The correct answer is option 3. "secondary structure".
Explanation:
The secondary structure of polypeptide and proteins refers to the second order of folding, including common structures such as beta sheets, alpha helix, as well as turns and loops. The hydrogen bonds between the oxygen and nitrogen atoms of the peptide bonds are crucial to secondary structures, as these hydrogen bonds form and stabilize the aforementioned common structures.
They must occur in the DNA, which is in each cells nucleus. More specifically the nucleolus of the cell. A mutation occurs in an allele, where a base triplet (or a codon) gets transcribed incorrectly.
