Two meters of DNA fit inside a nucleus six microns wide.
The Visitor walks along the spools that make this possible and reads the tags on them.
Thirty Million Spools
- DNA wraps around histone octamers (eight proteins: two each of H2A, H2B, H3, H4) at intervals of about 147 base pairs. Each unit is a nucleosome, and the page counts 30 million of them in every human nucleus.
- In 1996 David Allis and Bryan Turner independently proposed the histone code hypothesis: combinations of chemical marks on histone tails specify gene expression states.
- Three marks anchor the story. H3K27 acetylation activates. H3K27 trimethylation represses. H3K4 trimethylation marks active promoters.
- The bivalent state carries both an active and a repressive mark on the same promoter. The page reports 2,400 bivalent genes in human embryonic stem cells, poised for rapid activation or silencing.
- Mass spectrometry has catalogued well over 100 modified sites on human histones, spanning more than a dozen chemical types. One 2011 study alone identified 130 sites (Tan et al. 2011). Cells use a defined subset at any moment, and the page describes the code as heritable through cell division.
The strand winds over and under the spools while marks glow on the tails. The site page carries the three anchor marks and the bivalent state.
How many modified histone sites has mass spectrometry catalogued, and what does the bivalent state give a stem cell?
Well over 100 sites are catalogued, with 130 found in one 2011 study alone. The bivalent state gives a promoter poised for rapid activation or silencing, with removal of either mark determining cell fate.
Well over 100 catalogued histone sites carry readable combinations of marks that set each gene's state.
The Visitor lights node 03. Epigenetics and Histones now glow together.
See the map