What is denaturation and what causes denaturation in proteins?

Denaturation is something normal while you give such a stressful situation to or DNA that their quaternary, tertiary and secondary structures is lost.
As we said before, denaturation is an event that normally occurs in stressful situations such as heat , mixing with strong acid or base (acetic acid or sodium bicarbonate), radiation (gamma rays), organic salts and organic solvents (alcohol).
Firstly, I would like to highlight that DNA and proteins are the unique molecules that can be denaturalized . This is true due to their interactions between aminoacids (in proteins) or nucleotides (in DNA). We can find a whole range of interactions:
Disulfide bridges
Hydrogen bonds
Van der Waals Forces
Hydrophobic interactions
In order to understand how denaturation occurs, we briefly must learn how proteins are folded:
In case of proteins, these interactions gives a linear sequence of AA ( Primary Structure ) a reorganization in space in order to form alpha or beta helix ( Secondary Structure ). Furthermore, these alpha and beta helixes can be folded in a three-dimensional structure to form the Tertiary Structure and, amazingly, these three-dimensional structure of alpha helixes and beta helixes folded can interact between them and form Quaternary Structures .
Secondly, we are going to see how denaturation works:
In quaternary structure denaturation, protein sub-units are dissociated and/or the spatial arrangement of protein subunits is disrupted (normally due to lost hydrophobic interactions).
Tertiary structure denaturation involves the disruption of:
Disulfide bridges between cystein groups
Hydrogen bonds from polar amino acid side-chains (and the surrounding solvent)
Van der Waals interactions between nonpolar amino acid side-chains.
In secondary structure denaturation, proteins lose all regular repeating patterns such as alpha-helices and beta-pleated sheets, and adopt a random coil configuration.
Primary structure , such as the sequence of amino acids held together by covalent peptide bonds, is not disrupted by denaturation .
Last but not least, depending on the stressful interaction, the denaturation can be reversible or irreversible.
It is said when a denaturalized protein mixed with acid, you quit the acid and, spontaneously, the protein return to its native form. This process is called renaturation. However, this event usually doesn’t occurs.
References:
http://2012books.lardbucket.org/books/an-introduction-to-nutrition/s10-02-the-role-of-proteins-in-foods-.html
https://en.wikipedia.org/wiki/Denaturation_(biochemistry)
http://garfield.library.upenn.edu/classics1980/A1980JC93500001.pdf
http://www.neatorama.com/2008/02/26/herv-this-the-man-who-can-unboil-an-egg/!EW0FX (a funny article, just for laughs)