This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Methionine and Cysteine details, Subject name: Biochemistry
Disulphide bond is seen between- 
Select and explain. 
A. Lysine and cysteine, B. Cysteine and cystine, C. Cysteine and cysteine, D. Arginine and histidine
C: Cysteine and cysteine
Answer: C: Cysteine and cysteine 
Explanation: But the question ask disulphide bond "is seen between" or in other words which two amino acids react to form disulphide bond. Two molecules of Cysteine join together to form Cystine, which has a disulfide bond (see fig). Cysteine cannot form disulfide bond with cystine. So disulphide bond is formed between two Cysteine molecules and not between Cysteine and Cystine