Klenow fragment
Klenow Fragment is a large fragment of the DNA polymerase I enzyme that is used extensively in molecular biology for adding nucleotides to the 3' end of a DNA strand. The fragment is named after Danish biochemist Hans Klenow who first isolated it in the early 1970s. Unlike the complete DNA polymerase I enzyme, which possesses both 5'→3' polymerase activity and 5'→3' and 3'→5' exonuclease activities, the Klenow Fragment has only the 5'→3' polymerase activity and the 3'→5' exonuclease activity, making it useful for certain DNA sequencing and DNA labeling techniques.
Structure and Function[edit]
The Klenow Fragment is derived from the full-length DNA polymerase I by proteolytic cleavage, which removes the 5'→3' exonuclease domain of the enzyme. This leaves the 5'→3' polymerase and 3'→5' exonuclease activities intact. The 3'→5' exonuclease activity provides a "proofreading" function, allowing the enzyme to remove incorrectly incorporated nucleotides, thereby increasing the fidelity of DNA synthesis.
Applications in Molecular Biology[edit]
The Klenow Fragment has several applications in molecular biology, including:
- DNA sequencing: It is used in some methods of DNA sequencing, where its ability to synthesize DNA in the 5'→3' direction can be harnessed to extend primers annealed to a template strand.
- DNA labeling: The enzyme can be used to add labeled nucleotides to the 3' end of DNA fragments, which is useful in various types of DNA probes for hybridization experiments.
- Blunting of DNA ends: The Klenow Fragment can be used to convert sticky ends of DNA fragments into blunt ends, facilitating the cloning of DNA fragments into vectors that only accept blunt-ended fragments.
Advantages and Limitations[edit]
The Klenow Fragment offers high fidelity in DNA synthesis due to its 3'→5' exonuclease activity. However, its use is limited by its sensitivity to heat denaturation and the inability to displace strands during synthesis, unlike some other DNA polymerases such as Taq polymerase.
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