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

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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

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4:30pm, January 29, 2018
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.

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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.

Chromosome choreography

Time-lapse microscope images show the packing of chromosomes in a chicken cell’s nucleus into tiny capsules. Only minor changes happen after the first 15 minutes. 

Citations

J.H. Gibcus et al. A pathway for mitotic chromosome formation. Science. Published online January 18, 2018. doi:10.1126/science.aao6135.

Further Reading

S. Schwartz. The human genome takes shape and shifts over time. Science News. Vol. 188, September 5, 2015, p. 18.

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