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Loops or spirals? 150-year mystery of what happens to DNA in cell division solved

22 January 2018
Appeared in BioNews 934

How DNA is accurately split between cells when they divide has finally been solved by researchers.

Despite 150 years of study of the cell division process of mitosis, little was known about how DNA is arranged to make sure that each daughter cell receives a complete genome. Some scientists were convinced that the DNA was arranged into a spiral shape, while others argued that the DNA was in loops. Now, a new paper published in Science has shown that both sides of the argument are right.

Life is dependent on the ability of cells to divide their DNA equally into two new cells in mitosis. Whenever a living organism grows, replicates or repairs itself, it is reliant on successful cell division. Mistakes in this process can result in a cell missing parts of its genome or having too many copies of a certain bit of DNA. This can cause serious consequences including birth defects or formation of a tumour.

To study how cells manage to do this correctly the vast majority of the time, the team used chicken cells to track how chromosomes prepare themselves for cell division. They condense and change shape from what looks like fuzzy blob through the microscope to the more recognisable rod-like chromosome shape.

Combining what was seen in the cells with biochemical experiments and mathematical modelling, the team found that that two protein molecules known as condensins gather up DNA in each chromosome into organised loops arranged around a central helix, like stairs in a spiral staircase. This organised structure is easily divided during mitosis and helps control how much DNA each daughter cell gets.

'I find this extremely satisfying,' said Dr Job Dekker from the Howard Hughes Medical Centre, one of the researchers who led the study. 'I always aim for consilience. If you're confronted with datasets that supposedly tell you two different things, can you find a way for them both to be right?'

The findings are another step on the long road to fully understanding how chromosomes behave during mitosis. After a century and a half of investigation, 'it is brilliant to see decades of work come to fruition', said Tom Collins, a senior portfolio developer at the Wellcome Trust, which funded the research. 

'It's the beginning of a long journey towards practical applications and the next step is to take this knowledge of how the process works in healthy cells, and identify what can go wrong to cause cancer or birth defects.'

A pathway for mitotic chromosome formation
Science |  18 January 2018
DNA study casts light on century-old mystery of how cells divide
EurekAlert |  18 January 2018
Packing a Genome, Step-by-Step
Howard Hughes Medical Institute |  18 January 2018
Researchers crack DNA riddle to show how process of mitosis works
Wellcome Trust |  18 January 2018
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