Biochemistry

1. Sanger's Reagent Reaction

Sanger's method is a classic technique used for the determination of the N-terminal amino acid of a peptide or protein.

Reagent: Sanger's reagent (DNFB) reacts specifically with the free amino group ( ) of the N-terminal amino acid residue.

Mechanism: It forms a stable yellow derivative called a Dinitrophenyl (DNP)-peptide.

The general reaction is:

$ 6M \text{HCl} 100^\circ \text{C} \text{N} \text{R} \text{-CH}_3 \alpha \text{-CH}_3$ side chain is Alanine (Ala).

The product shown is thus Dinitrophenyl-Alanine (DNP-Ala).

4. Selecting the Correct Tetrapeptide

Since the reaction product is DNP-Alanine, the original tetrapeptide must have Alanine (Ala) as its N-terminal residue. We look for a tetrapeptide starting with Ala:

(A) Ala-Gly-Leu-Phe

(B) Asp-Phe-Leu-Pro

(C) Asp-Gly-Tyr-Phe

(D) Ala-Phe-Tyr-Pro

Both options (A) and (D) begin with Alanine (Ala).

Therefore, the tetrapeptide(s) that give this product are (A) Ala-Gly-Leu-Phe and (D) Ala-Phe-Tyr-Pro.

The correct answer is (A) and (D).

Step 1: Understanding the guanine-cytosine base pair.
Guanine and cytosine pair through three hydrogen bonds in the DNA double helix. The base pairing follows specific hydrogen bond rules: guanine forms hydrogen bonds with cytosine through two NH groups and one oxygen atom.

Step 2: Analyzing the number of hydrogen bonds.
Each hydrogen bond is formed as follows:

- One hydrogen bond between the NH group of guanine and the carbonyl oxygen of cytosine.

- Another hydrogen bond between the amino group of guanine and the N-H group of cytosine.

- The third hydrogen bond is formed between the N-H group of guanine and the N atom of cytosine.

Step 3: Conclusion.
The guanine-cytosine base pair contains 3 hydrogen bonds.