## 10.2 Molecular Scissors: Restriction Endonucleases **Restriction Endonucleases**: These are enzymes that recognize specific DNA sequences and cleave the phosphodiester bonds of both DNA strands within that sequence. They are crucial tools for cutting a gene of interest during genetic recombination. ### Natural Function: A Bacterial Defense System * Restriction endonucleases serve as a natural defense mechanism in bacteria against invading viruses (bacteriophages). * They "restrict" viral growth by digesting (cutting up) the viral DNA at specific recognition sites. * The bacterial cell's own DNA is protected from its own restriction enzymes through a process called *methylation*. ### Host DNA Protection: Methylation * Bacteria add a methyl group ($\text{CH}_3$) to nucleotides within their own restriction sites. * This modification is carried out by an enzyme called *methyltransferase*. * The added methyl groups block the restriction endonuclease from binding and cleaving the host's DNA, while the un-methylated viral DNA remains vulnerable. ### Nomenclature of Restriction Enzymes * Enzymes are named after the genus, species, and strain of the bacteria from which they were isolated. * **Examples:** * ***EcoRI***: Isolated from ***E***scherichia ***co***li (strain **R**Y13, **I** indicating the first enzyme identified). * ***Hind-II*** & ***Hind-III***: Isolated from ***H***aemophilus ***in***fluenzae (strain **d**). * ***XhoI***: Isolated from ***X***anthomonas ***ho***lcicola. ### Recognition Sites and Types of Cuts * ***Palindromic Sequences***: Restriction enzymes recognize specific sequences of 4 to 8 base pairs that are *palindromic*. This means the nucleotide sequence on one strand reads the same in the opposite direction on the complementary strand (e.g., 5'-GAATTC-3' on one strand and 3'-CTTAAG-5' on the other). * **Types of Cuts:** 1. **Staggered Cuts (Sticky Ends)**: The enzyme cuts the two DNA strands at different points within the recognition site, creating single-stranded overhangs called **sticky ends**. These ends are "sticky" because they can easily form hydrogen bonds with complementary sequences. * **Example (EcoRI)**: Produces sticky ends at the 5' end of each strand. 2. **Blunt Cuts**: The enzyme cuts both DNA strands at the same point, resulting in fragments with no overhangs. These are called **blunt ends**. * **Example (HaeIII)**: Cuts directly in the middle of its recognition sequence (GGCC). ### Possible Questions/Answers **Q: Why don't restriction endonucleases destroy the DNA of their host bacterium?** **A:** The host bacterium protects its own DNA by adding methyl groups ($\text{CH}_3$) to the nucleotides in its recognition sequences. This process, called methylation, blocks the restriction enzyme from binding and cutting the host's DNA. **Q: Why are restriction enzymes that create sticky ends more useful in genetic engineering?** **A:** Sticky ends are single-stranded overhangs that are complementary to each other. This allows a DNA fragment (like a gene of interest) and a cloning vector (like a plasmid) cut with the same enzyme to easily join together (anneal) through base pairing, which greatly facilitates the process of creating recombinant DNA.