B-, A-, and Z-form DNA
The textbook double helix is real, but it is not the only way DNA can twist. Like a piece of rope that can be wound a little tighter, a little looser, or even braided the other way, DNA adopts several distinct shapes, or conformations, depending on its sequence and surroundings.
Three named forms matter. B-form DNA is the familiar one: a right-handed helix, roughly ten base pairs per turn, and the dominant shape under normal cellular, watery conditions. A-form DNA is also right-handed but shorter and fatter, with the base pairs tilted; it appears when DNA is dehydrated and is the usual shape of double-stranded RNA and RNA-DNA hybrids. Z-form DNA is the oddball — a left-handed helix that zig-zags down its backbone (hence 'Z'), favoured by certain alternating sequences like repeated G-C and by negative supercoiling. All three pair their bases by the same A-T, G-C rule; they differ in how the helix is wound.
These forms matter because they puncture the idea that DNA is a single rigid stick. The molecule is dynamic and bendable, flexing to wrap around proteins, opening locally to be read, and shifting conformation with conditions. Z-DNA in particular is not a mere curiosity: short stretches form transiently behind moving polymerases and may play roles in gene regulation, a reminder that the shape of DNA is part of how it is used.
When a cell builds a double-stranded stretch of RNA, or makes an RNA copy paired to its DNA template, that hybrid duplex naturally adopts the squat A-form rather than the slender B-form, because the extra 2' oxygen on ribose forbids the B-form geometry.
Same base-pairing rule, three different twists.
Z-DNA is left-handed, but this does not reverse the genetic code or break base pairing — A still pairs with T, G with C. The handedness describes the direction the backbone spirals, not the information it carries.