At a microscopic level, every living thing is made up of cells. A bacterium only has one cell, but you have about 100 trillion.

Your 100 trillion cells may be one of about 200 different kinds, in different shapes and sizes. Each kind has its own role to play. In order to do this, the cells group together into different types of tissue, which in turn form organs and structures, like the heart. These organs work within systems, like the cardiovascular system, which allow the body to function. Our muscles are made of muscle cells, and our bones of bone cells. There are even very specialized types of cells for our fingernails and each of the delicate parts of our eyes.

Image: David M. Phillips, Visuals Unlimited, Inc. Blood cells.	Bone cells.	Image: David M. Phillips, Visuals Unlimited, Inc. Nerve cells.
Image: Fred E. Hossler, Visuals Unlimited, Inc. Skeletal muscle cells.	Image: Fred E. Hossler, Visuals Unlimited, Inc. Fat cells.	Skin cells.

Chromosomes, which are made up of DNA molecules, are found within the nucleus of the cell. A gene is one section of a DNA molecule that contains the code for the production of a protein.

Proteins, produced by our genes, are essential to life. Our bodies are constructed of them and depend on them. Protein absorbed from the food that we eat provides our bodies with the necessary building material and energy that is essential to its proper functioning. Mutations in genetic instructions can alter the production of proteins, which can lead to sickness or even death.		If you unrolled the DNA contained in only one of your cells, it would be 1.8 metres long.