Genes and body plans
To build something, you need instructions – for bodies, a genome
The basic body plan of humans is encoded within the DNA instructions inside our cells. But how we turn out depends on both internal and external stimuli – the genes we inherit from our parents, as well as what happens to us in the womb and the environmental influences to which we are exposed after birth.
The DNA in our genome is sometimes likened to a blueprint or a recipe book, as it contains the instructions needed to build an organism.
The assembly process is finely controlled in time and space – eyes, for example, are made only in the head and begin developing only when the head is prepared for them. As bodies are made of cells, embryo development is crucially about cell fate – making sure that a nose cell, say, is made only where and when it is needed.
It might seem astonishing that a four-letter DNA code can achieve such wonders. This point was reinforced when the Human Genome Project revealed that humans have just 23,000 or so genes – not the 100,000 many expected. Even this estimate may have to be adjusted downwards. How can so few genes build the rich complexity of a human?
The answer lies partly in genetic changes affecting proteins, but also in the sophisticated way genes are switched on and off. The fate of a cell – whether it turns into a neuron, or a kidney cell, or a white blood cell – depends on the set of genes active within it. Some genes also override the effects of others. This is known as epistasis. By exquisite control of when and where genes are active (transcribed to make proteins), cells are made when and where they should be, and collectively form tissues, organs and ultimately an entire body.
So how is this control exercised? Several mechanisms are known to be crucial.
Some genes have very powerful effects because they regulate other genes. One protein may activate tens or hundreds of genes, and thereby switch on an entire developmental programme. A classic example is the fruit-fly Antennapedia gene, which can turn a leg into an antenna (or vice versa).
Cells can exert a profound influence on their neighbours. Transplant the right bit of a tadpole, for example, and it will stimulate the growth of a new leg. Such organising cells secrete (or have on their surface) molecules that switch on (or off) genes in cells around them.
Some cells release signalling molecules that control tissue growth (morphogens). The concentrations of morphogens will be high next to the cell releasing them but will fall away with distance – creating a morphogen gradient. If cells respond differently to different concentrations, one morphogen can generate a range of cell types.Lead image:
Teresa Niccoll and Daniel St Johnston/Wellcome Images CC BY NC ND