged because of its phosphate groups, so histones and DNA attract each other strongly and the chromatin is tightly packed.Adding acetyl groups
Enzymes called histone acetyltransferases (HATs) add acetyl groups to the lysines in the histone tails. This:
1. removes the positive charge on the lysines
2. reduces the attraction between the histones and the negatively charged DNA
3. so the DNA is held less tightly and the chromatin becomes more loosely packed (euchromatin)
4. so transcription factors and RNA polymerase can bind to the promoter, and the gene is transcribed.Some acetylated histones are also recognised by proteins that help transcription to start.
Removing acetyl groups
Enzymes called histone deacetylases (HDACs) remove the acetyl groups. The positive charges return, the histones bind the DNA more tightly, the chromatin condenses into heterochromatin and the gene is switched off.The balance between HATs and HDACs controls which genes are available for transcription. Some drugs that inhibit HDACs are used to treat certain cancers, because they can switch back on genes that stop uncontrolled cell division.
Histones have 'tails' that stick out from the nucleosome. These tails contain many amino acids called lysine, which carry a positive charge. DNA is negatively charged because of its phosphate groups, so histones and DNA attract each other strongly and the chromatin is tightly packed.Adding acetyl groups
Enzymes called histone acetyltransferases (HATs) add acetyl groups to the lysines in the histone tails. This:
1. removes the positive charge on the lysines
2. reduces the attraction between the histones and the negatively charged DNA
3. so the DNA is held less tightly and the chromatin becomes more loosely packed (euchromatin)
4. so transcription factors and RNA polymerase can bind to the promoter, and the gene is transcribed.Some acetylated histones are also recognised by proteins that help transcription to start.
Removing acetyl groupsEnzymes called histone deacetylases (HDACs) remove the acetyl groups. The positive charges return, the histones bind the DNA more tightly, the chromatin condenses into heterochromatin and the gene is switched off.The balance between HATs and HDACs controls which genes are available for transcription. Some drugs that inhibit HDACs are used to treat certain cancers, because they can switch back on genes that stop uncontrolled cell division.