10.2 Molecular Scissors: Restriction Endonuclease

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 Escherichia coli (strain RY13, I indicating the first enzyme identified).
  • Hind-II & Hind-III: Isolated from Haemophilus influenzae (strain d).
  • XhoI: Isolated from Xanthomonas holcicola.

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.