C-DNA may facilitate homologous DNA pairing

Computational identification of ultra-conserved elements in the human genome: a hypothesis on homologous DNA pairing

Thousands of prolonged sequences of human ultra-conserved non-coding elements (UCNEs) share only one common feature: peculiarities in the unique composition of their dinucleotides. Here we investigate whether the numerous weak signals emanating from these dinucleotide arrangements can be used for computational identification of UCNEs within the human genome. For this purpose, we analyzed 4272 UCNE sequences, encompassing 1 393 448 nucleotides, alongside equally sized control samples of randomly selected human genomic sequences. Our research identified nine different features of dinucleotide arrangements that enable differentiation of UCNEs from the rest of the genome. We employed these nine features, implementing three Machine Learning techniques - Support Vector Machine, Random Forest, and Artificial Neural Networks - to classify UCNEs, achieving an accuracy rate of 82-84%, with specific conditions allowing for over 90% accuracy. Notably, the strongest feature for UCNE identification was the frequency ratio between GpC dinucleotides and the sum of GpG and CpC dinucleotides. Additionally, we investigated the entire pool of 31 046 SNPs located within UCNEs for their representation in the ClinVar database, which catalogs human SNPs with known phenotypic effects. The presence of UCNE-associated SNPs in ClinVar aligns with the expectation of a random distribution, emphasizing the enigmatic nature of UCNE phenotypic manifestation.

Remodeling of perturbed chromatin can initiate de novo transcriptional and post-transcriptional silencing

In eukaryotes, repetitive DNA can become silenced de novo, either transcriptionally or post-transcriptionally, by processes independent of strong sequence-specific cues. The mechanistic nature of such processes remains poorly understood. We found that in the fungus Neurospora crassa, de novo initiation of both transcriptional and post-transcriptional silencing was linked to perturbed chromatin, which was produced experimentally by the aberrant activity of transcription factors at the tetO operator array. Transcriptional silencing was mediated by canonical constitutive heterochromatin. On the other hand, post-transcriptional silencing resembled repeat-induced quelling but occurred normally when homologous recombination was inactivated. All silencing of the tetO array was dependent on SAD-6, fungal ortholog of the SWI/SNF chromatin remodeler ATRX (Alpha Thalassemia/Mental Retardation Syndrome X-Linked), which was required to maintain nucleosome occupancy at the perturbed locus. In addition, we found that two other types of sequences (the lacO array and native AT-rich DNA) could also undergo recombination-independent quelling associated with perturbed chromatin. These results suggested a model in which the de novo initiation of transcriptional and post-transcriptional silencing is coupled to the remodeling of perturbed chromatin.

Meiosis through three centuries

Meiosis is the specialized cellular program that underlies gamete formation for sexual reproduction. It is therefore not only interesting but also a fundamentally important subject for investigation. An especially attractive feature of this program is that many of the processes of special interest involve organized chromosomes, thus providing the possibility to see chromosomes "in action". Analysis of meiosis has also proven to be useful in discovering and understanding processes that are universal to all chromosomal programs. Here we provide an overview of the different historical moments when the gap between observation and understanding of mechanisms and/or roles for the new discovered molecules was bridged. This review reflects also the synergy of thinking and discussion among our three laboratories during the past several decades.

Meiosis: Dances Between Homologs

The raison d'être of meiosis is shuffling of genetic information via Mendelian segregation and, within individual chromosomes, by DNA crossing-over. These outcomes are enabled by a complex cellular program in which interactions between homologous chromosomes play a central role. We first provide a background regarding the basic principles of this program. We then summarize the current understanding of the DNA events of recombination and of three processes that involve whole chromosomes: homolog pairing, crossover interference, and chiasma maturation. All of these processes are implemented by direct physical interaction of recombination complexes with underlying chromosome structures. Finally, we present convergent lines of evidence that the meiotic program may have evolved by coupling of this interaction to late-stage mitotic chromosome morphogenesis.