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81.
The simple repeat poly(dT-dG).poly(dC-dA) common to eukaryotes is absent from eubacteria and archaebacteria and rare in protozoans 总被引:5,自引:0,他引:5
Genomic DNA from a wide variety of prokaryotic and eukaryotic organisms has
been assayed for the simple repeat sequence poly(dT-dG).poly(dC-dA) by
Southern blotting and DNA slot blot hybridizations. Consistent with
findings of others, we have found the simple alternating sequence to be
present in multiple copies in all organisms in the animal kingdom (e.g.,
mammals, reptiles, amphibians, fish, crustaceans, insects, jellyfish,
nematodes). The TG element was also found in lower eukaryotes
(Saccharomyces cerevisiae, Neurospora crassa, and Dictyostelium discoideum)
and at a much lower frequency in protozoans (Oxytricha fallux and
Tetrahymena thermophila). The sequence was also repeated in high copy
number in a higher plant (Zea mays) as well as at very high levels in a
unicellular green alga (Chlamydomonas reinhardi). Although the copy number
of the repeat per haploid genome was generally proportional to genome size,
there was a greater-than-1,000-fold variation in the number of
(TG)25/100-kb genomic DNA. By contrast, no eu-or archaebacterium--including
Myxococcus xanthus, whose life cycle is very similar to that of the slime
mold Dictyostelium discoideum, and Halobacter volcanii, whose genome
contains other repeated sequences-- was found whose genomic DNA contained
this sequence in detectable amounts. A computer search also failed to find
the TG element in human mitochondrial DNA.
相似文献
82.
Protein evolution in different cellular environments: cytochrome b in sharks and mammals 总被引:4,自引:0,他引:4
DNA sequences for the mitochondrial cytochrome b gene were determined for
13 species of sharks. Rates and patterns of amino acid replacement are
compared for sharks and mammals. Absolute rates of cytochrome b evolution
are six times slower in sharks than in mammals. Bivariate plots of the
number of nonsynonymous and silent transversions are indistinguishable in
the two groups, however, suggesting that the differences in amino acid
replacement rates are due primarily to differences in DNA substitution
rates. Patterns of amino acid replacement are also similar in the two
groups. Conserved and variable regions occur in the same parts of the
cytochrome b gene, and there is little evidence that the types of amino
acid changes are significantly different between the groups. Similarity in
the relative rates and patterns of protein change between the two groups
prevails despite dramatic differences in the cellular environments of
sharks and mammals. Poor penetrance of physiological differences through to
rates of protein evolution provides support for the neutral theory and
suggests that, for cytochrome b, patterns of evolution have been relatively
constant throughout much of vertebrate history.
相似文献
83.
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86.
The past decade has produced considerable debate over the feasibility of paleodemographic research, with much attention focusing on the question of reliability of age estimates. We show here that in cases where age is estimated rather than known, the traditional method of assigning individuals to age classes will produce biased estimates of age structure. We demonstrate the effect of this bias both mathematically and by computer simulation, and show how a more appropriate method from the fisheries literature (the "iterated age length key") can be used to estimate age structure. Because it is often the case that ages are also estimated for extant groups, we suggest that our results are relevant to the general field of anthropological demography, and that it is time for us to improve the statistical basis for age structure estimation. We further suggest that the oft noted paucity of older individuals in skeletal collections is a simple result of the use of inappropriate methods of age estimation, and that this problem can be rectified in the future by using maximum likelihood estimates of life table or hazard functions incorporating the uncertainty of age estimates. 相似文献
87.
Sarah Frankenberg Christy Hoong Alexander Knyshov Christiane Weirauch 《Organisms Diversity & Evolution》2018,18(2):211-224
Evolutionary biologists have long been intrigued by exaggerated morphologies tied to sexual or natural selection. In insects, relatively few studies have investigated the evolution of such traits at the genus level and above and have used comparative phylogenetic methods to do so. We here investigate the interspecific evolution of head length in the minute litter bug genus Nannocoris Reuter based on the first phylogenetic hypothesis of the group (25 ingroup species, five gene regions, 3409 bp) and ancestral state reconstruction. Head lengths in this speciose genus range from approximately one sixth of the total body length to more than a quarter of the body length, while the head and mouthpart (rostrum) lengths are correlated. Different species therefore possess a markedly different reach of the rostrum when extended. The analyses show that head length evolution in Nannocoris is plastic, with head length elongations and reductions occurring in several clades, derived from ancestors with moderately elongated heads. Evidence is provided that exaggerated head lengths evolved through elongation of either the genal (pricei group) or the tip (arimensis group) region of the head. The biology of species in the genus Nannocoris is unknown, but given the lack of sexual dimorphism of head lengths, we speculate that head evolution in this genus may be driven by natural selection, potentially in the context of prey capture. 相似文献
88.
89.
Yields of DNA double-strand breaks (dsb), i. e. the average number of dsb, N–, per relative molar mass, M
r
, and dose, D, produced by electrons and photons in the energy range 50 eV – 1 MeV were calculated. The experimental data of dsb induction
by ultrasoft x-rays and by photons agree well with the calculated yields of dsb as a function of photon energy. The dsb are
classified into simple and complex ones. Energy transfers of less than about 200 eV producing at least two ionizations generate
mainly simple dsb, while low-energy electrons with an initial energy between 200 and 500 eV induce preferentially complex
dsb. Assuming that dsb is the main DNA lesion leading to exchange-type chromosome aberrations (etca), three different mechanisms
have to be considered: 1) complex dsb on its own; 2) interaction between two dsb induced by the same primary particle; and
3) interaction between two dsb induced by different primary particles. Mechanisms 1) and 2) produce a linear term, whereas
mechanism 3) leads to a quadratic term for the yield of etca. The sum of contributions 1) and 2) to the yield of dicentrics
describes fairly well the non-trivial structure of the experimental data. The results suggest that interaction between complex
dsb does not contribute significantly to the formation of dicentrics via mechanism 3).
Received: 30 July 1995 / Accepted in revised form: 28 March 1996 相似文献
90.
Esophageal cancer involves multiple genetic alternations. A systematic codon usage bias analysis was completed to investigate the bias among the esophageal cancer responsive genes. GC-rich genes were low (average effective number of codon value was 49.28). CAG and GTA are over-represented and under-represented codons, respectively. Correspondence analysis, neutrality plot, and parity rule 2 plot analysis confirmed the dominance over mutation pressure in modulating the codon usage pattern of genes linked with esophageal cancer. 相似文献