Which structure indicates the specific sequence of amino acids in a polypeptide chain?

The primary structure of a protein refers to the specific sequence of amino acids that are linked together by peptide bonds to form a polypeptide chain. This sequence is unique for each protein and determines its overall structure and function. The primary structure is determined by the genetic code within DNA, which is transcribed into mRNA and subsequently translated into a sequence of amino acids.

This sequence is crucial because even a single change in the amino acid order can result in a protein that functions very differently, potentially leading to various biological effects. Understanding the primary structure is foundational, as it influences higher levels of protein structure, including secondary, tertiary, and quaternary structures, which are shaped by interactions between the amino acids in the chain.

The other choices represent different levels of protein structure: secondary structure pertains to local folding patterns such as alpha helices and beta sheets, tertiary structure refers to the overall three-dimensional shape of a single polypeptide, and quaternary structure involves the assembly of multiple polypeptide chains into a functional protein complex. However, none of these structures define the specific sequence of amino acids, which is what the primary structure does.