38. There are three main steps to DNA replication: initiation, elongation, and termination. In order to fit within a cell's nucleus, DNA is packed into tightly coiled structures called chromatin, which loosens prior to replication, allowing the cell replication machinery to access the DNA strands.

39. Cancer is primarily a disease of ageing epithelia, and of ageing individuals. We now possess detailed insights into the changes in cell regulatory genes and DNA repair systems which accumulate with time and which manifest in malignancy.

40. Proto-oncogene: A normal gene which, when altered by mutation, becomes an oncogene that can contribute to cancer. Proto-oncogenes may have many different functions in the cell. Some proto-oncogenes provide signals that lead to cell division. Other proto-oncogenes regulate programmed cell death (apoptosis). Proto-oncogenes (unmutated oncogenes) stimulate cell division in a regulated manner. Proto-oncogenes include growth factors, growth factor receptors, and cyclins. Oncogenes are mutated forms of these genes that result in unregulated stimulation of cell division. Tumor suppressor genes prevent cell division.

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