C-DNA, also known as C-form DNA, is one of many possible double helical conformations of

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

. DNA can be induced to take this form in particular conditions such as relatively low

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...

and the presence of certain

ions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

, such as Li⁺ or Mg²⁺, but C-form DNA is not very stable and does not occur naturally in living organisms. In 1961, it was found by Marvin, when he tried to repeat for the Li salt the higher water content pattern of the Na salt. What Marvin found is the semicrystalline C-DNA. "Semicrystalline" describes a diffraction pattern for which crystalline reflexions are seen at low resolution but continuous transform at higher resolution.

Structure

The C-DNA is a non-integral helix of slightly variable dimensions, with mean values of 3.32Å for the unit rise and 38.60° for the unit twist, giving about 9 1/3 rather that 10 unites per turn. There are some different models for C-DNA proposed over years. In 2000, van Dam and Levitt found that both C-DNA and B-DNA consist of two distinct nucleotide conformations, B-I and B-II. The ratio of B-II conformation in C-DNA is more than 40%, but in B-DNA the ratio is only about 10%. The B to C form transition in fibrous DNA may be interpreted in terms of B-I and B-II conformational changes. Figure in that paper shows the ideal modal of these two conformations published by them.

Counterion 160px, cation-exchange_resin.html" ;"title="Polystyrene sulfonate, a cation-exchange resin">Polystyrene sulfonate, a cation-exchange resin, is typically supplied with as the counterion. In chemistry, a counterion (sometimes written as "counter ...

s such as primary amides under basic conditions have been used in experiments to show the relationship between B and C forms of DNA. The overall shape and orientation of DNA is heavily dependent on its

primary sequence Biomolecular structure is the intricate folded, three-dimensional shape that is formed by a molecule of protein, DNA, or RNA, and that is important to its function. The structure of these molecules may be considered at any of several length sca ...

as well as

hydrogen bonding In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

between its base pairs, which stabilizes and maintains the double helix conformation. C-DNA was shown to hold its conformation in the absence of water and was able to form upon dehydration. Some amides under basic conditions and low humidity were shown to hold the C-form conformation, but smoothly transitioned to B-form DNA as the humidity was increased. This may suggest a strong correlation between C-form and B-form DNA, which was also seen using lithium salt at low humidity.

References

{{Nucleic acids DNA

Structure

See also

References