⚡ Core Concept: Stereochemistry of Nucleic Acids & Chromatin Compaction
Deoxyribonucleic acid (DNA) is a right-handed antiparallel double helix stabilized by Watson-Crick hydrogen bonding and base-stacking van der Waals interactions. In eukaryotes, approximately 2 meters of linear genomic DNA is compacted by a factor of 10,000 into chromatin through hierarchical association with histone octamers.
1. Structural Polymorphism: B-DNA, A-DNA, and Z-DNA
DNA adopts distinct structural conformations governed by hydration state, salt concentration, and base composition:
| Biophysical Parameter | B-DNA (Physiological Standard) | A-DNA (Dehydrated / RNA-DNA Duplexes) | Z-DNA (Alternating Purine-Pyrimidine) |
|---|---|---|---|
| Helical Sense | Right-handed | Right-handed | Left-handed (Zigzag backbone) |
| Base Pairs per Turn | 10.5 bp | 11.0 bp | 12.0 bp (6 repeating dimers) |
| Helical Pitch | 3.4 nm (0.34 nm / bp) | 2.8 nm (0.26 nm / bp) | 4.5 nm (0.37 nm / bp) |
| Sugar Pucker | C2′-endo | C3′-endo | C2′-endo (Pyr) / C3′-endo (Pur) |
| Glycosidic Conformation | anti for all bases | anti for all bases | anti for Pyrimidines; syn for Purines |
Figure 1: Hierarchical Chromatin Architecture from Double Helix to Metaphase Chromosome
Chromatin Biology
Figure 1: The four tiers of eukaryotic genome compaction: 2 nm naked DNA → 11 nm nucleosome beads-on-a-string → 30 nm solenoid/zigzag fiber → condensin-mediated metaphase chromosome scaffolding.
2. Histone Octamer Architecture & Epigenetic Post-Translational Modifications
The core nucleosome particle encloses 147 bp of DNA wrapped 1.67 left-handed superhelical turns around a cylindrical histone octamer composed of two H2A-H2B heterodimers and one (H3-H4)2 heterotetramer. Histone amino-terminal tails protrude outward and undergo dynamic epigenetic marks:
- Acetylation (HATs): Neutralizes positive lysine charges (e.g., H3K9ac, H3K27ac), relaxing chromatin into transcriptionally accessible euchromatin. Reversed by Histone Deacetylases (HDACs).
- Methylation (HMTs): Context-dependent; H3K4me3 marks active promoters, whereas H3K9me3 and H3K27me3 recruit HP1 and Polycomb repressive complexes to establish transcriptionally silent heterochromatin.
RNA Chemical Divergence
RNA contains ribose with a 2′-hydroxyl (-OH) group instead of 2′-deoxyribose and uracil in place of thymine. The 2′-OH makes RNA intrinsically susceptible to alkali-catalyzed transesterification cleavage and forces ribonucleotide duplexes into the compact A-form geometry with deep major grooves.
