The primary structure of a protein molecule has

Options

A. two ends
B. one end
C. three ends
D. no ends.

Explanation

To answer the question about the primary structure of a protein molecule, we can break down the information step by step:

### Step-by-Step Solution:

1. **Understanding Protein Structure**:
- Proteins have four levels of structure: primary, secondary, tertiary, and quaternary. Each level has distinct characteristics and functions.

2. **Definition of Primary Structure**:
- The primary structure of a protein refers to the linear sequence of amino acids that are linked together by peptide bonds. This sequence determines the protein's unique characteristics.

3. **Composition of Proteins**:
- Proteins are made up of 20 standard amino acids. The specific order of these amino acids in the polypeptide chain is crucial for the protein's function.

4. **Formation of Polypeptide Chains**:
- When amino acids are joined together, they form a long chain known as a polypeptide. The arrangement of these amino acids is what constitutes the primary structure.

5. **Characteristics of Primary Structure**:
- The primary structure is stable and maintains its integrity. It has two distinct ends: the amino (N) end and the carboxyl (C) end.

6. **Notation of Ends**:
- The N-terminus (amino end) is denoted as the "N" end, while the C-terminus (carboxyl end) is denoted as the "C" end. This notation helps in understanding the directionality of the polypeptide chain.

7. **Conclusion**:
- Therefore, the primary structure of a protein molecule has two ends, which are the amino end and the carboxyl end.

### Final Answer:
The primary structure of a protein molecule has **two ends**: the amino end (N-terminus) and the carboxyl end (C-terminus).

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