page_title: DNA
section_title: Supercoiling
hierarchical_section_title: DNA / Properties / Supercoiling
caption_attribution_description: Deutsch: Von links nach rechts: Strukturmodelle der A-, B- und Z-DNA mit jeweils 12 Basenpaaren. English: From left to right, the structures of A-, B- and Z-DNA. The structure a DNA molecule depends on its environment. In aqueous enviromnents, including the majority of DNA in a cell, B-DNA is the most common structure. The A-DNA structure is dominates in dehydrated samples and is similar to the double-stranded RNA and DNA/RNA hybrids. Z-DNA is a rarer structure found in DNA bound to certain proteins.
context_page_description: Deoxyribonucleic acid is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses. DNA and ribonucleic acid are nucleic acids. Alongside proteins, lipids and complex carbohydrates, nucleic acids are one of the four major types of macromolecules that are essential for all known forms of life.
  The two DNA strands are known as polynucleotides as they are composed of simpler monomeric units called nucleotides. Each nucleotide is composed of one of four nitrogen-containing nucleobases, a sugar called deoxyribose, and a phosphate group. The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. The nitrogenous bases of the two separate polynucleotide strands are bound together, according to base pairing rules, with hydrogen bonds to make double-stranded DNA. The complementary nitrogenous bases are divided into two groups, pyrimidines and purines.
context_section_description: DNA can be twisted like a rope in a process called DNA supercoiling. With DNA in its "relaxed" state, a strand usually circles the axis of the double helix once every 10.4 base pairs, but if the DNA is twisted the strands become more tightly or more loosely wound. If the DNA is twisted in the direction of the helix, this is positive supercoiling, and the bases are held more tightly together. If they are twisted in the opposite direction, this is negative supercoiling, and the bases come apart more easily. In nature, most DNA has slight negative supercoiling that is introduced by enzymes called topoisomerases. These enzymes are also needed to relieve the twisting stresses introduced into DNA strands during processes such as transcription and DNA replication.
caption: From left to right, the structures of A, B and Z DNA
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