Circular double helix

Mitochondria are about the size of bacteria. They have a diameter of 0.2 to 0.5 gm and are 0.5 to 7 p.m long. They are bounded by two lipid bilayers, the inner one being highly folded. These folds are called cristae. The innermost space of the mitochondrion is called the matrix. They have their own DNA in the form of at least one copy of a circular double helix (Chap. 7), about 5 p.m in overall diameter it differs from nuclear DNA in its density and denaturation temperature by virtue of being richer in guanosine and cytosine (Chap. 7). The different density from nuclear DNA allows its separation by isopycnic centrifugation. Mitochondria also have their own type of ribosomes that differ from those in the cytoplasm but are similar to those of bacteria. 

Many bacteria and viruses contain circular double-helix DNA which is often coiled into a superhelix which can be right- or left-handed (this supercoUing presumably occurs only after removal of the histones) (Figure 11.37). 

The unmodified and complementary oligonucleotides were also synthesized, in order to detect thermodynamic and spectroscopic differences between the double helices. Circular dichroism spectra revealed that the covalently bound anthracene does not stack in the centre of the DNA double helix. Mutagenic activity by intercalative binding of the anthracene residue is thus unlikely. Only in vitro and in vivo replication experiments with site-specifically modified... 

Small-angle X-ray scattering (SAXS), circular dichroism (CD), and UV spectroscopy at different temperatures were used to investigate the nature of calf-thymus DNA in aqueous solution, in the presence of [Me Sn] " (n = 1-3) species. The results demonstrate that the [MeSn(IV)] moiety does not influence the structure and conformation of the DNA double helix, and does not degrade DNA, as indicated by agarose gel electrophoresis. Inter alia, the radii of gyration, Rg, of the cross section of native calf-thymus DNA, determined by SAXS in aqueous solution in the presence of [Me Sn] " (n = 1-3) species are constant and independent of the nature and concentration of the [Me Sn] species. 

Kuhn H., Demidov V., Frank-Kame-NETSKii M. D. An earring for the double helix Assembly of topological links comprising duplex DNA and a circular oligonucleotide. J. Biomol. Struct. Dyn. 2000 2 221-225. 

Mitochondrial DNA and the DNA of most prokaryotes are closed circular structures. These molecules may exist as relaxed circles or as supercoiled structures in which the heUx is twisted around itself in three-dimensional space. Supercoiling results from strain on the molecule caused by under- or overwinding the double helix ... 

Electron microscopy shows that DNA consists of either linear or circular structures. The chromosomal DNA in bacteria is a closed circle, a result of covalent joining of the two ends of the double helix (Figure 10.11). Note the presence of supercoils, branch points, intersections, and the generally thin and open structure. The chromosomal DNA in eukaryotic cells, like ours, is believed to be linear. 

One of the most exciting biological discoveries is the recognition of DNA as a double helix (Watson and Crick, 1953) of two antiparallel polynucleotide chains with the base pairings between A and T, and between G and C (Watson and Crick s DNA structure). Thus, the nucleotide sequence in one chain is complementary to, but not identical to, that in the other chain. The diameter of the double helix measured between phosphorus atoms is 2.0 nm. The pitch is 3.4 nm. There are 10 base pairs per turn. Thus the rise per base pair is 0.34 nm, and bases are stacked in the center of the helix. This form (B form), whose base pairs lie almost normal to the helix axis, is stable under high humidity and is thought to approximate the conformation of most DNA in cells. However, the base pairs in another form (A form) of DNA, which likely occurs in complex with histone, are inclined to the helix axis by about 20° with 11 base pairs per turn. While DNA molecules may exist as straight rods, the two ends bacterial DNA are often covalently joined to form circular DNA molecules, which are frequently supercoiled. 

The DNA double helix may be arranged in space, in a tertiary arrangement of the strands. The two strands of DNA wind around each other. In a covalently closed circular DNA, this means that the two strands can t be separated. Because the DNA strands can t be separated, the total number of turns in a given molecule of closed circular DNA is a constant, called the Linking Number, or Lk. The linking number of a DNA is an integer and has two components, the Twist (Tw), or number of helical turns of the DNA, and the Writhe (Wr), or the number of supercoiled turns in the DNA. Because L is a constant, the relationship can be shown by the equation ... 

Circular dichroism(CD) is the differential absorption of left- and right-handed circularly polarized light, due to molecules that have optical handedness or optical activity (this happens for most molecules of biological interest—for example, the a helix, [1 sheet, and random coil regions of proteins and the double helix of nucleic acids have recognizable CD spectral signatures). 

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