Supercoils in human DNA.

The three-dimensional structure of a double-stranded DNA molecule may be described by distinguishing the helical turns of the DNA duplex from any superhelical turns that might be superimposed upon the duplex turns. There are characteristic changes in the hydrodynamic properties of superhelical DNA molecules when they interact with intercalating agents. The hydrodynamic properties of nuclear structures released by gently lysing human cells are changed by intercalating agents in this characteristic manner. The characteristic changes are abolished by irradiating the cells with gamma-rays but may be restored by incubating the cells at 37 degrees C after irradiation. These results are interpreted as showing that human DNA is supercoiled. A model for the structure of the chromosome is suggested

Mapping sequences in loops of nuclear DNA by their progressive detachment from the nuclear cage.

Nuclear DNA is organised into loops, probably by attachment to a supramolecular structure. We describe a method which enables us to map the position of sequences within a loop relative to the point of attachment. Nuclear DNA is isolated unbroken by lysing HeLa cells in 2M NaCl to release structures which retain many of the morphological features of nuclei. Their DNA is supercoiled and so must remain unbroken and looped during lysis. Nucleoids are digested to various degrees with a restriction endonuclease and the cages - and any associated DNA - sedimented free from unattached DNA. The cage-associated DNA is purified and completely fragmented using the same restriction endonuclease. Equal weights of fragmented DNA are separated by gel electrophoresis, transferred to a filter and the relative amounts of the alpha, beta and gamma globin genes on the filter determined by hybridisation to the appropriate probes. The alpha genes, unlike the beta and gamma genes, resist detachment from the cage and so must lie close to the point of attachment to the cage. Our ability to map these genes implies that sequences cannot be attached at random to the cage; rather, specific sequences must be attached, so looping the DNA

Review of the distribution, causes for the decline and recommendations for management of the quokka, Setonix brachyurus (Macropodidae: Marsupialia), an endemic macropod marsupial from south-west Western Australia

The former and current distribution of the quokka, Setortix brachyurus, was mapped from published and all available unpublished records. At the time of European settlement the quokka was widespread and abundant and its distribution encompassed an area of approximatelyThe former and current distribution of the quokka, Setortix brachyurus, was mapped from published and all available unpublished records. At the time of European settlement the quokka was widespread and abundant and its distribution encompassed an area of approximately 41 200 km2 of south-west Western Australia inclusive of two offshore islands, Bald Island and Rottnest Island. Historical reports indicated an extensive population decline occurred in the 1930s. The decline continued, with a previously undocumented decline apparent in the period from 1980 to 1992. However, this decline may be an artefact of the time scales used for mapping and may well equate with a previously reported decline lor a suite of south -west mammals in the 1970s. By 1992 the quokka´s distribution had been reduced to an area of approximately 17800 km2. An increased awareness of the presence of the quokka on the mainland has resulted in numerous reportings of quokka presence since 1992, has confimled the existence of several populations at the northern extent of the quokka´´s known geographic range and indicated the cmrent, 2005, distribution to be similar to that in 1992. However, survey and population estimates at six of these mainland locations from the northem jarrah forest indicated low abundance. There have been no population estimates elsewhere on the mainland. Two populations have been reported tiom the Swan Coastal Plain, but neither has been confirmed extant. Predation by the introduced fox, Vulpes vulpes, is implicated as a major cause of the quokka´s initial decline, while ongoing predation, habitat destruction and modification through altered tire regimes have contributed to the continued decline. Specific conservation management actions are recommended, namely: (i) Implementing an active adaptive management program in the northern jarrah forest to determine quokka population response to habitat manipulation through the use of fIre, fox baiting and pig control; (ii) Surveying the Stirling fumge and Green Range populations with emphasis placed on determining population size and population genetic structure; (iii) Surveying the reported occurrences from the Swan Coastal Plain, with emphasis on unambiguously determining presence. If confirmed, priority should he directed to assessing population size and determining the management requirements to ensure persistence of the population; (iv) Surveying southem forest and south coast populations to assess quokka population size, the extent of movement between sllbpopulations and assessment of the range of habitat types used by quokkas. The latter should be combined with spatial analyses of known extant populations and suitable and potentially suitable habitat; (v) Determining the role of tire in establishing and maintaining preferred habitat of southern forest and south coast populations; and (vi) Establishing a program to assess the potential effects from management operations