DNA structure and organization

DNA Components - The Genetic Bricks

• Core Components: The fundamental units of DNA.

  • Deoxyribose Sugar: A pentose sugar lacking a 2'-OH group, providing stability.
  • Phosphate Group: Creates a phosphodiester backbone; confers a negative charge.
  • Nitrogenous Base: Encodes the genetic sequence.

• Base Types:

  • Purines (2 rings): Adenine (A), Guanine (G).
    • 📌 Mnemonic: PUre As Gold.
  • Pyrimidines (1 ring): Cytosine (C), Thymine (T).
    • 📌 Mnemonic: CUT the PYe.

• Assembly:

  • Nucleoside: Base + Sugar
  • Nucleotide: Base + Sugar + Phosphate (mono-, di-, or tri-)

⭐ Thymine (in DNA) is essentially methylated Uracil (in RNA). This small change significantly increases DNA's genetic stability.

Double Helix - The Twisted Ladder

• A right-handed helix composed of two antiparallel polynucleotide chains.
  • One strand runs 5' → 3'; the opposing strand runs 3' → 5'.
  • The hydrophilic sugar-phosphate backbone faces the aqueous exterior.
  • Hydrophobic nitrogenous bases are stacked inside.
• Complementary Base Pairing (Chargaff's Rules):
  • Adenine (A) forms 2 hydrogen bonds with Thymine (T).
  • Guanine (G) forms 3 hydrogen bonds with Cytosine (C) (stronger bond).
  • 📌 Mnemonic: Pure As Gold (Purines); Cut The Py (Pyrimidines).
• The helical structure creates two distinct grooves:
  • Major Groove: Wider, allows proteins to access bases.
  • Minor Groove: Narrower.

⭐ B-DNA is the most common physiological form, a right-handed helix with approximately 10.5 base pairs per turn.

DNA Packaging - Chromatin & Coils

• Nucleosome: The fundamental unit of chromatin. Consists of a core histone octamer [2x(H2A, H2B, H3, H4)] wrapped by approximately 147 base pairs of DNA.
  • Histones are rich in positively-charged amino acids (Lysine, Arginine), which bind to the negatively-charged DNA phosphate backbone.
• H1 Histone: A linker histone that binds the nucleosome and the "linker DNA" segment, helping to compact nucleosomes into a more condensed fiber.
• Chromatin Types:
  • Euchromatin: Less condensed, appears lighter on EM. Transcriptionally active and accessible. 💡 Histone Acetylation makes DNA Active.
  • Heterochromatin: Highly condensed, appears darker on EM. Transcriptionally inactive. 💡 Histone Methylation makes DNA Mute.

⭐ Barr bodies, which are inactive X chromosomes in females (Lyonization), are classic examples of facultative heterochromatin.

High‑Yield Points - ⚡ Biggest Takeaways

• DNA is a right-handed double helix with antiparallel strands linked by phosphodiester bonds.
• Purines (A, G) pair with pyrimidines (C, T). G-C pairs have 3 H-bonds (stronger) versus A-T pairs with 2 H-bonds.
• DNA wraps around histone octamers to form nucleosomes, the fundamental unit of chromatin.
• Euchromatin is transcriptionally active and accessible, while heterochromatin is highly condensed and inactive.
• Telomeres protect chromosome ends; centromeres are crucial for segregation during cell division.