Restriction Enzymes, DNA Palindromes and Ligation

Nucleases can remove or cut DNA

Nucleases are enzymes that cleave phosphodiester bonds in nucleic acids. Exonucleases remove nucleotides sequentially from accessible DNA or RNA ends; endonucleases cut within a nucleic acid strand. Restriction endonucleases are a particular group, typically associated with bacterial defence against foreign DNA. Recognition of a defined DNA sequence allows cleavage at or near a particular site. Methylation of matching sites in host DNA can protect the producing bacterium in a restriction–modification system.

The word restriction historically describes the ability of some bacteria to restrict invading bacteriophages. It is useful to distinguish the native protective system from the laboratory application: researchers deliberately use purified restriction enzymes to cut DNA into predictable fragments. A nuclease alone cannot insert DNA into a plasmid. Further components, especially a carrier vector and ligase, are needed to join fragments and maintain them.

Names and recognition sites

An enzyme's name often refers to the organism and strain from which it was first isolated. EcoRI, for example, derives from Escherichia coli, with a strain designation and Roman numeral reflecting the discovery sequence. HindII and other historically characterised restriction enzymes provided key evidence that recognition and cutting can be sequence dependent. Each enzyme has a particular specificity; some recognise short sites and others more complex motifs.

Many familiar type II restriction enzymes recognise palindromic double-stranded sites. For EcoRI the paired strands can be written with their opposite polarities as 5′-GAATTC-3′ and 3′-CTTAAG-5′. Read both strands in the 5′→3′ direction and the sequence is GAATTC in each case. This is a DNA palindrome. It is not correct to read both displayed strands left to right without accounting for their antiparallel orientations. Not every restriction enzyme follows the same simple palindromic pattern.

EcoRI makes staggered cuts and cohesive ends

EcoRI recognises the six-base-pair site GAATTC and cuts the upper strand between G and A while cutting the complementary strand at the symmetric position: 5′-G|AATTC-3′ and 3′-CTTAA|G-5′. The vertical marks denote cleavage of the sugar–phosphate backbones, not breaking of every hydrogen bond at once. The resulting DNA ends have single-stranded 5′ overhangs of AATT. These protruding stretches are commonly called sticky or cohesive ends.

Two DNA fragments with complementary overhangs can temporarily base-pair through hydrogen bonds. This helps align ends from an insert and a vector. The resulting association is not yet a permanently joined DNA backbone: DNA ligase must seal compatible nicks through phosphodiester-bond formation. When source DNA and vector are cut using enzymes that create suitable compatible ends, the molecules can be assembled into recombinant DNA.

DNA ligase creates a continuous backbone

DNA ligase catalyses the formation of a phosphodiester bond between adjacent nucleotides with the correct chemical ends. In normal DNA repair and replication, ligases seal nicks in DNA strands. In recombinant DNA work, the same chemistry joins vector DNA and a selected fragment. Most widely used laboratory ligases require an appropriate cofactor, commonly ATP for T4 DNA ligase. Ligase cannot solve every end-compatibility problem: incompatible ends may need alternative construction strategies.

Successful ligation also does not prove that the desired insert is in the correct orientation or that only one insert was incorporated. A vector may re-close without an insert or acquire an incorrect fragment. For this reason, investigators distinguish DNA cutting, molecular joining and identification of correct recombinant constructs as separate stages. Selection of transformed cells and verification of their DNA follow later.

The two sources of DNA must be considered together

A source fragment may carry an interesting coding sequence, while the vector carries an origin of replication and selectable marker. Cleaving both at compatible sites creates a possible physical connection. If a recognition site occurs several times in the vector or source, additional fragments can be created; this can complicate reconstruction. Suitable cloning strategies often prefer well-characterised unique sites or other carefully designed methods of assembly.

Restriction endonucleases are not the only route to recombinant DNA in modern research, but they provide an especially clear example of specificity. The enzyme reads a nucleotide pattern; cleavage creates a chemical end; complementary ends help align pieces; ligase seals the backbone; and a compatible vector preserves the product in a host. Each step answers a different molecular problem.

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