Reciprocal recombination (crossing over) between homologous chromosomes (homologues) is believed to be required for proper meiosis I segregation. The mechanisms that control recombination so that each pair of homologues undergoes crossing over are not known. In Saccharomyces cerevisiae and humans, small chromosomes have higher meiotic reciprocal recombination rates (cM/kb) than large chromosomes, and in S. cerevisiae, rates of reciprocal recombination have been shown to respond directly to chromosome size. One set of recent experiments, however, suggested that size-dependent control of crossing over was not a general feature in all strains or on all chromosomes. As it has been proposed that size-dependent control of recombination is an essential feature of the mechanism guaranteeing crossing over between homologues, the investigator plans to find out if it indeed occurs in all strains and chromosomes and, if it does not, to begin to determine why it might not be occurring. The decreased rates of recombination on large chromosomes have been proposed to be due to increased crossover interference. Interference is defined by the apparent inhibition of additional meiotic reciprocal recombination events observed near the site of a crossover. The molecular mechanism of interference and how it might respond to chromosome size are not known. It has been suggested that the density of meiotic recombination-inducing double-strand break (DSB) sites is greater on small chromosomes than on large ones. If this is indeed the case, then chromosome size-dependent control of recombination (and, perhaps, interference) might act by controlling DSB formation. To test this idea, the investigator plans to analyze DSB formation on chromosome I constructs of different sizes. The investigator has also discovered that the most distal euchromatic DNA undergoes meiotic recombination at a higher rate than more interior sequences, suggesting that chromosome position also regulates recombination. The investigator will determine whether chromosome position does actually affect recombination rates.
The next part of this project involves the meiotic synaptonemal complex (SC). The precise function of this structure is not known, but it has been proposed that it may serve as a regulator of meiotic recombination. The investigator has produced a functional, fluorescent ZIP1-GFP fusion protein that labels SCs in living cells. He has used this protein to show that SCs undergo dynamic movements and changes in their distribution. The role of these movements is unknown but they could play a role in facilitating recombination. He now proposes to label axial elements with REC8-CFP and, through the use of fluorescence video microscopy, examine the dynamics of pairing and SC movements. These studies should provide new insights into the molecular mechanisms controlling recombination and chromosome synapsis, processes that lead to proper meiotic chromosome segregation.
In most species, proper chromosome segregation is essential for the production of normal gametes (e.g. sperm, eggs, pollen, and ascospores). Identifying mechanisms that ensure proper chromosome segregation will provide a better understanding of how meiotic divisions occur. A better understanding of these mechanisms could lead to greatly improved plant breeding and animal husbandry. Having this research carried out at UMDNJ in Newark will also be important for the scientific education of both graduate and medical students at that institution, and for the undergraduates from neighboring institutions (NJIT, Rutgers) who will actively participate in the project. Furthermore, UMDNJ's location has been instrumental in the investigator's significant success in enabling gifted underrepresented minority students to participate in research. He will continue this effort with the aim of producing outstanding scientists from UMDNJ's environs. The opening of Newark's Science High School one block from his laboratory should further increase his and his laboratory's impact upon Newark's secondary school population as well.
Meiosis is an almost universal process that is required for eukaryotic sexual life cycles whereby cells which normally contain two homologous or near identical copies of each chromosome undergo a specialized form of nuclear division that produces germ cells such as sperm and eggs which contain only a single copy of each chromosome. Fusion of these germ cells produces a zygote cell which again contains two homologous copies of each chromosome. This cell divides to produce more cells or a fully developed organism, some or all of these cells can then undergo meiosis to produce more germ cells. During meiosis homologous chromosomes initiate recombination, pair and form a tight associative structure called the synaptonemal complex (SC) where recombination is completed. Recombination between homologous chromosomes is required to insure that germ cells get only a single copy of each homologous chromosome. We are studying the control of meiotic recombination and the dynamics of nuclei and chromosomes during the stages at which these processes occur in living wild-type and mutant yeast cells. We continue to examine the formation, structure and rearrangement of SCs by fluorescence microscopy. We previously found that the meiotic nucleus and the chromosomes contained therein underwent dramatic movements that we believe are essential for this process to occur efficiently. Chromosomal movements are largely dependent upon tethering chromosomes to the nuclear periphery. Based on our studies we believe the chromosomal tethers are complex and utilize both the ends and interstitial regions of the chromosomes. The end tethers have begun to be characterized but little is known about tethering that appears independent of the chromosome ends. We have been concentrating on trying to find genes that are involved in these complex movements in order to learn more about these associations. Based on our studies, similar chromosome behavior has been observed in higher plant s and mammals and there is little doubt that our studies in yeast, a simple model eukaryote is required in order to further our knowledge of all organisms that require meiosis as an essential component of their life cycle. These studies can aid agriculture by enabling more efficient plant and animal breeding as well as a better understanding of factors that might be used to prevent birth defects.
