Proteins from different sources differ from one another primarily because of what feature of their polypeptide chains?

Introduction / Context:
This question tests understanding of what gives each protein its unique identity. Proteins are made from only about twenty standard amino acids, yet there are thousands of different proteins in living organisms, each with its own structure and function. The key reason for this diversity lies in the order in which amino acids are arranged along the polypeptide chain, known as the primary structure. Recognising this concept is central to biochemistry and molecular biology.

Given Data / Assumptions:

• Proteins are long chains of amino acids connected by peptide bonds.
• The options mention peptide bonds, sugar sequences, nucleotides, and amino acid sequences.
• Proteins are not made directly from sugars or nucleotides; these belong to carbohydrates and nucleic acids respectively.
• Peptide bonds are of the same chemical type in most proteins.

Concept / Approach:
The primary structure of a protein is the specific sequence of amino acids from the amino terminus to the carboxyl terminus. Even small changes in this sequence can alter the way a protein folds into its secondary and tertiary structure and thus change its activity. The chemical nature of a peptide bond is essentially the same in all proteins. Proteins do not consist of long chains of sugars or nucleotides; such chains form polysaccharides and nucleic acids. Therefore, the main feature that makes one protein different from another is the particular order of amino acids in its polypeptide chain.

Step-by-Step Solution:
Step 1: Identify the basic building blocks of proteins. Proteins are polymers of amino acids joined by peptide bonds. Step 2: Consider the nature of peptide bonds. The peptide bond is a standard amide linkage and does not change its chemistry from one protein to another. Step 3: Examine the role of sequences. Different proteins have different orders of amino acids, which determine their shape and function. Step 4: Compare with the options. Only the option mentioning amino acid sequence correctly describes what varies between proteins.

Verification / Alternative check:
Biology texts describe how the sequence of amino acids is coded by the sequence of nucleotides in DNA. Mutations change the DNA sequence, which can change the amino acid sequence and thereby change the protein. Experimental techniques such as peptide sequencing and mass spectrometry focus on determining the exact order of amino acids to distinguish proteins. These examples confirm that amino acid sequence is the key distinguishing feature of each protein molecule.

Why Other Options Are Wrong:
Option A: Peptide bonds are chemically similar in all proteins; they do not differ in type from protein to protein. Option B: Proteins are not defined by sequences of sugar molecules; that description fits polysaccharides. Option C: Nucleotides are the building blocks of DNA and RNA, not of proteins, so this statement does not apply.

Common Pitfalls:
Students sometimes confuse different classes of biological macromolecules and may think that all biopolymers are built from nucleotides or sugar units. Another pitfall is to focus on peptide bonds because they are often mentioned during protein synthesis, but the bond type is the same in all proteins. Remember the simple idea that a protein is defined by its amino acid sequence, just as a word is defined by the order of letters.

Final Answer:
Proteins differ from one another mainly because the sequence of amino acids in the polypeptide chain differs from protein to protein.