Gene Expression

Transcription

The two major questions are: 1. how is a single protein made starting from a chromosome, which contains the information for many different proteins. 2. how is the amount of proteins regulated although there are only two gene copies for each protein present in the chromosomes (since there are two identical copies of each chromosome except for the sex chromosomes). To achieve this, copies from single genes are made and the number of copies is tightly regulated. These copies exist in form of Messenger RNA.

To understand transcription we need to have a look at the structure of a Gene on the chromosome first. A gene contains all the coding sequence, which is located in what is called Exons interrupted by non-coding sequences called Introns, which are later excised (see also next chapter). Upstream from these sequences there is the promotor, which is the binding site for the RNA-synthesizing enzyme RNA Polymerase and one or more upstream sequences, which are binding sites for transcription factors. These are a lot of new terms, so explanations will follow now.

How is messenger or mRNA synthesized? The enzyme doing this is called RNA polymerase and consists of several different proteins, which are also called cofactors. RNA polymerase can bind to certain DNA regions, promotors, which are the starting points for mRNA synthesis. This process is called Transcription. However, if there is no further regulation, the enzyme would synthesize mRNA for all genes at the same time. Therefore, there is a mechanism, which regulates the synthesis of mRNA. The enzyme needs other factors to be able to initiate transcription. The regulation comes in form of Transcription Factors. These are proteins, which recognize very specific DNA sequences located upstream of the start site for RNA polymerase of a particular gene. Only when a transcription factor has bound to such a specific sequence, RNA polymerase can start synthesizing mRNA. The RNA polymerase moves then along the DNA and the mRNA is being synthesized until its full length. To the ´top´ or 5´ end of the mRNA a structure is added called Cap. We will here more about this in the lecture about ´Translation´.

Of course as expected there are many transcription factors, which recognize different sequences, and genes have different upstream sequences. It gets even more complicated, since many genes are regulated by more than one transcription factor. To make the confusion complete, transcription factors can also inhibit transcription of some genes, but that would go much too far for the scope of this tutorial.

It is clear that such a system is sensitive to changes, since the amount of a mRNA for a particular protein depends on the amount and activation status of transcription factors. Whoops, did I mention ´amount and activation´? Yes, I did, which means that transcription factors themselves are regulated. As we have already discussed previously, there are protein kinases and these enzymes modify proteins by attaching phosphate residues (negatively charged). Once modified, proteins behave differently from their unphosphorylated status and are either active or not active. Thus a transcription factor can be phosphorylated and this can cause its activation for example. It now binds to its specific DNA sequence and allows genes to be transcribed. In cancer cells certain protein kinases are constantly activated. Do you see the point? Transcription of certain genes would also continue constantly and be deregulated. mRNA Processing At this stage the RNA is not yet ready for translation into proteins. It has to be further processed. Let´s have look once more at the mRNA. The RNA still contains stretches inside the coding region (in black), which are called Introns and which are excised. The RNA also contains stretches at the 3´end, which are cut off as shown in the next figure. All mRNA´s with a few exceptions contain a sequence at the 3´end, which consists only of the nucleotide Adenine (A) and is called PolyA sequence. This sequence is added to the 3´end of the mRNA by an enzyme. The final step now is the removal of the introns, which is a complicated process executed by a large complex of proteins called the Splicosom, since this process is called splicing. Now finally the mRNA is ready to be translated, which we will be shown in the next lesson.

Let´s review this lesson. Genes are transcribed by RNA polymerase with the help of transcription factors. The product of transcription is mRNA. Transcription is usually changed in cancer cells due to mutations. To view the process of transcription again let´s look at a little animation. The RNA is then processed in several steps until it is mature. What comes next? After the synthesis of mRNA´s and their processing they are translated into proteins. The next lesson deals with the mechanism of Translation.