Why is Curly Hair, Curly?

Updated: Dec 16, 2020

We were all born with different hair textures and types, but have you ever wondered why your hair is curly, wavy or straight? What is the chemistry behind your hair?





The structure of hair

To understand why hair has a particular shape, we need to know what the hair is made up of and the differences of curly hair structure and straight hair structure.


Hair is made up of 95% keratin. Keratin is a strong helicoidal protein which protects the hair, and makes it insoluble. Being a protein, it is made up of several amino acids, most notably: cysteine.


There are three bonds which hold the hair structure together:

  • Hydrogen bonds

  • Salide/ionic bonds

  • Disulphide bonds

Hydrogen and ionic bonds can be easily broken through mechanical or gentle chemical action (water), however disulphide bonds are much stronger.


Cystine-linked hair curls

Cystine is responsible for these disulphide bonds.





The amino-acid's R group is CH2-SH. The SH group allows for the formation of disulphide bonds between strands of proteins.

When two cystine molecules are in close proximity, they will undergo an oxidation reaction to form disulphide bonds.





Where these disulphide bonds accumulate on the hair will dictate what shape your hair will take.

Straight hair has a fairly even distribution of disulphide bonds throughout it's structure, whereas curly hair has more disulphide bonds on one side, giving it a slight irregular shape - hence a curly forms.


Angle of growth curls

Another reason why you may have curly hair, is the way the follicle is positioned on your scalp, hence the angle and shape of the hair.





If your hair follicle is vertical to your scalp, your hair will grow out straight, with no compression, so the cross-section of the hair will remain circular - giving you beautiful pin-straight hair.


The higher the angle of the hair follicle, the wavier/curlier the hair will be.

If the hair leaves the scalp at a very high angle, it will have an oval cross-section - compared to a circular cross-section of straight hair.



Is it all down to your genes?

So, in conclusion - yes it's all down to genetics.

Your DNA contains the base sequences which influence the type of amino acid sequence your hair will have.

And the angle at which hair follicles grow hair is also dictated by your genome.

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