This text is describing one of the deadly diseases in the world. How a cancer is formed in the human body is written to show where it comes from and how it mutates. The description of that cancer formation then is considered as an example of explanation text.
Explanation text is very similar to procedure in English writing genres. Both procedure and explanation text tries to show reader how the object discussed is formed or why it happens. However they are different from each other in the way that are written. Procedures text are composed in imperative sentences while the explanation is written in the mode affirmative sentences.
Cancer: Example of Explanation Text
What is cancer? It is actually a group of more than one hundred separate diseases. Most of us are fear from cancer It is reasonable because next to heart disease, cancer is the second leading cause of death.
Cancer cells come from normal cells because of mutations of DNA. Those mutations can occur spontaneously. The mutations may be also induced by other factors such as: nuclear and electromagnetic radiation, viruses, bacteria and fungi, parasites, heat, chemicals in the air, water and food, mechanical cell-level injury, free radicals, evolution and ageing of DNA, etc. All such factors can produce mutations that may start cancer.
Cancer cells are formed continuously in the organism. It is estimated that there are about 10,000 cancer cells at any given time in a healthy person. Why do some result in macroscopic-level cancers and some do not? First, not all damaged cells can multiply and many of them die quickly. Second, those which potentially divide and form cancer are effectively destroyed by the mechanisms available to the immune system.