Which type of DNA is commonly found inside the cell?

Concept: DNA is polymorphic. The same Watson–Crick base-paired duplex can settle into several regular helical conformations, and which one it adopts is decided by hydration, the ionic environment and the base sequence rather than by the genetic message it carries. Fibre-diffraction studies distinguished the right-handed A, B and C families and the left-handed Z family.

Application: the interior of a living cell is an aqueous, moderately salty environment, so chromosomal DNA stays fully hydrated. Under that condition the duplex adopts the B conformation: right-handed, about 10 base pairs per turn, a rise of about 3.4 Å per base pair (pitch about 34 Å), a diameter of about 20 Å, base pairs nearly perpendicular to the helix axis, and a wide major groove beside a narrow minor groove. This is the Watson–Crick model of 1953, and it is the form that the great majority of cellular DNA takes.

Contrast: each of the other regular conformations needs a condition that the inside of a cell does not normally provide.

• A-DNA: right-handed, obtained when the duplex is dehydrated — fibres near 75% relative humidity, or high salt and alcohol; about 11 base pairs per turn, rise about 2.6 Å, base pairs tilted roughly 20° to the axis. Inside cells it appears only locally, for example in RNA–DNA hybrid regions.

• C-DNA: right-handed, reported from lithium-salt fibres near 66% relative humidity; about 9.3 base pairs per turn. It is a fibre form studied in the laboratory rather than a bulk conformation of cellular DNA.

• Z-DNA: left-handed with a zig-zag phosphate backbone, favoured by alternating purine–pyrimidine tracts such as CG repeats at high ionic strength or under negative supercoiling; about 12 base pairs per turn. Inside cells it occurs only as short, transient stretches.

Cross-check: dehydration, lithium-salt fibres and alternating purine–pyrimidine tracts at high salt are laboratory or purely local conditions, whereas full hydration at moderate ionic strength is the everyday state of DNA inside a cell. The type of DNA commonly found inside the cell is therefore B-DNA.