Most polypeptides undergo further modification, either during or after synthesis to assume their functional structures.

Observe the animation to the right.

This is a protein produced by the Insulin gene. What we want is only the green parts.

The yellow part is a section of the polypeptide which direct the destination of the molecule.

We cut the polypeptide, removing the yellow part off, creating pro insulin. At this stage the insulin molecule is inactive. The molecule remains in this form until needed.

When we cut out the rest and leave the green, we are left with an active insulin molecule.

Proteins can also be modified by adding carbohydrates to them, such as Glyco-proteins. These proteins around found throughout the cell membrane.

Their modification starts in the Endoplasmic reticulum and is continued in the Golgi apparatus.