Here’s how cells rapidly stuff two meters of DNA into microscopic capsules

Two proteins form loops and a spiral helix that facilitate speedy packing

simulation of bundled chromosomes

PACK IT UP  Before dividing, a cell bundles each of its chromosomes (gray and colored strings in this simulation of a single chromosome) into a tightly-packed, orderly cylinder. A protein that creates a central, spiral scaffold (red links) is partly responsible for the efficient packing, a new study finds.

A. Goloborodko and J. Gibcus, with input from K. Samejima, B. Earnshaw, L. Mirny and J. Dekker

Frequent fliers, take note. Scientists have figured out how cells quickly pack long chromosomes into compact, organized bundles — a key step before cells divide. The new finding unifies two competing ideas about the process: whether it involves winding chromosomes into a spiral staircase or into a set of loops. It turns out cells use two different ring-shaped proteins called condensins to do both actions, imaging and computer simulations reveal.

Normally, chromosomes sit unspooled in a cell’s nucleus. But when a cell prepares to undergo mitosis — a type of cell division — those strings of DNA must condense into easy-to-transfer cylinders. It’s a formidable task: A cell must cram about two meters of DNA into microscopic packages without tangling the genetic material like a string of holiday lights.

Condensin II shapes a chromosome into large loops and then forms a helical scaffold for the loops to wind around. Condensin I subdivides large loops into smaller nested loops that allow for more space-efficient packing.

Together, the two proteins deftly stuff the chromosome into a densely packed cylinder, scientists report online January 18 in Science. Most of that condensing process happens in about 15 minutes, says study coauthor Job Dekker, a Howard Hughes Medical Institute investigator at the University of Massachusetts Medical School in Worcester.

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