摘要:

AbstractThe widespread occurrence of genetic circularity suggests a selective advantage to map circularityper se.Circularity permits gene clustering relations not possible in linear maps; that is, every gene in a circular map can have two nearest map neighbors, two next nearest, etc. It seems possible that map circularity is a consequence of the selective forces responsible for the clustering of genes of related function. Certain features of the pattern of crossing‐over in various fungi suggest that circularity of linkage maps is there to be found. A concurrence of high frequencies of second‐division segregation and negative chromatid interference across the centromere, both of which phenomena have been reported, is a feature of a simple hypothetical crossover pattern that generates circular linkage m

ISSN:0021-9541    

DOI:10.1002/jcp.1040700403

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractDifferent species of viruses contain linear or circular DNA molecules. The circular molecules are either single‐chained or circular duplexes. The linear molecules from various species are always duplex. However, they may be either unique or circularly permuted collections of sequences. All species of linear duplexes that can be successfully tested can be shown to be terminally repetitious. The two temperate phages that have been studied (λ and P22) are unique and permuted collections, respectively. Shortly after infection both of these molecules form closed helical rings (superhelices). Certain virulent phages show no evidence of superhelix formation. How unique and permuted collections are produced at maturation is a puzzle. In this respect, it is of interest that P22 is a generalized transducing phage, whereas λ is a specialized transducing

ISSN:0021-9541    

DOI:10.1002/jcp.1040700404

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractExperiments will be described which demonstrate that the product of purified “Qβ‐replicase” is fully competent to serve both as a template and as a program for the synthesis of complete virus particles. The use of mutant RNA has permitted the demonstration that nucleic acid is the instructive agent in the replication process and hence satisfies the definition of a self‐duplicating entity. Methods have been devised to examine the product both physically and biologically with a minimum of manipulation and with complete recovery of product and input template. A detailed analysis of every interval of synthesis has thus become possible. These technical advances have permitted us to demonstrate the existence of a latent period prior to the appearance of the first new complete infectious RNA molecules. Further, this latent period is accompanied by an eclipse of the input templates as infectious agents. The use of electrophoretic separation on acrylamide gels has yielded a detailed account of both templates and early product during the latent period, with the consequent identification of the intermediate stages. The resulting data and their implications for the mechanism of RNA replication will be d

ISSN:0021-9541    

DOI:10.1002/jcp.1040700405

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractEver since the structure of DNA was made clear by Watson and Crick and their guess about the general mechanism of DNA replication shown to be correct by Meselson and Stahl, there has been great uncertainty about the mechanics of the process that expeditiously separates the two strands of the double helix during DNA replication. These uncertainties are most acute in the case of the bacterial chromosome, because it is apparently both the longest DNA molecule known (and therefore the one most subject to mechanical problems) and, in terms of nucleotides per second, the most rapidly replicated. Nevertheless, there are some facts that bear upon the mechanical aspects of DNA replication in bacteria (and elsewhere), and these are reviewed in this article.

ISSN:0021-9541    

DOI:10.1002/jcp.1040700406

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractIt seems likely that the transfer of genetic material observed upon mating of male strains ofEscherichia coliK12 is due to (or is at least concomitant with) a particular replication of the donor DNA. The detailed mechanism of genetic transfer is discussed in the light of different, recently published experimental results.Although these data do not lead to a clear and definite conclusion at the present time, some points are now clearly established:(1) The DNA molecules transferred and recovered after mating from the recipient cells are synthesized during (and not before) mating; one strand of these DNA molecules is a parental DNA strand which comes from the parental donor cell.(2) The replication of the DNA seems to be required for transfer; it is not yet clearly established whether this replication has to take place in the male cells, in the female cells, or in both. However, some preliminary results suggest that a replication is necessary in the male cells only, and no clear evidence has been given to refute such an hypothesis.(3) One of the most interesting aspects of this problem is the identification of the mechanism which controls the transfer replication. The Hfr chromosome results from the fusion of two replicons (bacterial chromosome and sex factor), and an attractive hypothesis is that the chromosomal replicon controls the vegetative replication of the whole, and the sex factor controls transfer replication. Although this model has not yet been firmly demonstrated, it is supported by some arguments.

ISSN:0021-9541    

DOI:10.1002/jcp.1040700407

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractBoth meiotic and mitotic replication of chromosomal and cytoplasmic DNAs in a unicellular green alga,Chlamydomonas reinhardi, have been studied by isotopic transfer experiments coupled with density‐gradient centrifugation analysis. It has been shown that during the vegetative cycle (1) the replication mode of chloroplast DNA, as well as γ DNA, is semiconservative; (2) in synchronized culture these two cytoplasmic DNA satellites replicate coordinately with a high degree of synchrony; (3) the replication of chromosomal DNA is independent of and separable from that of the two cytoplasmic DNAs. It has been shown that during the sexual cycle (1) the chloroplast DNA replicates semiconservatively during zygote maturation, at which time there is neither chromosomal DNA replication nor nuclear division; (2) another DNA component (M‐band DNA) replicates extensively and appears in large quantity; (3) meiosis occurs during zygote germination and is accompanied by one round of semiconservative chromosomal DNA replication; (4) the M‐band DNA disappears in the early germination period, and the degraded substance is not incorporated into the newly replicated chromosomal DNA. A possible origin of the M‐band DNA and a few properties of the cytoplasmic DNA satellites have been elucidated. Genetic recombination and regulation of incoordinate replication in an eucaryotic system have been discussed in terms of the data

ISSN:0021-9541    

DOI:10.1002/jcp.1040700408

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractWell‐separated prophage markers in anE. coliλ/F′ (λ) partial diploid recombine recipro

ISSN:0021-9541    

DOI:10.1002/jcp.1040700409

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractThe discovery of the phenomenon of nonhomologous pairing in the female ofDrosophila melanogasterprovided a genetic tool for analyzing chromosome behavior, which has made it possible to establish a temporal order of meiotic events that includes two types of pairing, one preceding exchange (exchange pairing) and one following exchange (distributive pairing). Studies with free X‐duplications support the assumption that the initial pairing for exchange involves parasynapsis rather than short, effectively paired regions. Recognition at exchange pairing is correlated with the extent of euchromatic homology in the duplication up to a physical length between 11 μ and 25 μ of the salivary gland chromosome, at which point full recognition (equivalent to the same length of homology carried on a normal X chromosome) is achieved. Recognition at distributive pairing is correlated with total size of the chromosome, is independent of homology, and is restricted to chromosomes which have not undergone exchange with an independent homologue. Temperature‐treatment and labeling studies indicate that there is a rough coincidence between the temperature‐sensitive period for recombination and that of DNA replication. Temperature‐induced recombinants have been shown to be meiotic, not oogonial, in origin; and marked sensitivity to temperature likewise appears to reside in the early oocyte rather than in the oogonia. The temperature responses of interstitial and proximal chromosomal regions reveal a chronological and a quantitative difference, with the later and most pronounced response in the region spanning the centromere. If temperature acts directly, exchange pairing is initiated at the interphase stage; whereas the excellent correlation between segregation behavior and mitotic metaphase length of noncrossover chromosomes suggests that distributive pairing occurs between greatly condensed chromosomes, late in

ISSN:0021-9541    

DOI:10.1002/jcp.1040700410

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractSeveral multifactor crosses have been performed in which all segregating genotypes were identified. The genetic map of T4 and mapping techniques are re‐evaluated in light of the finding that double‐mutant recombinants occur less frequently than those identified as wild‐type. This difference is attributable to heterozygotes which exaggerate the latter class. Analysis of the crosses suggests that the mathematical mapping functions so far devised overcorrect for clustering of multiple crossovers, and in so doing predict more crossovers than actually occur. An 8‐factorrIIcross in which crossover estimates were based exclusively on double‐mutant recombinants shows that the high negative interference (HNI) observed in mass lysates is not an artifact of the method used to enumerate recombinants. Experiments to investigate the cause of HNI include parallel 8‐factorrIIcrosses between alternating point mutations and deletions. One cross was performed in the presence of inhibitory concentrations of fluorouracil deoxyriboside, the other in its absence. Based on double‐mutant recombination frequencies, neither case showed a difference in the coefficient of coincidence for insertion of a point mutant between two deletions and for insertion of a deletion between two point mutants. Either insertion heteroduplexes are not involved in formation of double crossovers, or deletion heteroduplexes (not observed among progeny phages) are efficiently repaired. Analysis of single‐burst progenies of cells infected with a partial phage genome and a helper phage shows a 6.8‐fold increase in recombination frequency near the ends of the partial phage genome. It is suggested that the HNI observed in mass lysates is a population genetics artifact accounted for by the increased frequency of recombination near ends

ISSN:0021-9541    

DOI:10.1002/jcp.1040700411

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY

摘要:

AbstractRecombination‐deficient mutants ofEscherichia colihave been isolated as members of a class showing reduced ability to mother recombinants when crossed with a given Hfr strain. All those showing less than one‐tenth the ability of their immediate ancestors in mothering merodiploids have been eliminated from consideration, as have those strains of reduced fertility with only one donor strain. The remainder are all more sensitive to ultraviolet light (UV) than were their immediate ancestors; differences in levels of sensitivity are evident. Mutants have been isolated from two distantly related ancestors, and the sets of mutants from each are divisible into three phenotypic groups. The strains in the first group are the least UV‐sensitive (UVs), and lambda lysogens of these show normal spontaneous induction. The strains in the second and third groups are more UVsthan those in Group 1, and lysogens of these show abnormally low spontaneous induction. Groups 2 and 3 are separable by their levels of fertility with the Hfr strain (KL16) found to transfer therec+allele of some of the strains early in conjugation. The strains more fertile with KL16 are in Group 2, and those less fertile are in Group 3. Two mutants (JC1553 and AB2463) of different parentage but both in phenotypic Group 2 have been analyzed by transduction; and a single mutation probably determines both their mutant UVsand Rec−phenotypes. When the mutants of different parentage are arranged together, a continuous ranking of fertilities is observed. Preliminary complementation tests show no complementation between those mutations carried by strains in Group 2 and Group 3 which have been tested. Complementation has been observed between those mutations carried by strains in Group 1 and those carried by strains in Groups

ISSN:0021-9541    

DOI:10.1002/jcp.1040700412

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1967

数据来源: WILEY