to the expression of oncogenes, which are pivotal in cancer genetics. Accurate delineation and prognosis of a cancer may depend upon careful cytogenetic analysis. Cytogenetics is still of great importance when monitoring changes in somatic cell hybrids. The role of chromosome genetics within modern biology is continually changing and evolving, and thus a knowledge of cytogenetics is of ever-increasing value. Chromatin structure 1 and replication Summary It is now a well-known fact that DNA is the hereditary material and that the vast majority of the DNA of an organism is housed in organelles called chromo­ somes. However, the length of DNA contained in a cell is far greater than the size of the cell within which it exists; for example, the shortest human chromosome consists of 1. 4 cm of DNA which must be compacted into a cell only a few micrometres across. To achieve this, DNA has to undergo several levels of packing, a process that is a mixture of DNA-DNA and DNA­ protein interactions. The most compact form of DNA can be seen under the light microscope as a meta­ phase chromosome during mitotic cell division. DNA must also be faithfully replicated to ensure accurate cell function and maintenance of the organ­ ism as well as ultimately, the reproduction of the species. Our current knowledge of these complex processes is very limited, with huge gaps, in particular, concern­ ing the links between the molecular and organelle level of organization.