If you have ever tried to construct robots, you know how difficult it is to master all the technical problems to at least make them a little flexible, reliable and intelligent. We can construct robots that are stronger, faster, bigger and more exact than we are, but we cannot even get anywhere near the flexibility and efficiency of a very "simple" little insect. We cannot construct any robot that can do everything that this insect does, even if we could make it as big as we wanted. We do not even know exactly how this little creature achieves this perfection, because its workings are extremely complex even though it is so small compared to us. But that does not mean that we have no idea.
Biology has made great steps during the last decades. The rapidly growing knowledge of genetics has made it possible to take some cells from a mature sheep and give it an identical twin brother by letting them develop. According to medical scientists, it seems to be only a matter of years until we do not need artificial hearts or kidney donations any more because we can breed any spare parts for the human body without incompatibility problems. It is an open question if this is a good development: Immortality approaches. But it is really astonishing what is possible already now. The combination of genetics and microbiology makes it possible to use viruses as little helpers for the sake of human sanity. They can give the doctor a good helping hand by being able to give the human body cells more power in combatting a disease4. How does this wonderful cooperation work?
For a cell to have more power here means to be able to execute functions that could not be executed before. How is it possible that cells get new abilities? How does a cell get its abilities? These questions are easy to answer if you know what DNA is. To put it very simply, it is a long chain of interconnected molecules that is found in every cell and contains all the information about what the cell could possibly do. This chain is like a long slice of paper with a text written on it. Every word5 (which in the case of DNA is called a gene) of the text has a special meaning. The meaning of most of the genes is a description of how to construct the molecules of a specific substance. These substances are called proteins. The language in which the text is written only contains four different letters, but as one word can be very long6, there is a vast number of different words which all describe the structure of different proteins. The whole text is called genome7. So we have a genome which consists of many genes which each code for a specific protein. And this genome (which is made of DNA) is contained in every cell.
What is at first astonishing is the fact, that even though the body of higher animals and humans consists of thousands of different cell types with very different functions, shapes and sizes, they all contain the same genome and have all developed out of one single cell. Why and how did they change their shape, size, location and function? And why are they different even though they contain the same genome? Does the genome not describe what the cell does after all? Yes, it does. But it describes, what the cell could possibly do. What it actually does and how it looks depends on which of the genes are read and which of the proteins described by them are being produced. And that depends on which genes are activated. So genes can be active or inactive and the function of the cell depends on which genes are active and which are not.
You remember that a virus can give human cells more power? It achieves this goal by inserting specific genes that are needed for that power into the cell genome. Viruses reproduce by inserting their genome into a cell genome8 and letting the cell produce new viruses. So the doctor only9 has to replace most of the virus genome with the genes that are needed in the human body cells and the virus then takes the part of inserting it into the cells.
When a creature grows from a single egg-cell, the cells get more and more by dividing, which means making an exact copy of themselves, and sometimes they change their type because different genes get activated. But what activates or inactivates genes?
Not every gene codes for a protein. There are also some genes that have a quite different function. They can activate or inactivate other genes. And whether they do it or not depends on the substances that they find in the cell. Such a regulator gene might for example activate two coding genes (called structural genes) only if the concentration of a substance x in the cell exceeds a certain value. This regulation of gene activity is called gene regulation. To make it even more interesting, this substance x is very often a protein which can be produced either by the cell itself or by other cells in its neighbourhood. And many proteins can leave the cell where they are produced, diffuse to other cells and get into them. So it is possible that a protein produced by one cell activates a gene of another cell which therefore produces another protein which in turn inactivates the gene of the first cell which allowed the production of the first protein. You see that this mechanism enables very complex interactions between different genes as well of one cell as of different cells. And because the activation of genes also depends on other substances that are unequally distributed in the growing cell cluster of a developing creature, the cells differentiate to different types and start their intricate interactions10.
And the fantastic or even unbelievable thing about these interactions and the growth and differentiation is: everything makes a lot of sense. The resulting cell cluster is a perfectly functioning creature where all the cells work together to make this creature live. The cooperation of cells in the body of living beings is one of the most amazing if not the most amazing example of small units that can each only execute a limited function but work together to achieve a higher goal. And it is very improbable11 that the units know what they do, want to cooperate, have a higher goal in mind or have been told by somebody to do what they do for achieving the goal12. Instead, this cooperation simply emerges on the basis of the information contained in the genome. So you could say: The genome is the somebody who tells the cells what to do. But how could the genome be constructed so that this hypercomplex interaction results in just this perfect functioning body? (And it even does under very different environmental conditions and with many variations of the genetic code. This means that it is very reliable and insensitive to most errors.)