The sequence of amino acids, i.e., the positional information in a protein - which is the first amino acid, which is second, and so on - is called the primary structure of a protein. A protein is imagined as a line, the left end represented by the first amino acid, and the right end represented by the last amino acid. The first amino acid is also called as N-terminal amino acid. The last amino acid is called the C-terminal amino acid. A protein thread does not exist throughout as an extended rigid rod. The thread is folded in the form of a helix (similar to a revolving staircase).