Nucleic Acids: Structure of DNA and RNA

Nucleotide components:

1. Nitrogenous base (purine or pyrimidine)
2. Pentose sugar (deoxyribose in DNA; ribose in RNA)
3. Phosphate group(s)

Nucleoside = base + sugar (no phosphate) Nucleotide = base + sugar + phosphate

Bases:

Purines (double ring — fused pyrimidine + imidazole): Adenine, Guanine (both in DNA and RNA)

Pyrimidines (single ring): Cytosine (both), Thymine (DNA only), Uracil (RNA only)

Memory: "CUT the PY-ramid" → Cytosine, Uracil, Thymine are pyrimidines

Ribose vs Deoxyribose:

Watson-Crick base pairing:

A --- T (DNA): 2 hydrogen bonds G === C (DNA): 3 hydrogen bonds A --- U (RNA): 2 hydrogen bonds

Chargaff's Rules (double-stranded DNA only): $[A] = [T], \quad [G] = [C], \quad [A]+[G] = [T]+[C] \text{ (purines = pyrimidines)}$

Melting temperature relationship:

$T_m \propto \% \text{G-C content}$

Higher G-C → more H-bonds → higher Tm (more energy needed to separate strands)

$T_m \approx 69.3 + 0.41 \times (\% \text{G+C}) \text{ (°C, for long DNA in 0.1M NaCl)}$