Hannah Horrell, of Waitaki Girls' High School, asks :-

When an embryo is dividing, how do cells know what function they are going to perform?

Murray Grigor, of the Centre for Gene Research at Otago University, responded.

The fertilised egg is, of course, a single cell that contains in its DNA all the information needed for the development of the mature animal.

Early in embryonic development the cells begin to show differences which indicate that they are destined to develop into specific organs and tissues. The differences amongst the cells in these tissues result from the fact that only a certain set of genes are being expressed, that is are active, in them. The term differentiation is used to describe this process. The control of this has puzzled scientists for a long time and now we are starting to understand what is going on.

Firstly the fertilised egg is not symmetrical and this asymmetry can signal to the cells their first decisions as to their fate. Subsequently, cells can "talk" to one another by producing molecules that act as signals to modify gene expression in target cells. Sometimes they send signals to themselves that ensure they continue to develop along appropriate lines. Studies with the fruit fly Drosophila have provided much of the information about which genes are activated at particular times within the development period. These genes contain regions that can bind specific regions of the DNA controlling the expression of other genes. They are often named after developmental abnormalities detected resulting from mutations in specific genes. For example, one gene is called antennapedia, because it can lead to the formation of a leg (pedia) where an antenna should normally be. Although humans don't have antennas, we do have similar genes that are believed to play a different but critical role in human embryonic development.