How do histones relate to gene expression?
In humans and most eukaryotes, nuclear DNA is often kept repressed by default, bound in the structure of a supercoil. This is a coiled coil of DNA that is wrapped around histones at regular intervals of around 146 base pairs. Each of the histone "beads" on the DNA "string" is referred to as a nucleosome.
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For genes to be expressed, the DNA to be transcribed into RNA by RNA polymerase. Since most DNA is in the compressed supercoiled state, the promoter sequence is almost completely inaccessible to the polymerase and the transcription factors. However, among the regions where topoisomerases have untwisted the DNA, the properties of the nucleosome determine whether a gene will be expressed.
Can histone marks predict gene expression?
Certain modifications to the histone proteins that compose the nucleosome, such as methylation, acetylation, or lactylation of specific places on the protein can change how tightly bound the DNA is to the histone core. These particular modifications are part of the epigenome, which is used by multicellular organisms to allow for cell differentiation. The epigenome is copied to the newly synthesized DNA strands during cell replication, ensuring that descendants of the differentiated cell will be the same type.
Since epigenetic markers like histone modifications affect the accessibility of DNA, in many cases they can serve the determining factor for how much will certain genes be expressed. This competition involves using the frequency and positions of histones in the region of gene promoter sequences in order to predict expression values