Chromosomes single strand model

Figure 22.19. Mechanistic models for the formation of chromosomal aberrations. Regions of sequence homology (e.g., repeats and pseudogenes) are shown as black boxes with the arrow indicating orientation. HR, homologous recombination NHEJ, nonhomologous end joining SSA, single-strand annealing.

Most trivalent metal cations would be anticipated to cause DNA collapse (aggregation) into condensed toroidal structures that are considered to be a model for DNA packaging in viruses and chromosomes. [Co(NH3)6] + induces this effect at five times lower concentration than spermidine, [+H3N-(CH2)4-NH3+-(CH2)3-NH3+], whcrcas divalent metal ions and putrescine, [+H3N-(CH2)4-NH3+], are incapable of causing DNA condensation in aqueous solution. 7302 polyvalent metal compounds, including Ap+ ions, also can relax supercoiled DNA, probably through single-strand DNA cleavage (see Section 7). 

Figure 4.7. Representations of some unusual higher order DNA structures triple helix (triplex), in which a third DNA strand hydrogen-bonds in the major groove of a double-stranded DNA a hairpin, in which a single strand of DNA hydrogen-bonds with itself to make a stem and nonpaired bases are pushed out to form a loop a tetraplex (quadruplex), in which (in this case) a single strand of DNA folds over on itself in an unusual G-G-G-G hydrogen bonding pattern, important in telomeric structure at the ends of chromosomes and a Holliday junction, in which four single strands of DNA base pair as usual but form this unusual complex structure, believed to be a good model for DNA recombination.

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