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The occurrence of chaos in basic Lotka-Volterra models of four competing species is studied. A brute-force numerical search conditioned on the largest Lyapunov exponent (LE) indicates that chaos occurs in a narrow region of parameter space but is robust to perturbations. The dynamics of the attractor for a maximally chaotic case are studied using symbolic dynamics, and the question of self-organized critical behaviour (scale-invariance) of the solution is considered.

DNA with base pair mismatches was prepared by annealing mixtures of genetically marked DNA from bacteriophage lambda. This heteroduplex DNA was used to transfect bacteria under conditions minimizing recombination. Genetic analysis of the progeny phages indicates that: (i) Mismatch repair occurs, usually giving rise to a DNA molecule with one chain with the genotype arising from repair and one parental chain. (ii) The frequency of repair of a given mismatch to wild type depends on the marker, ranging from 3 to 20%. (iii) Excision tracts may extend several hundred nucleotides but are usually shorter than about 2000 nucleotides. (iv) In Rec-mediated bacteriophage crosses, recombination of markers closer than about 10-3 nucleotide pairs frequently occurs by mismatch repair within heteroduplex DNA. (V) The average amount of heteroduplex DNA formed in a Rec-mediated recombination event is a few thousand nucleotide pairs.