Proteins

Proteins are organic molecules which are resposible to most checmical 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 containing about 200 amino acids. 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 $\alpha$-helices and $\beta$-sheets. 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.

  

Figure: protein structure

The whole complex structure (primary to quaternary) is determined solely by the primary sequence of amino acids (and therefore, is defined by the genetic material itself).