Team:Groningen/Summerschool Biobricks


Short guide to genetic constructs

by Glenn Alan Mulder
Every organism exists out of one or multiple cells. All cells, no matter what type they are, can exist and maintain themselves due to the constant renewal of the proteins of which they are composed and the continuous dismantling of proteins they use to make new proteins of. This dismantling is done by enzymes (which are a specific type of protein). These enzymes, just like the cells own proteins, which it needs in order to exist, are created according to something not unlike a cellular blueprint, known as DNA.

DNA is basically written in groups of three base-pairs. Each sequence of three base-couples, called codons, codes for a specific amino-acid, which are the basic bricks for proteins. This means that a line of seven groups of codons stands for a sequence of seven amino-acids. In this way, al long strand of codons can code for a specific protein.

Since bacterial DNA is one long string of base-pairs, and an amino-acid-sequence is not that long, the cell needs a mechanism to 'know' where to start and stop the sequence and thus know where the beginning and end of a protein exactly is. For that two of the codons have special meanings. One is the Start- codon, which means that the cell will know where to start the amino-acid-sequence. The other is the Stop-codon, indicating where the cell has to stop the amino-acid-sequence and finish the protein.

Now another thing complicates the creation of proteins. Not every protein has to be made all the time. Some proteins only need to be made once a certain substance is detected. Only if the substance is present the cell should make the protein. For this problem the cell has a simple solution. On some places on the DNA there are places where the transcriptase (the protein that is responsible for the reading of DNA and making sure that a protein is formed) is capable of binding. On these places it could bind if the cell is under the right conditions for the making of the proteins that are written in the strand following this binding-site.

So if the cell is exposed to the right conditions, the transcriptase is capable of binding to the DNA, and starts transcribing it as soon as it finds a start-codon untill it finds a stop-codon and then starts transcribing again as soon as it finds a start-codon. If the cell is exposed to other conditions, a different protein binds to the DNA at the bind-site, and thus prevents the transcriptase of binding. This bind-site is called the Promotor.

However, the transcriptase is not supposed to transcribe the entire DNA just because it could bind on one spot. Therefore the transcriptase should at a certain point, after it has transcribed all the needed codons, encounter a codon that tells it to stop transcribing and to unbind. This codon is called the Terminator.

So in essence, once a promotor is activated, a transcriptase binds there and follows the DNA strand until it reaches a start-codon, where it starts to transcribe until it finds a stop-codon. It keeps doing this until it comes in contact with a terminator, where the transcriptase releases the DNA.

Synthetic biologists use this knowledge to make their own working genetic divices. A sequence existing out of a Promotor, several [start-codon - protein coding sequence - stop codon] sequences and a Terminator that responds under a specific condition by setting in motion the chain of reactions that produces a protein is called a genetic construct.