Nucleic Acids
Nucleic acids (DNA and RNA) are polymers of nucleotides, each consisting of a pentose sugar, nitrogenous base, and phosphate group. They store and transmit genetic information.
Components
- Purines: Adenine (A) and Guanine (G) — double-ring structure
- Pyrimidines: Cytosine (C), Thymine (T — in DNA only), Uracil (U — in RNA only) — single ring
- DNA sugar: 2'-Deoxyribose
- RNA sugar: Ribose
DNA Structure
Watson-Crick double helix (B-form, right-handed): two antiparallel strands held by hydrogen bonds between complementary bases (A=T: 2 H-bonds; G≡C: 3 H-bonds). The strands are antiparallel (3'→5' and 5'→3'). The backbone is sugar-phosphate. Chargaff's rules: [A]=[T], [G]=[C].
Types of RNA
- mRNA (messenger): Carries genetic code from DNA to ribosomes for translation. Contains 5' cap and poly-A tail in eukaryotes.
- tRNA (transfer): Adaptor molecule; carries amino acids; cloverleaf structure; anticodon pairs with mRNA codon.
- rRNA (ribosomal): Most abundant RNA; structural and catalytic component of ribosomes (ribozyme activity).
- hnRNA: Heterogeneous nuclear RNA — primary transcript in eukaryotes; processed to mRNA by splicing.
- snRNA, miRNA, siRNA: Regulatory functions.
Nucleotides as Signaling Molecules
ATP: Universal energy currency. cAMP: Second messenger (from adenylyl cyclase). NAD+, FAD, CoA: Coenzyme forms of nucleotides. GTP: Required for protein synthesis elongation and tubulin polymerization.
DNA Organization
In eukaryotes, DNA is organized into chromatin. Nucleosome = 147 bp of DNA wrapped around histone octamer (H2A, H2B, H3, H4 ×2). Linker histone H1 connects nucleosomes. Heterochromatin = condensed, transcriptionally inactive. Euchromatin = loosely packed, active.
Clinical
- Xeroderma pigmentosum: Defective nucleotide excision repair → UV-induced DNA damage → skin cancer
- HGPRT deficiency (Lesch-Nyhan syndrome): Purine salvage pathway impaired → hyperuricemia, self-mutilation