correctly complete the following statement: The side chains ('-R groups') of amino acids in a beta-sheet: A. alternate pointing "up" and "down" for each successive amino acid in the primary sequence. B. point in one direction in a parallel beta sheet and the opposite direction in an antiparallel beta-sheet. C. always point toward the direction of water. D. point in random directions. E. all point in opposite directions

A.  

Explanation:

The β-sheet is a twisted pattern in which the protein strands are laterally linked through hydrogen bonds. These β-sheet motifs are segments of 3 to 10 amino acids that are specially configured to form beta antiparallel strands. In β-sheet regions, the hydrogen bonds are localized among carbonyl and amino groups of the polypeptide backbone, and side chains (i.e., the R groups) are extended up and down in the plane of the β-sheet.