Watson and Crick’s model drew on X-ray diffraction evidence, including Franklin’s and Wilkins’s work, and Chargaff’s observations. Two sugar–phosphate chains face opposite directions. A–T pairs have two hydrogen bonds and G–C three; purine–pyrimidine pairing supports an approximately uniform width.

The textbook B-DNA model has about10 base pairs per3.4nm turn, with 0.34nm spacing along the axis. Base stacking also contributes to stability. Complementarity makes copying possible: each parental strand can specify a new partner. DNA→RNA→polypeptide describes transcription and translation; RNA→DNA is reverse transcription.

A promoter and termination signals define a transcription unit with its transcribed region. RNA polymerase reads one template strand for that transcript and makes complementary RNA using U rather than T. RNA matches the coding strand when both are reported5′→3′, apart from U/T.

Bacterial RNA polymerase works with initiation factors such as sigma; termination has several mechanisms, including rho-dependent termination. In eukaryotes, nuclear polymeraseII produces an mRNA precursor, I major rRNAs, and III tRNA/5S rRNA among other products. Pre-mRNA processing includes splicing,5′ capping and3′ polyadenylation in the illustrated pathway; mature mRNA can then exit the nucleus.

VNTR loci contain tandem repeat units whose copy number can vary between alleles. Shared flanking sites with different repeat counts can yield fragments of different lengths. Such variation can be inherited and need not alter a protein.

Historical DNA fingerprinting involved isolation, restriction digestion, gel separation, Southern transfer, labelled-probe hybridisation and detection. Smaller DNA fragments generally travel farther through a gel toward the positive electrode. Modern methods often amplify STR loci. Multiple loci and statistical context are needed to interpret consistency.