The main types of organization of genetic material in eukaryotes

Molecular biological mechanisms of speciation

Growing recognition that much of the evolutionary history of eukaryotic genomes reflects the operation of turnover processes involving repetitive DNA sequences has led to the recent formulation of models describing speciation as a consequence of such turnover. These models are of three general kinds: those attributing hybrid infertility to the process of transposition, those attributing hybrid infertility to mispairing between chromosomes of divergent repetitive DNA composition, and those assuming that change in repetitive DNA's can reset coordinated gene regulation. These models are discussed with respect to the kinds of evidence needed for their corroboration and to their significance for questions related to macroevolutionary punctuated equilibria and genetic revolutions.

EBV-based plasmid DNA rearrangements after transfection of eukaryotic cells

The cDNA encoding influenza virus (A/Udorn/307/72 strain) M2 protein was subcloned into the EBV-based vector pREP9. Three continuous kidney cellular lines of different origin were transfected with recombinant plasmid pREP9-M2. One and 5 months after transfection plasmid DNA rearrangements were detected by means of restriction analysis of recovered plasmids and their hybridization with an influenza-virus-specific radioactive probe. Deletions were the most frequent type of pREP9-M2 mutations. PCR with primers corresponding to cellular genome and plasmid DNA followed by Southern blot analysis with the [(32)P]-labeled M2-fragment allowed host DNA rearrangements to be revealed in transfected cells.

DNA sequences homologous to long double-stranded RNA. Transcription of intracisternal A-particle genes and major long repeat of the mouse genome

Long double-stranded RNA (dsRNA-A) (Kramerov, D.A., Ryskov, A.P. and Georgiev, G.P. (1977) Biochim. Biophys. Acta 475, 461-475) from Ehrlich ascites carcinoma cells was used for the search for mobile dispersed genes in the mouse genome. Two kinds of genomic sequences hybridizing to dsRNA-A were cloned. They were designated A1 and A2. The A1 sequence was identified as the gene for intracisternal A particles, while the A2 sequence was found to be the major long repetitive sequence of the mouse genome. Melted dsRNA-A hybridized equally well to both DNA strands of A1 and A2 sequences while total poly(A)+ RNA bound preferentially to one of them. Thus, a partially symmetrical transcription took place in the case of A1 and A2 elements. The analysis of transcripts of A1 elements in Ehrlich carcinoma cells revealed RNAs with sizes of 9.5 kb, 6.8 kb and 6.0 kb. In plasmocytoma MOPC 21 cells, instead of the 6.0 kb RNA, two other kinds of RNA with sizes 5.3 kb and 7.8 kb were found. These transcripts poorly coincided with the four known variants of intracisternal A-particle (IAP) genes. It seemed that at least some of the described RNAs were transcribed from some minor non-identified variants of IAP genes. The A2 transcripts were practically restricted to nuclei, their sizes being heterogeneous.

Transcription of repetitive DNA in condensed plant chromatin

Polyadenylated transcripts homologous to highly repetitive DNA were found in root tips of Vicia faba by Northern blot hybridization. Electron microscope autoradiography using [3H]uridine as a probe revealed transcription of condensed chromatin in various higher plants. This is consistent with the general rule that highly repetitive DNA is located within condensed chromatin, but it is new that this chromatin fraction is active in RNA synthesis to a considerable amount. Semi-quantitative comparison of the intensity of transcription in species with widely differing 2C DNA contents by means of light microscope autoradiography revealed an inverse relationship between the amount of 2C DNA (and condensed chromatin), and the rate of RNA synthesis.

The role of plant hormones in higher plant cellular differentiation. I. A critique

Primary growth and morphogenesis in higher plants can be explained mechanistically in terms of primary types of cellular differentiation, namely, phenomena of cell division, primary wall growth, intercellular bonding and polarity. Plant hormones fulfil essential roles in regulating these types of differentiation, and it is well established that plant hormones can initiate primary growth and morphogenesis. Secondary and terminal types of cellular differentiation largely determine the usefulness of plants to man; however, regulators of these types remain poorly characterized. Secondary and terminal types need not differentiate in order for primary growth and morphogenesis to occur, and there is no conclusive evidence that factors regulating primary growth and morphogenesis also initiate subsequent types of differentiation.