The use of enzymes to manipulate DNA
Enzymes play a crucial role in gene technology, acting as molecular tools that allow scientists to cut, join, and replicate DNA with precision. This section will explore some of the key enzymes used to manipulate DNA in a laboratory.
Use this page to revise the following concepts of nucleic acids:
Restriction Enzymes/Endonucleases
Restriction enzymes, also called endonucleases, are enzymes that make targeted cuts to specific DNA sequences. This targeted cutting of DNA is essential for many DNA manipulation technologies, such as gene cloning .
Often referred to as molecular scissors, restriction enzymes recognize and cut DNA at specific sequences, known as recognition sites. These sites are typically short, palindromic sequences of 4-8 base pairs. Each restriction enzyme is highly specific and binds only to its unique recognition sequence. For example, the enzyme EcoRI recognizes the sequence 5’-GAATTC-3’.
Once the enzyme binds to its recognition site, it cleaves the DNA. This cut can result in two types of ends:
- Sticky ends: The enzyme cuts the DNA in a staggered manner, leaving single-stranded overhangs. These overhangs can easily pair with complementary sequences, making them useful for joining different DNA segments in genetic modification and transformation. An example is the restriction enzyme EcoRI, which produces sticky ends. The recognition sequence for EcoRI is shown in the image below, as well as the sticky (overhanging) ends it creates.
- Blunt ends: The enzyme cuts straight across both strands of DNA, leaving no overhangs. The enzyme SmaI is an example of an endonuclease that produces blunt ends.
There are both advantages and disadvantages of sticky and blunt ends when used in gene cloning. The table below explains each.
| Advantages | Disadvantages | |
|---|---|---|
| Sticky ends | Sticky ends can easily pair with complementary sequences through base pairing (hydrogen bonding), making ligation more efficient. This ensures precise and specific joining of DNA fragments. The complementary overhangs ensure that only the correct DNA fragments are joined, reducing errors in cloning. | Sticky ends produced by different restriction enzymes are often not compatible, limiting flexibility in combining fragments unless the same enzyme is used for all fragments. Single-stranded overhangs are more prone to degradation by nucleases, which can affect the success of the cloning process. Ligation depends on the presence of complementary overhangs, which can limit experimental design if appropriate enzymes are unavailable. |
| Blunt ends | Any two blunt ends can be ligated together, regardless of the source or the restriction enzyme used. This provides more flexibility in designing experiments. Blunt ends are less prone to degradation since they lack single-stranded overhangs. | Blunt ends lack base pairing to guide the ligation, so ligation relies entirely on DNA ligase , which is less efficient compared to sticky ends. Blunt-end ligation can occur in either orientation, leading to a higher chance of incorrect insertion of DNA fragments. |
DNA ligase
DNA ligase works by joining two DNA strands together through the formation of a phosphodiester bond in the sugar-phosphate backbone of DNA. This process is essential for repairing breaks in DNA and creating recombinant DNA in genetic engineering.
DNA ligase binds to the broken ends of DNA, such as those created by restriction enzymes or during DNA replication. It can join both sticky ends (with overhangs) and blunt ends (straight cuts), although ligating blunt ends is less efficient.
The enzyme catalyses a reaction between the 3'-hydroxyl group of one DNA strand and the 5'-phosphate group of the adjacent DNA strand. This forms a covalent phosphodiester bond, sealing the gap and restoring the integrity of the DNA backbone.
DNA ligase in the creation of recombinant plasmids
Plasmids, which are small circular strands of DNA found in bacteria, are often used in gene technology. First, the plasmid is cut with a restriction enzyme, such as EcoRI. The gene to be inserted into the plasmid is also cut with the same restriction enzyme. The two are mixed in the presence of DNA ligase to form a recombinant plasmid, as shown below.
DNA Polymerase
DNA polymerase is an essential enzyme in gene technology, as it is responsible for synthesising new DNA strands by adding nucleotides complementary to a template strand during DNA replication. In biotechnology, different types of DNA polymerases are used for various applications, including:
- Polymerase Chain Reaction (PCR)
- DNA sequencing
- Editing DNA
DNA polymerase is used by organisms to replicate their DNA during DNA replication. Genetic engineers commonly use Taq Polymerase, which is a DNA polymerase from the bacterium Thermus aquaticus, as it can withstand very high temperatures.
DNA polymerase enables the rapid synthesis of large amounts of DNA, which is essential for techniques like PCR. By enabling DNA amplification, sequencing, and editing, DNA polymerase has become crucial in genetic engineering, medical diagnostics, and molecular biology research.
