Anyone who has ever had to brush long hair will know that trying to get the knots out can be a nightmare. But mathematicians have now proved what many have suspected for some time – that the key to freeing the tangles is beginning at the ends and moving upwards the roots. Harvard researchers created a model that simulated two helically entwined filaments (similar to a strand of DNA) to represent a tangle of hair, and analysed different ways of'brushing' it so the hairs became free. Their results, published in the journal Soft Matter, revealed short brush strokes that start at the'free' end of the hair and move towards the'clamped' end are most effective. Experiments and simulations show the'tine' (representing a prong of the brush) moving along the double helix from the clamped end towards the free end'Using this minimal model, we study the detangling of the double helix via a single stiff tine (prong) that moves along it, leaving two untangled filaments in its wake,' said Plumb-Reyes, a graduate student at SEAS. 'We measured the forces and deformations associated with combing and then simulated it numerically.'

The human genome has its own proofreaders and editors, and their handiwork is not as haphazard as once thought. When DNA's double helix is broken after damage from, say, exposure to X-rays, molecular machines perform a kind of genetic "auto-correction" to put the genome back together -- but those repairs are often imperfect. Just as your smartphone might amend a misspelled text message into an incoherent phrase, the cell's natural DNA repair process can add or remove bits of DNA at the break site in a seemingly random and unpredictable manner. Editing genes with CRISPR-Cas9 allows scientists to break DNA at specific locations, but this can create "spelling errors" that alter the function of genes. This response to CRISPR-induced damage, called "end joining," is useful for disabling a gene, but researchers have deemed it too error-prone to exploit for therapeutic purposes.