1 bp = 650 Da (avg) = 0.34 nm length
DNA is a polymer of nucleotides. Each base pair (bp) in dsDNA has an average molecular weight of ~649 Da (including the phosphate backbone and two bases). Single-stranded DNA averages ~330 Da per nucleotide. The distance between base pairs in the B-form helix is 0.34 nm, with 10 bp per helical turn (3.4 nm pitch). Human chromosomal DNA: 3.2 billion bp = ~2 meters long per cell. A typical plasmid: 3-10 kb. PCR products: 100-10,000 bp. Lambda phage: 48.5 kb. Knowing bp count allows calculation of: gel electrophoresis migration, DNA concentration from OD260 (1 OD260 = 50 µg/mL for dsDNA), and number of moles for cloning and ligation reactions.