Could somebody explain the different levels of protein structure?

1. Primary: refers to the unique sequence of amino acids in the protein. All proteins have a special sequence of amino acids, this sequence is derived from the cell's DNA.

2. Secondary: the coiling or bending of the polypeptide into sheets is referred to the proteins secondary structure. alpha helix or a beta pleated sheet are the basic forms of this level. They can exist separately or jointly in a protein.

3. Tertiary: The folding back of a molecule upon itself and held together by disulfide bridges and hydrogen bonds. This adds to the proteins stability.

4. Quaternary: Complex structure formed by the interaction of two or more polypeptide chains.

<a rel="nofollow" href="https://http://highered.mcgraw-hill.com/sites/dl/free/0072943696/291136/protein_denaturation_final.swf" target="_blank">https://http://highered.mcgraw-hill.com/sites/dl/free/0072943696/291136/protein_denaturation_final.swf</a>

I found a video on youtube that helped me quite a lot:
Protein Structure and Function - Part 1 (kristinagremski)