Question: When you are a grown up, how do your cells know when to stop growing

Hayley Lees answered on 29 Dec 2013:

Hi Joey 🙂

We grow into adults from children, not because our cells are getting bigger (though they do a bit), but because they divide (split into two) and make more cells. We all start off as a fertilised egg cell – this splits into two cells, then they both split into two to make four cells, and these split to make eight cells, and these split to make 16….and so on, until an entire baby has been made, just from one cell!

You are right – our cells do stop doing some things when we are grown up. They stop following the instructions that they were following when you were a developing embryo, or a baby, or a child. They know when they have done everything they need to to (like how many times they need to divide, where in the body they need to divide etc). These instructions are encoded in our DNA, and we have a full copy of DNA in (nearly) every cell.

Some cells still divide even when we are grown up – cells such as skin cells need to still divide because they get damaged easily and so our body sheds them and replaces them with brand new ones (the dead ones that we lose make up much of the dust you will find in your house!). But most of our brain cells have stopped dividing by the time we are 20 years old!

There’s another thing that can tell our cells to stop dividing, other than the instructions in our DNA. DNA is a really long chain of letters (these letters are called nucleotides) and at the ends of the chain is something called a telomere. This is basically an extra bit of DNA, which gets shorter and shorter every time a cells divides and makes a new copy of the DNA for the new cell. When this telomere gets too short, the cell knows it should not divide any more. This is what can happen when we get older, and scientists think that this could be one of the reasons why we age (get wrinkly skin, grey hair, bruise easily). Cells can no longer divide and so damaged cells don’t get repaired.

I hope this has answered your question. If it is too complicated or if you want even more information, just ask 🙂