A new study finds that proteins known as linker histones control the complex coiling process that determines whether DNA will wind into long and thin chromosomes, made up of many small loops, or short and thick chromosomes with fewer large loops.
Now, new research has identified proteins called linker histones as the factor that controls whether DNA winds into long and thin chromosomes, made up of many small loops, or short and thick chromosomes with fewer large loops. The findings, published in eLife, are the first to describe how chromosome shape is tuned by linker histones at the molecular level.
«The linker histone was once thought to impact only a narrow range of the genetic material,» says Rockefeller’s Hironori Funabiki. «We have now shown that it controls the number of loops in the chromosome and its ultimate shape, a much larger regulation space than expected.»
Beyond «beads on a string»
Genetic material is organized around a nucleosome — often depicted as a bead on a string, with a length of DNA «string» wound around a central protein «bead.» The string is clamped to its bead by a sort of protein clip — the linker histone — which is also involved in folding multiple nucleosome beads into chromatin fibers. These fibers form chromosomes after they are ratcheted through a molecular motor, the condensin, that organizes chromatin into loops.
Chromosomes come in a wide range of shapes across species and cell types, largely based on the size of each chromatin loop. Funabiki draws an example from the familiar (and frustrating) experience of coiling wired earphones. If you wrap them into many tiny loops, the headphones will fit neatly into your pocket. If, however, you wind the wires into only a few large loops, the earphones form a bulky mass. Similarly, a greater number of small loops will give rise to longer, thinner chromosomes; a few large loops of chromatin will form shorter, thicker chromosomes.
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