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DNA structure Knowledge-Based Study

Biology · Molecular Genetics · Biology · DNA Structure & Replication

Biology topic explanations on DNA structure.

Study notes

  1. Components of a Nucleotide

    Deoxyribonucleic acid (DNA) is a polymer composed of repeating monomer units known as nucleotides. Each nucleotide consists of three covalently bonded components:

    • A five-carbon sugar: In DNA, this sugar is deoxyribose, which lacks an oxygen atom at the 2' carbon position compared to ribose.
    • A phosphate group: Attached to the 5' carbon of the deoxyribose sugar, conferring a negative electrical charge to the molecule.
    • A nitrogenous base: Attached to the 1' carbon of the deoxyribose sugar via a glycosidic bond.

    Nitrogenous bases fall into two structural categories:

    • Purines: Double-ring structures comprising Adenine (A) and Guanine (G).
    • Pyrimidines: Single-ring structures comprising Cytosine (C) and Thymine (T).

    Phosphodiester Linkages and Directionality

    Individual nucleotides join to form a continuous strand through condensation reactions. A covalent bond, called a phosphodiester bond, forms between the phosphate group attached to the 5' carbon of one nucleotide and the hydroxyl (-OH) group on the 3' carbon of the adjacent nucleotide.

    This alternating arrangement of deoxyribose sugars and phosphate groups forms the sugar-phosphate backbone of DNA. The nitrogenous bases project inward away from this backbone.

    Because one end of the strand terminates in an unbound 5' phosphate group and the other end terminates in an unbound 3' hydroxyl group, every single DNA strand possesses intrinsic directionality (5' to 3').

    Antiparallel Strands and Complementary Base Pairing

    Naturally occurring cellular DNA exists primarily as a right-handed double helix consisting of two polynucleotide strands. The architectural features of this double helix include:

    • Antiparallel orientation: The two strands run in opposite directions; one strand runs from 5' to 3', while the paired complementary strand runs from 3' to 5'.
    • Complementary base pairing: Bases on opposing strands form specific non-covalent hydrogen bonds across the central axis:
      • Adenine pairs exclusively with Thymine via two hydrogen bonds (A = T).
      • Guanine pairs exclusively with Cytosine via three hydrogen bonds (G ≡ C).
    • Chargaff's rules: Due to complementary base pairing, the total amount of adenine equals thymine, and the total amount of guanine equals cytosine (A = T and G = C). Consequently, the purine-to-pyrimidine ratio is 1:1.

    Forces Stabilizing the Double Helix

    The structural integrity of DNA is maintained by two primary intermolecular forces:

    • Hydrogen bonding: The cumulative effect of millions of hydrogen bonds between paired bases holds the two strands securely together while remaining easily breakable during replication and transcription.
    • Base stacking interactions: Hydrophobic interactions and van der Waals attractions between adjacent, stacked planar base pairs stabilize the core of the helix, shielding nonpolar rings from the aqueous environment.

    The negatively charged hydrophilic sugar-phosphate backbones face outward toward the surrounding aqueous solution, minimizing thermodynamic strain and facilitating interactions with nuclear proteins such as histones.