Genes are like recipes for the cell, they tell it how to make proteins. You have a protein to do everything in your body, for example, one to make a colored pigment for your eyes, another to protect that protein so that is can do its job, another to direct it the right place, another to get rid of the protein when its done its job and the list goes on and on. Now you would think that DNA would be quite complicated to be able to contain so much information but in fact it's really simple. DNA is made up of 2 strands that stick together to make a double helix.
You can imagine a strand of DNA as a string of beads, with four types of bead (known as bases) making it up- we call them A, T, G and C. They like to be in pairs so if you have a G on one strand, it will stick to a C on the other, and vice versa. A and T will always stick together too. We say that the strands are complementary to each other and it is this property of DNA that allows it to copy itself whenever the body makes a new cell.
What happens when DNA replicates itself is that the 2 strands separate and the machinery in the cell reads each strand, a base at a time and every time it sees a G, it sticks a C on the new complementary strand, an A for every T, a C for every G and a T for every A etc.
