The organization of alphoid repeated sequences on human nucleolus-organizing (NOR) chromosomes 13, 21, and 22 has been investigated. Analysis of hybridization of alphoid DNA probes to Southern transfers of restriction enzyme-digested DNA fragments from hybrid cells containing single human chromosomes shows that chromosomes 13 and 21 share one subfamily of alphoid repeats, whereas a different subfamily may be held in common by chromosomes 13 and 22. The sequences of cloned 680-base-pair EcoRI fragments of the alphoid DNA from chromosomes 13 and 21 show that the basic unit of this subfamily is indistinguishable on each chromosome. The sequence of cloned 1020-base-pair Xba I fragments from chromosome 22 is related to, but distinguishable from, that of the 680-base-pair EcoRI alphoid subfamily of chromosomes 13 and 21. These results suggest that, at some point after they originated and were homogenized, different subfamilies of alphoid sequences must have exchanged between chromosomes 13 and 21 and separately between chromosomes 13 and 22.One of the models for genomic change that mediates the evolution of new species or generates major changes within a species involves periodic reorganizations of the genome accompanied by amplification of different families of repetitive DNA. The alphoid family of repetitive DNA is found exclusively in primates and has been studied in human and several monkey and ape species. It is believed that different families of this repeat arose prior to the emergence of several of these species, after which the alphoid families have remained relatively unchanged (1). Although the separation of the branches leading to the great apes and humans took place 6-8 million years ago (2), the most significant human evolution has probably taken place within the last few million years. One might therefore expect to find within the human genome families of the alphoid repeat that have been amplified relatively recently, and evidence for one such family has already been reported (3). Recent studies (4)(5)(6) indicate that human alphoid DNA is organized into chromosome-specific subfamilies, formed by the amplification of segments composed of tandemly arranged related copies of the 170-basepair (bp) (monomeric) or 340-bp (dimeric) repeat units.Chromosome specificity of subfamilies of alphoid DNA implies that transfer of sequences between nonhomologous human chromosomes occurs very rarely. However, one group of chromosomes, the nucleolus organizing (NOR) chromosomes, appears to undergo recombination between nonhomologues more frequently than do other chromosomes (7,8), and thus some alphoid families might be expected to be held in common between the NOR chromosomes. We report here studies on the alphoid repetitive DNA in three different human NOR-bearing chromosomes-13, 21, and 22. These chromosomes hold in common a related subfamily of alphoid sequences that has diverged =25% from the average sequence of alphoid repeats. The sequences of tetramers of the basic unit of this alphoid family are indisti...

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From observations on the partial disintegration of isolated human metaphase chromosomes we propose that human metaphase chromatids have a rather simple organization based on the folding and coiling of a long, regular, hollow cylindrical structure with a diameter of about 4000 A. This cylindrical structure, the unit fiber, is postulated to be a super-solenoid formed by the coiling of a 300 A solenoid, itself composed by coiling the basic string of nucleosomes. The structure of a human chromatid would thus be a hierarchy of helices, the contraction ratio of each coil, in ascending order of size, being approximately 7,6,40, and 5. This model appears to explain the estimated mass/unit length and accounts for many of the known features of human mitotic chromatids.It has been known for a long time that chromosomes consist of both DNA and protein, but the general nature of the association of the DNA with histones resulting in the formation of the basic string of nucleosomes has only recently been elucidated (see, for example, refs. 1 and 2). This packing of the DNA in the primary chromatin fiber results in a condensation in length of the original DNA duplex by a factor of about 7. A further packing of the DNA in interphase chromatin has very recently been proposed.

SUMMARYThe base composition of purified DNA from 18 species of the genus Candida was determined from the denaturation temperature, Tm. A great genetic heterogeneity was found with a mean molar guanine +cytosine (% GC) composition ranging from 35-1-5706 %.Candida albicans, C. tropicalis, C. clausenii and C. stellatoidea showed essentially identical base composition 35-1-35.7 % GC. The highest values (54-1-57-6 % GC) were noted for C. catendata, C. brumptiiand C. zeylunoides.These three species also showed a compositional distribution of their DNA distinctly higher than the other species examined. The taxonomic and phylogenetic affinity between C. catenulata, C. brumptii and C. zeylanoides and the C. albicans group must on the basis of these investigations be seriously reconsidered. The rest of the species examined showed a wide range of mean % GC between the two clusters already mentioned. A variable degree of genetic affinity between these species and the C. albicans group cannot be excluded on the basis of the present examinations. Knowledge of DNA base compositions would appear to be of definite significance in the taxonomy of yeasts.

The rate of introduction of neutral mutations is lower in man than in other primates, including the chimpanzee. This species is generally regarded as our closest relative among the great apes. We present here an analysis of sequences of X chromosomal alphoid repetitive DNA from man and the great apes, which supports the closer relationship between man and chimpanzee and indicates a considerably increased rate of recombination in the human repeat DNA. These results indicate that the ‘molecular clock’ is running more quickly in man.