We usually have a laboratory group of 10-12 individuals, whom you'll meet if you click on the UP button above and then on Our Lab Group.  The group includes Pieter Dijkwel, who is an expert on the nuclear matrix and a faculty member in Biochemistry and Molecular Genetics, and James Larner, who is a radiation oncologist interested in DNA damage, repair, and cell cycle checkpoints.  Listed below are the projects that our extended laboratory group works on.                                                                                                                                                                        About 20 years ago, we established a methotrexate-resistant cell line (CHOC 400) that has amplified one allele of the dihydrofolate reductase (DHFR) locus about 1,000 times.  Since the unit of amplification (amplicon) is ~250 kb in length, we reasoned that it should contain at least one origin of replication, and possibly more.  Thus, the amplicon is comparable to a high copy number virus or plasmid, and this feature facilitated the localization of the origin of replication in the DHFR locus.  This was the first mammalian chromosomal origin to be identified, and it turns out to be very complex.  Read more about its properties by clicking on the button marked Replication.                                                                                                                                                                                   Although we initially used the CHOC 400 cell line as a tool to identify origins, we got interested in how this cell line amplified the DHFR locus in the first place - not because drug-resistance markers like DHFR are amplified very often, but because almost all biopsied human tumors have amplified at least one oncogene.  Using the DHFR gene as an easily selectable amplification marker, we have studied this process for many years.   With the aid of high resolution fluorescence in situ hybridization (FISH), we discovered that DNA sequence amplification is initiated by chromosome breaks.  To read about this fascinating subject, click on the button marked Amplification.                                                                                                                        As it turns out, all cells (both normal and cancerous) suffer chromosome breaks, but these are usually quickly repaired.  If they are not repaired,various checkpoints are activated, which normally direct the cell along a program leading to cell death.   These checkpoints operate in the G1 period, during the DNA synthetic (S) period, and prior to entry into mitosis.  One (or more) of these pathways is usually non-functional in cancer cells, allowing amplification to occur.  We are interested in one of the cell cycle checkpoints (the S-phase-dependent or SDS) pathway that leads to transient arrest of DNA synthesis after DNA damage is incurred.   To read more about this important aspect of our research program, click on the button marked Checkpoints.