Lesley Gilder, of Christchurch, asks :-

What causes curly hair?

Lyn Dowsett, a biochemist at Otago University's School of Medical Sciences, responded

Curly hair has to do with the chemical bonds in the protein that makes up hair - a-keratin. Keratins, particularly a-keratin, have long sequences of amino acids (often more than 300) which forms a helical structure.

Pairs of these helical structures then coil about each other in a left-hand coiled-coil structure. These are then attracted to another coiled coil so two keratin helices will stick together. These four-chain structures then associates with another four-chain structure to form the hair filament. One of the amino acids which make up these chains, is cysteine which has a sulphur group which is able to make connections with other sulphur groups on other coiled coils.

The more interactions a filament has with another filament, the more tightly coiled the coiled-coil becomes. Hence, curly hair has more interactions than straight hair. The process of `perming' hair introduces more accessibility of the sulphur on the amino acid, cysteine, to make these connections. Finger and toe nails have many many bonds between filaments and are thus, very hard.