In the primary structure of protein

Options

A. Left end represents `rarr` 1 st amino acid (C- terminal amino acid)
B. Right end represents `rarr` Last amino acid (N -terminal amino acid)
C. Left end represents `rarr` 1 st amino acid (N- terminal amino acid)
D. Right end represents `rarr` Last amino acid (C- terminal amino acid)

Answer

### Step-by-Step Solution:

1. **Understanding Primary Structure**: The primary structure of a protein refers to the linear sequence of amino acids that make up the protein. This sequence is crucial as it determines the protein's overall structure and function.

2. **Amino Acids and Peptide Bonds**: Proteins are composed of 20 different amino acids, each with a unique side chain. The amino acids are linked together by peptide bonds, which are formed during protein biosynthesis.

3. **N-terminal and C-terminal**: In a polypeptide chain, the left end of the chain is known as the N-terminal, which represents the first amino acid in the sequence. Conversely, the right end of the chain is known as the C-terminal, which represents the last amino acid.

4. **Analyzing Options**:
- Option 1 states that the left end represents the first amino acid, which is correct (N-terminal).
- Option 2 incorrectly states that the right end represents the last amino acid as the N-terminal, which is wrong.
- Option 3 correctly states that the left end represents the first amino acid (N-terminal) and the right end represents the last amino acid (C-terminal).
- Option 4 incorrectly states that the left end represents the last amino acid, which is wrong.

5. **Conclusion**: Based on the analysis, the correct answer is that the left end represents the first amino acid (N-terminal) and the right end represents the last amino acid (C-terminal).