When you next look into a mirror ask yourself if you are the same person as you were yesterday. Well, of course you are.
Even people who last met you ten years ago can still recognise you and call you by your name. Although they might add, “My, how you have grown!”
And yet, if we could see under your skin, we would find that you are not the same. One of the biggest mysteries in biology is how we can change all of the time, whilst still staying the same.
Your skin cells live for about two weeks, so every month they are completely replaced. Red blood cells live for about 100 days and about two million are made in your body in every second.
Some of the chemicals in your cells exist for only minutes or seconds.
There is an energy store called ATP, which is needed for muscle contraction. ATP is made and broken down within 15 seconds. Cells need glucose to make ATP and this explains why muscle cells need a continuous supply of glucose to stay alive. This comes from our food.
Even large organs, like the liver, are replaced regularly. You grow a new liver every year. The cells in the alveoli of your lungs are renewed every eight days. Even the bone cells in our skeleton are replaced every three months. Your entire skeleton is remade every ten years.
So, when your friend sees you after ten years and calls out your name, there is not a single part of your body that was the same as when you last met. You have been completely remade and remodelled. And the same is true of your friend.
So, how can this be? New cells are made when one cell divides to make two cells. The information in the genome is copied before cells divide, so the new cells always receive the same information as the old cells.
The new cells use this information to grow bigger and to develop. So, you stay the same because of how your new cells use the information in their genomes.
Living organisms are alive because they actively remake themselves. No man-made machine can do this. Which is, perhaps, just as well.
- In what ways have you changed in the last ten years?
- In what ways have you stayed the same?
- Why do need to eat food everyday?
The genome is divided between a number of 
How can this hart’s tongue fern grow on a vertical cliff face about two metres from the ground.
Like fungi, another ancient group, ferns produce spores. They are the brown dots on the underside of this fern leaf. Spores are light and float in the air like particles of dust.
This is a fern, but it is not the mature adult form. It has tiny roots that grow into the humus, drawing nutrients from it. 
The gametophyte makes many small sperm that swim in the water on the surface of the plant. They swim towards eggs, which are much larger. This photograph shows a fern sperm fertilising a fern egg.