DNA – The Genetic Material of Life

1. Structure of DNA

• Location: Primarily found within the nucleus of a cell, specifically within chromosomes. It also exists in certain organelles like mitochondria and chloroplasts.
• Chemical Composition: Composed of the primary elements: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), and Phosphorus (P).
• Structural Principles:
  • Polymeric: DNA is a polymer, made up of repeating units called monomers.
  • Monomers: The monomers of DNA are **nucleotides**, each consisting of:
    • A phosphate group (H3PO4)
    • A deoxyribose sugar (C5H10O4)
    • One of four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).
• Dimensions:
  • Length of each nucleotide: 3.4 Å (Angstroms).
  • Weight of each nucleotide: 300 daltons (Da).
• Bonding:
  • Between nucleotides within a strand: Phosphodiester bonds.
  • Between nucleotides on opposing strands: Hydrogen bonds.

2. Spatial Structure of DNA

• Model: DNA is structured as a double helix, with two polynucleotide chains wound around each other in opposite directions (5′-3′ and 3′-5′).
• Base Pairing Rules: The nitrogenous bases on the two strands pair according to the complementary base pairing rules:
  • Adenine (A) pairs with Thymine (T) via two hydrogen bonds.
  • Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.
• Diversity: Although there are only four nucleotides, the number, composition, and sequence of these bases along the DNA strand are highly diverse, resulting in the uniqueness of each organism’s genetic makeup.

3. Characteristic Parameters of DNA

• DNA Length: L = N/2 x 3.4 Å (where N is the total number of nucleotides).
• DNA Weight: M = N x 300 Da.
• Number of Turns (Cycles): C = N/20 or C = L/34.
• Number of Hydrogen Bonds: H = 2A + 3G.
• Number of Covalent Bonds between Nucleotides:
  • Within each strand: (N/2 – 1) x 2
  • In the entire DNA molecule (between sugar and acid): (N/2 – 1 + N/2) x 2
• Number of Each Type of Nucleotide: A + G = N/2
• Percentage of Each Type of Nucleotide: %A + %G = 50%

4. DNA Replication Mechanism

• Purpose: DNA replication produces two identical DNA daughter molecules from a single parental DNA molecule, ensuring the stable transmission of genetic information from one generation to the next.
• Steps:
  • Step 1: Unwinding of the DNA molecule to expose the two single strands.
    • The 3′ strand is synthesized continuously.
    • The 5′ strand is synthesized discontinuously, forming Okazaki fragments that are later joined by the enzyme ligase.
  • Step 2: Synthesis of new strands in the 5′-3′ direction, based on the complementary base pairing rules, with the assistance of specific enzymes.
  • Step 3: Two identical DNA daughter molecules are generated from one parent DNA molecule.

Conclusion

DNA is a vital genetic information carrier, playing a central role in life. Understanding its structure, function, and replication mechanism is foundational for gaining a deeper understanding of life and applying this knowledge in practical applications.