DNA system provided the first clear suggestion that DNA carries genetic information. In 1933, while studying virgin sea urchin eggs, Jean Brachet suggested that DNA is found in the cell nucleus and that RNA is present exclusively in the cytoplasm. At the time, "yeast nucleic acid" (RNA) was thought to occur only in plants, while "thymus nucleic acid" (DNA) only in animals. The latter was thought to be a tetramer, with the function of buffering cellular pH. In 1937, William Astbury produced the first X-ray diffraction patterns that showed that DNA had a regular structure. In 1943, Oswald Avery, along with
Eukaryotic DNA replication formation of single-stranded DNA occurs frequently, more often during replication stress. ATR-ATRIP is able to arrest the cell cycle to preserve genome integrity. ATR is found on chromatin during S phase, similar to RPA and claspin. The generation of single-stranded DNA tracts is important in initiating the checkpoint pathways downstream of replication damage. Once single-stranded DNA becomes sufficiently long, single-stranded DNA coated with RPA are able to recruit ATR-ATRIP. In order to become fully active, the ATR kinase rely on sensor proteins that sense whether the checkpoint proteins are localized to a valid site of DNA replication stress. The RAD9-HUS1-Rad1