Refer to the Following Paragraph and Figure 3

Multiple Choice

Refer to the following paragraph and Figure 3.1 to answer the following questions.

Figure 3.1
Since structure correlates so well with function,biochemists are constantly looking for new ways to probe the complex structure of proteins in order to understand what they do and how they do it.One of the most powerful techniques in existence today is X-ray crystallography.The main difficulty with this technique is getting the protein to crystallize.Once crystallized,the protein is bombarded with X-rays to create a pattern that can be analyzed mathematically to determine the three-dimensional structure of the protein.This analysis has been performed by Krzysztof Palczewski on the protein rhodopsin,which is a light-sensitive protein found in species ranging from ancient bacteria (archaea) to humans.The structure (schematically shown above,where each letter represents an amino acid) is characterized by a single polypeptide chain with several α-helical segments that loop back and forth across the cell membrane.Another notable feature is the disulfide bond (-S-S-) that can be seen at the bottom of the third transmembrane segment.[Figure adapted from K.Palczewski et al. ,Science 289 (2000) : 739.]
-Refer to Figure 3.1.Which level of structure is being maintained by the disulfide bond?

A) primary
B) secondary
C) tertiary
D) quaternary