Lung cancer cell division. Coloured scanning electron micrograph (SEM) of a lung cancer cell during cell division (cytokinesis). The two daughter cells remain temporarily joined by a cytoplasmic bridge (centre). Cancer cells divide rapidly in a chaotic, uncontrolled manner. They may clump to form tumours, which invade and destroy surrounding tissues. Lung cancer is often associated with smoking tobacco and exposure to industrial air pollutants. It causes a cough and chest pain and may spread to other areas of the body. Treatment includes removal of affected parts of the lung, with radiotherapy and chemotherapy. Magnification unknown. STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY.