IB DP Biology: SL复习笔记1.4.3 Cancer

• 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

• Mutagens are agents that alter the genetic material of an organism
  • E.g. biological organisms (viruses), radiation (X-rays, UV light) or chemical substances (tar in tobacco smoke)
• If mutagens cause cancers they are called carcinogens
• Carcinogens are any agents that may cause cancer

Types of tumour

• 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

• Scientists studying the incidence and distribution of certain cancers identified links between smoking and cancer
• However, it was only when laboratory investigations showed that cigarette smoke contained more than 4000 chemicals, at least 40 of which were carcinogens, that a correlation was established
• There is a positive correlation between smoking and cancer. The more cigarettes smoked per day the higher the chance of developing certain cancers (e.g. lung and mouth)