The human body is made up of billions of cells. Cells are the tiny building blocks of our tissues and organs. We all started life as a single cell. That cell made an internal copy of itself (replication) and then divided into 2 cells. Those 2 cells then also replicated and divided, so the 2 cells became 4 cells. The 4 cells replicated too and divided into 8 cells, and so on. Cells specialise to perform particular tasks. Some cells will cluster together to form a finger, for example. Others create skin and heal the skin when it is wounded. Cells get old and die after a certain amount of time (“programmed cell death”, or apoptosis), and replication ensures that new cells are made to take their place. When they are acting normally, cells “know” which other cells to join up with and stick to – and they also know when to stop replicating and die. Each type of cell has a particular role and set of knowledge or instructions in their DNA (genes). Our cells know how to make the right number of fingers on our hand for us (and they know that fingers should only grow on our hands). Each finger is covered with skin and each finger has a fingernail. If we cut our finger, the skin cells will start replicating and create new skin to heal the wound. If we lose a fingernail, our cells can grow a new one. But the cells will not create extra fingers, even if we lose one. The rules are clear for those cells, and they keep to the rules.
