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What is the difference between hurricanes and typhoons? Hurricanes and typhoons are tropical cyclones. A cyclone is a low- pressure area in the atmosphere in which winds spiral inwards. A cyclone may cover an area as large as the United States. A tornado is a special, intense cyclone that measures 90 metres to 2,400 metres across.

A tropical cyclone develops over tropical or subtropical waters. Severe tropical cyclones, with wind speeds of 119 kilometres an hour, are called hurricanes or typhoons, depending on where they form.

Hurricanes form over the north Atlantic Ocean or the northeastern Pacific Ocean, and typhoons form over the western Pacific Ocean. These storms can bring winds of up to 290 kilometres an hour, heavy rain and violent thunder and lightning. They measure 320 kilometres to 480 kilometres across.

Typhoons begin near the equator and move westward, growing in intensity and size. They advance slowly, at about 16 kilometres to 24 kilometres per hour. But the circular winds around the centre of the storm are very strong, often reach ing speeds of 240 kilometres an hour.

What are genes? Genes are instructions that control the processes that keep us alive. These instructions, or genes, are built inside a substance called deoxyribonucleic acid - DNA.

A single molecule of DNA contains two spiral strands - the double helix - connected by thousands of special links. This DNA is carried in a chromosome. Chromosomes consist of a mixture of nucleic acid and proteins which support the DNA molecule. In humans, there are 23 pairs of chromosomes present in every cell (making a total of 46), except in gametes (eggs in females and sperm in males) which only form the correct number of chromosomes when joined in a fertilised egg.