Microbiology Procedure

Index >> Deoxyribose Nucleic Acid - DNA >> Forms of DNA

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Deoxyribosenucleic Acid - DNA Nucleic Acids History of Nucleic Acids Genetic Material of Organisms Structure of DNA Nucleosides and Nucleotides Nucleosides of RNA and DNA Ribonucleosides and Ribonucleotides Deoxyribonucleosides and Deoxyribonucleotides Unusual Bases in DNA

Forms of DNA

	Forms of DNA - DNA can exist in the A, B, C and D forms. Sugar puckering is the most important characteristic for distinguishing the DNA forms. The A form has 3' -exo puckering, the Band C form 3' -exo puckering, and the C form 2'-endo puckering. The B form (B-DNA) is the structure proposed by Watson and Crick and is the native conformation of DNA in solution.
The B-form X-ray diffraction pattern is obtained at high humidity (>66%) and in the presence of excess salt. It consists of a right-handed antiparallel double helix of sugar-phosphate backbone, with purine-pyrimidine base pail roughly perpendicular to the axis of the helix. The tilt of a base pair to the normal of the helix is 6.3°. One turn of the helix consists of 10 base pairs (to-fold helix). The rise of the helix per base pair (h) is 3.37A. The A form (A-DNA) has 11 base pairs (11-f :>l d helix). The base pairs are considerably tilted from the axis of the helix (ỳ = 20.2°).
Because of this displacement the depth of the deep groove is increased and that of the shallow groove decreased. The axial rise (h) is only 2.S6A. The B-form is observed at lesser hydration and salt content than the A form. Reduction of hydration of the B form below 66%, with excess of salt still present, results in the C-form (C-DNA).The helix symmetry of the C form is 28/3 or 9*1/3 -i.e. there are fewer residues per turn than in the B form.
The axis of the helix is located in or near the minor groove, and there is a pronounced negative tilt of the base pair plane (1=7.8°). The helix rise per base pair is 3.32A. The D form poly (dA-dT) and poly (dG-dC) has base pairs per turn of the helix (S-fold symmetry). As in the C form the base pairs are displaced backwards relative to the axis of the helix ( = 16.7). The helix rise per base pair is 3.03A. The B form is the metabolically stable configuration. It undergoes transition to the A, C or D conformations at relative humidity lower than 92%, depending upon the nature of the counterion, the nucleotide sequence and the concentration of excess salt. D-A transition has been observed at low relative humidities.
	The A form is, however, metastable and rapidly returns to the D form. RNA always occurs in the A form only. RNA-DNA hybrids are also in the A form leading to the belief that DNA must undergo a B-A transition during transcription.

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Single Stranded DNA Localization of Nucleic Acids Forms of DNA Characteristics of Different Forms of DNA RL Helix and Z-DNA Palindromic DNA Repetitive DNA (Satellite DNA) Satellite DNA of Drosophila Virilis

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