25. Indicate which one of the following statements about nucleic acids and protein structures is correct.
    (1) Hydrogen bonding between the bases in the major and minor grooves of DNA is absent
    (2) Both uracil and thymine have a methyl group but at different positions.
    (3) The backbone dihedral angles of α-helices and ß- sheets are very similar. Only the hydrogen bonding pattern is different.
    (4) A ß-turn is formed by four amino acids. The type of ß-turn is determined by the dihedral angles of the second and third amino acid.

Understanding Nucleic Acid and Protein Structures: Key Concepts and Differences

Introduction

Nucleic acids (DNA and RNA) and proteins are fundamental macromolecules in biological systems. Their structures play a critical role in genetic information storage, transmission, and enzymatic functions. This article explores key structural concepts and identifies the correct statement from a multiple-choice question about nucleic acids and protein structures.

Analysis of the Given Statements

Statement 1: Hydrogen bonding between the bases in the major and minor grooves of DNA is absent.

• Incorrect. Hydrogen bonding occurs between base pairs (A-T and G-C) in both the major and minor grooves of DNA. Proteins, such as transcription factors, recognize specific DNA sequences by interacting with exposed hydrogen bond donors and acceptors in these grooves.

Statement 2: Both uracil and thymine have a methyl group but at different positions.

• Incorrect. Thymine (T) has a methyl group at the 5th carbon, whereas uracil (U) lacks a methyl group entirely. Uracil is found in RNA, while thymine is found in DNA. The presence of a methyl group in thymine provides stability to DNA by reducing spontaneous deamination.

Statement 3: The backbone dihedral angles of α-helices and β-sheets are very similar. Only the hydrogen bonding pattern is different.

• Incorrect. The backbone dihedral angles (ϕ, Ψ) of α-helices and β-sheets differ significantly:
  • α-Helices: ϕ ≈ -60°, Ψ ≈ -45°
  • β-Sheets: ϕ ≈ -120° to -140°, Ψ ≈ 120° to 135°
  • Thus, both hydrogen bonding patterns and dihedral angles differ between these secondary structures.

Statement 4: A β-turn is formed by four amino acids. The type of β-turn is determined by the dihedral angles of the second and third amino acids.

• Correct. β-turns are short secondary structure motifs composed of four amino acids. They allow a polypeptide chain to reverse direction and are stabilized by hydrogen bonds. The dihedral angles (ϕ, Ψ) of the second and third residues determine the specific type of β-turn (Type I, II, etc.).

Correct Answer: (4) A β-turn is formed by four amino acids. The type of β-turn is determined by the dihedral angles of the second and third amino acids.

Key Concepts in Nucleic Acid and Protein Structures

1. DNA Structure and Hydrogen Bonding

• DNA has a double-helical structure with hydrogen bonds between complementary bases.
• Major and minor grooves allow proteins to recognize specific sequences.

2. Uracil vs. Thymine

• Uracil (RNA): No methyl group, making RNA more prone to degradation.
• Thymine (DNA): Contains a methyl group at C5, providing extra stability to DNA.

3. Differences Between α-Helices and β-Sheets

• α-Helices are right-handed coils stabilized by intrachain hydrogen bonds.
• β-Sheets consist of strands connected by inter-strand hydrogen bonds.
• The backbone dihedral angles are different in these two structures.

4. Role of β-Turns in Protein Folding

• β-Turns allow tight turns in polypeptide chains, aiding in compact folding.
• The second and third residues’ dihedral angles determine the β-turn type.

Conclusion

Understanding nucleic acid and protein structures is essential for grasping molecular biology concepts. Among the given statements, the correct answer is Statement 4, as β-turns are composed of four amino acids, and their dihedral angles determine their type. This knowledge is crucial in protein structure analysis, drug design, and molecular modeling.