1. Recognition and Cleavage:
* Restriction endonucleases are enzymes that recognize specific, short DNA sequences called restriction sites. These sites are typically 4-8 base pairs long and are palindromic (read the same backwards and forwards).
* Once a restriction enzyme encounters its target sequence within the invading DNA, it cuts the DNA molecule at that site, breaking the phosphodiester bonds between nucleotides.
* The precise point of cleavage within the recognition site can vary, leading to sticky ends (overhanging single-stranded DNA) or blunt ends (double-stranded breaks with no overhang).
2. Protecting the Host DNA:
* Bacteria protect their own DNA from being cut by their own restriction enzymes through a process called methylation.
* Specific enzymes called methylases add methyl groups to the same bases within the restriction site, preventing the restriction enzyme from binding and cleaving the host DNA.
3. Degrading Foreign DNA:
* When a virus or other foreign DNA enters a bacterium, the restriction endonucleases cleave the foreign DNA into fragments, preventing it from replicating and causing harm.
* This degradation effectively disables the invading DNA, protecting the bacterium from infection.
In summary: Restriction endonucleases are like the bacterial immune system, acting as molecular scissors to cut and destroy foreign DNA, while their own DNA is protected by methylation. This defense mechanism is essential for the survival of bacteria in a world constantly bombarded by invading viruses.
Beyond defense: Restriction endonucleases have also become invaluable tools in molecular biology for:
* Genetic engineering: Scientists use restriction endonucleases to cut and paste DNA fragments, allowing them to create new DNA sequences and study gene function.
* DNA fingerprinting: By analyzing the restriction fragment length polymorphisms (RFLPs) generated by specific restriction enzymes, scientists can identify individuals or establish relationships between them.
