What is the purpose of restriction enzymes

Restriction enzyme

Abbreviation: REN
Synonyms: restriction endonuclease
English: restriction enzyme

1 definition

Restriction enzymes or. Restriction endonucleases, short REN, are small proteins that recognize and specifically cut specific DNA sequences. They are also known as a molecular cutting tool. The antagonists of the restriction enzymes are the so-called ligases.

2 nomenclature

As a rule, the first letter denotes the generic name of the prokaryotic cell from which the restriction enzyme originates. The following two letters indicate the type of bacterium. The next capital letter defines the serotype or the strain denomination. If more than one type of restriction enzyme occurs in one and the same prokaryotic cell, this is indicated with a Roman numeral.

3 classification

In the case of restriction enzymes, a general distinction is made between endo - and exonucleases. An endonuclease cleaves a nucleic acid within the polynucleotide strand, whereas an exonuclease cleaves terminal nucleic acids, i.e. at the end of a polynucleotide strand. Furthermore, restriction enzymes are divided into 3 types.

3.1 Type I

These types of restriction enzymes cut quite randomly and usually unpredictably far from the recognition sequence. They need an energy supply in the form of adenosine triphosphate and transfer methyl groups.

3.2 Type II

These restriction enzymes are the most common and widely used RENs. They cut exactly on the palindromic recognition sequence, which has a twofold symmetry axis and is usually 4 to 8 base pairs long. A palindromic sequence is characterized in addition to the 2-fold axis of symmetry that it has an identical coding, which is comparable to the word Otto, regardless of the side read from. These RENs do not require any energy supply in the form of ATP and also have no methyltransferase activity. Once started, the REN cuts all existing palindromic sequences. When it comes to cutting, there are two ways in which it can be cut. It either cuts the sequence 'smoothly' (blunt ends) or 'sticky' (sticky ends). Smooth means that two strands are cut in the middle. The sticky cutting variant does not cut opposing points on the two strands, but rather opposing points on the double strand by an equal amount. From a terminal point of view, this results in the same end sequences for each fragment. Thus, these individual fragments can associate with each other and form a new double strand. Of the type II REN, 2500 pieces are now known, which have a total of almost 200 different recognition sequences.

3.3 Type III

Type III restriction enzymes cut approx. 20-25 base pairs further away than the recognition sequence. ATP is required for this and methyltransferase activity is observed.

4 Occurrence and function

Restriction enzymes are present in both eukaryotic and prokaryotic cells. In prokaryotic cells, the DNA is modified in defined areas by a modification methyltransferase with methyl groups. The methyl transferase serves as a catalyst here. The REN recognize the same defined areas as the methyltransferase. In the absence of a modification to one of the two polynucleotide strands, the restriction enzyme cuts out the unmodified, apparently foreign nucleotide. This is usually done by docking a DNA polymerase to the end generated by the REN. After the correction, the two fragments are linked by a ligase. Most foreign insertions caused by phages are not methylated. This recognition site uses the REN to correct such dangerous DNA changes. Thus, the REN is an effective method for prokaryotes to defend themselves against viruses. One of the two strands of DNA is always methylated. Thus, during replication between the different mitotic phases, at least the parent strand is methylated and thus protects the daughter strand from an endonuclease.

Restriction enzymes play a very important role in genetic engineering in the areas of biological protein synthesis and in analytical methods. In genetic engineering, they are used to smuggle foreign DNA into another genome and thus induce an organism to synthesize foreign proteins and / or to look at the subsequent effects on the organism.

In analysis, restriction enzymes are used in DNA analyzes. These are converted into a specific fragmentation pattern using a restriction enzyme and can then be separated and analyzed using gel electrophoresis. In many books they only talk about type II REN, as these have the highest efficiency and other types are unprofitable for research and industrial purposes.