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Humans are multicellular organisms. There are over a trillion cells in your body. They are specialised to do different jobs, even though they all came originally from just one single fertilised egg cell, or zygote. When fertilised, an egg cell copies itself to make two cells. This was your first experience of mitosis. Soon after, these two cells copy themselves to make four and so on. The process continues throughout your life to grow new cells and replace damaged ones.
The cells produced by this process, called cell division or mitosis, are all genetically identical. So each one of your trillion cells holds the same genetic information. This is contained in the DNA that is packaged into 23 pairs of chromosomes inside the nucleus of each body cell.
Cells are specialised and arranged to work together to make a fully-functioning human. Use the animation to see how the body is built up of organ systems, tissues and cells. Look inside the cells to see the structure of chromosomes and the DNA that controls cell activities.