Is DNA Left-Handed?
No, the most common form of DNA is right-handed. While left-handed DNA exists, it's not the prevalent form found in living organisms.
DNA, or deoxyribonucleic acid, is a molecule that carries the genetic instructions for all living organisms. Its structure is a double helix, resembling a twisted ladder. The "handedness" refers to the direction of the twist. Imagine holding your right hand with your thumb pointing upwards; if the helix twists in the same direction as your fingers curl, it's right-handed. Left-handed DNA would twist in the opposite direction.
The Prevalence of Right-Handed DNA
The standard form of DNA found in most living organisms is the B-DNA, a right-handed double helix. This is the form most commonly depicted and studied. References such as the Promega Connections article "Left-Handed DNA: Is That Right?" confirm this fact, stating that while both left-handed and right-handed double helices exist, the former is less common. The article further explains that the chirality (handedness) of DNA arises from the chiral nature of its building blocks, nucleotides.
Left-Handed DNA: Z-DNA
Left-handed DNA, specifically known as Z-DNA, does exist and has been structurally characterized (as noted in articles like the one from PNAS on the "Structure of a left-handed DNA G-quadruplex" and Wikipedia's entry on Z-DNA). However, it's less stable and represents a less common conformation. Research suggests it might play a role in certain biological processes, including cancer and autoimmune diseases (as highlighted in the article from The Scientist, "Left-Handed DNA Has a Biological Role Within a Dynamic Genetic Code"). The discovery of Z-DNA's structure was a significant milestone, as shown in the article from Nature, "Molecular structure of a left-handed double helical DNA fragment at 2.0-Å resolution". Further research, such as the cryo neutron crystallography study analyzing water structure around Z-DNA (DOI: 10.1093/nar/gkab264) also contributes to our understanding of this less prevalent form.
In summary, while left-handed DNA exists and has been studied extensively, the predominant form in all living organisms is the right-handed B-DNA. The reason for this preference is an area of ongoing research, with hypotheses involving cosmic rays influencing the initial bias (as discussed in Quanta Magazine's article, "Cosmic Rays May Explain Life's Bias for Right-Handed DNA").
