Denaturation of proteins is brought about by physical changes (like temperature) or chemical changes (like pH). During denaturation, the secondary and tertiary structures are destroyed, which involves the uncoiling of the helical structure, leading to the loss of the protein's biological activity. However, the primary structure remains intact. The statement 'A protein is formed from amino acids linked by peptide bonds' describes the fundamental primary structure of a protein, but it is not a statement detailing the process or effects of denaturation.