CIE A Level Biology复习笔记5.1.6 How Tumours Form

• Cancers demonstrate how important it is that cell division is precisely controlled, as cancers arise due to uncontrolled mitosis
• Cancerous cells divide repeatedly and uncontrollably, forming a tumour (an irregular mass of cells)
• Cancers start when changes occur in the genes that control cell division. A change in any gene is known as a mutation. If the mutated gene is one that causes cancer it is referred to as an oncogene
  • Mutations are common events and don’t lead to cancer most of the time
  • Most mutations either result in early cell death or result in the cell being destroyed by the body’s immune system
  • As most cells can be easily replaced, these events usually have no harmful effect on the body
• The mutations that result in the generation of cancerous cells do not result in early cell death or in the cell being destroyed by the body’s immune system
• This means that the harmful mutation occurring in the original cell can be passed on to all that cell’s descendants
• A typical tumour contains around a thousand million cancerous cells by the time it is detected

• Carcinogens are any agents that may cause cancer (eg. UV light, tar in tobacco smoke and X-rays). If the agent causes cancer it is described as carcinogenic
• Some tumours (such as warts) do not spread from their original site – these are known as benign tumours and do not cause cancer
• Some tumours spread through the body, invading and destroying other tissues – these are known as malignant tumours and cause cancer
  • Malignant tumours interfere with the normal functioning of the organ / tissue in which they have started to grow (eg. they may block the intestines, lungs or blood vessels
  • Malignant tumour cells can break off the tumour and travel through the blood and / or lymphatic system to form secondary growths in other parts of the body
  • The spreading of cancers in this way is known as metastasis
  • Metastasis is very dangerous as it can be very difficult to detect, locate and remove secondary cancers

Stages in the development of cancer