This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure and function of protein, Subject name: Biochemistry
In leucine zipper model, Leucine residue is seen after every 
Select and explain. 
A. 3 amino acids, B. 6 amino acids, C. 7 amino acids, D. 12 amino acids
C: 7 amino acids
Answer: C: 7 amino acids 
Explanation: *An a helix in which there is a periodic repeat of leucine residues at every seventh position. *This occurs for eight helical turns and four leucine repeats. * Similar structures have been found in a number of other proteins associated with the regulation of transcription in mammalian and yeast cells. * It is thought that this structure allows two identical monomers or heterodimers (eg, Fos-Jun or Jun-Jun) to "zip together" in a coiled-coil and form a tight dimeric complex *This protein-protein interaction may serve to enhance the association of the separate DNA binding domains with their target <img class="fr-dib" style="width: 300px;" src=" /> Ref : Biochemistry by U. Satyanarayana 3rd edition Pgno : 574