In plants, animals and fungi, DNA is wrapped around spools of histone proteins; one spool with its DNA is a nucleosome (Luger et al. 1997). Small tags on histone tails are added by writer enzymes and removed by erasers. Acetyl tags tend to open the DNA for reading; complexes such as Polycomb add methyl tags that keep genes silent (Allis & Jenuwein 2016). Established: Replicated many times; textbook knowledge.
Because these marks sit on top of the DNA sequence and can be copied when cells divide, they are called epigenetic. A plant uses them to remember winter: Polycomb marks build up on a flowering gene during the cold, so the plant flowers only in spring (Angel et al. 2011). Strong evidence: Several independent studies agree.
Complexes that close. Other protein complexes, such as Polycomb, add different tags, such as some methyl groups, and pack the spools tightly. Factors cannot reach the DNA, so the gene stays off. Same DNA sequence, different packaging: this is epigenetics.
A nucleosome: DNA wrapped around a histone spool
Rotate with a finger or mouse. On a phone you can place it in your room (AR).
- H3 K18: lysine 18 of histone H3, a seat for acetyl or lactyl tags
- H3 K27: lysine 27 of histone H3, often tagged to switch genes on or keep them off
- DNA: 147 base pairs wrap about 1.65 times around the spool
Nucleosome core particle, PDB 1KX5 (Davey et al. 2002, J Mol Biol). Model: khushisingh.in, CC BY 4.0.
In my research: Histone marks in the early embryo →
Sources
- Luger K, Mäder AW, Richmond RK, Sargent DF, Richmond TJ (1997) Crystal structure of the nucleosome core particle at 2.8 Å resolution. Nature 389:251–260. doi:10.1038/38444
- Allis CD, Jenuwein T (2016) The molecular hallmarks of epigenetic control. Nature Reviews Genetics 17:487–500. doi:10.1038/nrg.2016.59
- Angel A, Song J, Dean C, Howard M (2011) A Polycomb-based switch underlying quantitative epigenetic memory. Nature 476:105–108. doi:10.1038/nature10241