Question: How would you explain when in sexual reproduction, we start as one cell and then split to become many?

Helen Tunbridge answered on 18 Jun 2013:

Oh what a lovely question! Really great, this is a really interesting topic.
BTW. Sorry! This is quite a long answer, I got a little excited!! I’ll put asterisks next to the bits that are most relevant to your actual question!!

**So as I’m sure you know, we start off as a single egg, which has half the genetic material (DNA) needed to start making a human. When the egg is fertilised by a sperm, the other half of the DNA is provided and cell division can happen. The now complete egg divides in two to make two cells stuck together, then those two divide again to make four, then eight and so on until there is a small bundle of cells 32 which has the very catchy name of a morula. At the morula stage, all the cells are the same, but from this point lots of interesting changes start to happen.**

I won’t go into what all the changes are, because there are a lot of them and it’s quite complicated, but basically the cells start to change their shape and function depending on where in the morula they were. This is the first stage in deciding which of these early cells will become things like the placenta and embryonic sac, and which cells will develop into you!
**Of the cells that become you, some gradually change into gut cells, some into skin cells and some into brain cells. This is all dependent on where these cells are and which chemicals they are bathed in. Gradually, structures form which become your arms, legs, head, organs and so on until an incredibly complex little baby has been formed, all from one egg!**

*The reason this can happen is that ALL the information needed to make any cell is kept in every cell. The thing that makes each cell different is that different bits of the DNA are ‘read’. You can imagine this like having a book of short stories – you have all the words and stories in the one book, but depending on which bit of the book you read out, you’ll end up hearing a different story. It’s incredible!* (This bit is less about the cells dividing, but it explains how all the cells are able to become different and specialised)