The third level involves the actual organization of the DNA in chromosomes. This is done by a set of proteins known as histones , that form an organized series of nucleosomes , which repeatedly pack into coils to form chromatin. The degree of packing determines the accessibility of genes for transcription and so their expression. A fourth level only occurs occasionally, and involves the complete inactivation of entire chromosomes. Female mammals are the best example, since they have one of their two X chromosomes inactivated in every cell, witness calico cats .
Cancer results from loss of control of division of cells.
We now know that this requires a massive series of genetic insults to the DNA of single somatic cells, commonly with accompanying retroviral infection. A combination of activation of oncogenes , and inactivation of suppressor genes allows cell division to get out of control and a single cell forms a tumour. Ultimately additional changes cause some of the cells of a tumour to lose their adhesion for each other and move through the lymph and blood systems to form satellite tumours elsewhere, a process called metastasis , at which point the cancer becomes life-threatening. Click here to return to the Biology 104 home page.