Proteins are organic molecules which are responsible for most chemical
reactions performed in the cell, and thus, are essential for all cell functions.
A protein is a polypeptide - a macromolecule composed of building blocks called
amino acids attached end to end in a linear string. There are 20 amino acids,
with an average protein chaining about 200 of these. Proteins have a complex
structure, which can be thought of as having four logical levels. The amino
acid sequence of a protein's chain is called its primary structure. Different
regions of the sequence form local regular secondary structures, such
as 
-helices which are single stranded
helices of amino acids, and 
-sheets which
are planar patches woven from chain segments
that are almost linearly arranged. The tertiary
structure is formed by packing such structures into one or several domains.
The final, complete, protein may contain several protein chains arranged in
a quaternary structure.
The whole complex structure (primary to quaternary) is practically determined solely by the primary sequence of amino acids (and therefore, is defined by the genetic material itself). The three dimensional structure is generally the one with the minimal free energy.
