Q.What is meant by the primary structure of a protein? Why is it described as the most fundamental level of a protein's structure?

The primary structure of a protein is the precise sequence in which its amino acids are joined, one after another, by peptide bonds along the polypeptide chain, conventionally read starting from the free amino (N-terminus) end through to the free carboxyl (C-terminus) end.

It is described as the most fundamental level of protein structure because it is the base layer that every other level of structure is built upon: the specific pattern of attractions (and repulsions) created between amino acid side chains and backbone atoms, as fixed by that particular sequence, is exactly what determines how the chain subsequently folds into its higher-order shapes. Change the underlying amino-acid sequence, and the folded shape -- and often the biological function -- that results can change alo …

Define primary structure as sequence-only information, then explain 'fundamental' by pointing out that every subsequent leve …

Confusing primary structure with the protein's actual 3D shape -- primary structure is only the linear sequence information; the 3D folded shap …