DNA double helix (Watson-Crick model, 1953): Two antiparallel polynucleotide strands wound around a common axis. Sugar-phosphate backbone on outside; bases point inward. Right-handed helix with major and minor grooves.

Base pairing: Adenine = Thymine (2 hydrogen bonds); Guanine triple-bond Cytosine (3 hydrogen bonds). This complementary base pairing is the basis of DNA replication and transcription.

Chargaff's rules (empirical, explained by Watson-Crick model):

1. %A = %T and %G = %C (in any dsDNA)
2. A + G = T + C (purines = pyrimidines)
3. $\frac{A+T}{(G+C)}$ ratio is species-specific but constant for a species

Key calculations: If %A = x, then %T = x, %G = %C = (50 - x). Total purines = total pyrimidines = 50% always.

Exception: These rules apply ONLY to double-stranded DNA. In single-stranded DNA or RNA, %A need not equal %T (or %U).

Nucleoside = base + sugar (adenosine, guanosine, cytidine, thymidine). Nucleotide = nucleoside + phosphate (AMP, GMP, CMP, TMP). DNA backbone: 3'→5' phosphodiester bonds between adjacent nucleotides.