Curly vs. straight: what gives?

April 24, 2006 2:02 PM

What is the cause of curly hair, and further, why does the curliness of hair differ? Proteins? If so, is it genetic? How do "hair-straightening" products work?

Forgive the triviality of the post.

posted by Frankieist to science & nature (6 comments total)

Lyn Dowsett, a biochemist at Otago University responds :-

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 sulpher group which is able to make connections with other sulpher 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 sulpher on the amino acid, cysteine, to make these connections. Finger and toe nails have many many bonds between filaments and are thus, very hard.
posted by eatdonuts at 2:11 PM on April 24, 2006

Hair is curly through straight (on a bit of a spectrum) based on how flat or round the hair shaft is. Flat hair, like curling ribbon, is curly. It is genetic. Genetics of hair.

Hair-straightening works in various ways, in some cases to 'puff up' the hair to make it rounder and straighter. Down by the chemicals on this page are descriptions of what some of the various chemicals do specifically to have effects on the hair.

Chemo can also cause people's hair to grow in different; I presume it's because the genetics are somewhat co-dominant and when you kill off the fast growing (cancer/hair) cells, you get slightly different expressions the next time they grow in.
posted by cobaltnine at 2:13 PM on April 24, 2006

eatdonuts is right about the cysteine-cysteine disulfide bonds. A permanent wave essentially breaks the bonds with a reducing agent, reshapes the hair, then re-sets the bonds with an oxidizing agent.

Here is a site with crude diagrams that says what I just said.
posted by jenovus at 2:27 PM on April 24, 2006

I recall from high school that the texture of hair was determined by the shape of the hair folicle, round for straight hair, oval for curly hair and somewhere in-between for wavy hair.
posted by sgobbare at 2:33 PM on April 24, 2006

Tag-on question: what's up with my hair? It starts out fairly curly (i.e., when a new hair first sprouts from its follicle it's curly), then as it continues to grow it straightens out to a gentle wave. You can see the difference along a single strand of hair. It's not just the weight of the hair straightening it out, since if you hold a loose strand upside down, hanging from the tip instead of the root, the difference in curliness persists. What's going on here? apart from making my life one long bad hair day, that is ...
posted by Quietgal at 5:18 PM on April 24, 2006

I have the opposite problem, Queitgal, which has its own problems. I wish everyone (women mainly) would stop "doing" their hair so we could all have one big sigh of relief and just have bad hair days together. :)
posted by evening at 5:33 PM on April 24, 2006