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

22 January 2018

By Dr Rachel Huddart

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.'

Science | 18 January 2018
EurekAlert | 18 January 2018
Howard Hughes Medical Institute | 18 January 2018
Wellcome Trust | 18 January 2018


28 November 2016 - by Dr Rachel Huddart 
DNA only accounts for about half of the total volume of chromosomes, as shown by new 3D images of human chromosomes...
15 December 2014 - by Simon Hazelwood-Smith 
A study into the 3D structure of the human genome has revealed the locations of DNA 'loops' at unprecedented resolution...
12 September 2011 - by Sarah Pritchard 
Researchers based in the USA and the Netherlands report study findings that may explain why women with mutations in the BRCA1 gene are more susceptible to breast and ovarian cancer...
29 March 2010 - by Dr Will Fletcher 
A recent study has lent more weight to the view that 'Junk DNA' may be anything but junk. A joint effort by the European Molecular Biology Laboratory (EMBL) in Heidelberg, Germany and Stanford University, California, US, has uncovered large differences between the non-coding DNA of different individuals, which may be associated with differing levels of disease risk and other traits too...

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