Gene Expression

The Genetic Code

In the next sections we will learn how genes are expressed and how signals are transduced from the outside into the nucleus to induce growth or to induce events causing the cell to stop growing. We will have a closer look at the cell cycle machinery which is changed in cancer cells and leads to uncontrolled growth. To understand all this, however, we have to first understand how the cellular information is encoded in the DNA. Therefore, this chapter is about the Genetic Code.

In the last chapter we have learnt that chromosomes are located in the nucleus. Chromosomes consist of long rows of a substance called DNA. We will not go into the chemistry of DNA, which would be beyond the scope of this tutorial. So just keep in mind that DNA consists of four different building molecules, which are connected with each other as shown in the Figure.The name of these blocks is A,T,G, and C. The order of the A´s, T´s, G´s and C´s appears to be random but as you will soon see it is not. There are two other important observations here: 1. DNA is linear starting from one end which we call the 5´-end and ending at the 3´-end. This is related to the chemistry of the building blocks. 2. DNA is usually double-stranded and the complementary strand is paired in the opposite direction. The rule is: guess it and then go with the mouse over the Figure. Right! ´A´ will pair with ´T´ and ´G´ will pair with ´C´.

Before we now continue we must have a closer look at proteins. Proteins consist of 20 different building blocks called Amino Acids (Important: protein sequences like DNA sequences are linear.). All amino acids have an Amino- and a Carboxy-terminal end. These are chemical groups which can be connected as shown in the next Figure. The bond connecting amino acids is called Peptide Bond. There are small peptides consisting of only a few amino acids and large ones called Polypeptides or proteins consisting of hundreds or even several thousand of amino acids.

Let´s go back to the DNA now and have a closer look at Fig. 2. Again there is DNA but only one strand is shown, because only one strand of the two strands will code for a particular protein. All we have to do now is a simple calculation. There are 20 amino acids and 4 Bases. If one base codes for one amino acid then there are only four possibilities. This is not sufficient to cover all 20 amino acids. If 2 bases code for one amino acid there are 16 possibilities. Again that is not sufficient. What is left? Of course 3 bases code for one amino acid resulting in 64 possibilities. And exactly that was experimentally confirmed. However, this also means that several different codons code for the same amino acid, which was experimentally confirmed. For some amino acids there exists only one codon and for others there exist up to 6 codons. Three codons, however, do not code for any amino acid and these are called Stop Codons. They are important for the termination of protein synthesis as we will see later.

What is next? We will now be able to understand what is a Mutation. So the next chapter is about mutations.