Which type of protein structure involves parts being formed but not yet fully bonded?

The secondary structure of proteins refers to the local folded shapes that form within a protein due to hydrogen bonding between the backbone of the protein chain. In this stage, elements like alpha helices and beta sheets are established, which provide an initial level of organization. However, these structures are not yet fully bonded in the sense of having the complete three-dimensional shape or the full array of interactions that will be present in the mature protein.

While the primary structure denotes the sequence of amino acids in a protein, it does not describe any folded or organized regions. The tertiary structure involves the overall three-dimensional shape of a single polypeptide chain and includes interactions between the R groups of amino acids, leading to a more stable and defined conformation. Lastly, the quaternary structure refers to the arrangement of multiple polypeptides into a multi-subunit complex, which is a final structural arrangement.

Thus, the secondary structure is key in signaling the beginning of folding while components are still forming and getting organized through non-covalent interactions, making it the right choice in this context.