Helicases are responsible for the unwinding of double-stranded DNA to produce single-stranded DNA. They are responsible for the DNA unwinding during DNA replication, recombination, and repair. The unwinding of double-stranded DNA starts at the origin of replication and continues to form a structure known as the replication fork. The breakdown of hydrogen bonds between the two DNA strands requires energy in the form of ATP. The helicases also trap unwound bases to prevent the reannealing of DNA.
Key Areas Covered
1. What are DNA Helicases
– Definition, Features
2. How Does the DNA Get Unwind and Stay Unwind
– Process of DNA Unwinding
Key Terms: DNA Helicase, DNA Replication, Initial DNA Melting, Origin Recognition, Replication Fork
What are DNA Helicases
DNA helicases are the fundamental components of DNA replication. The main function of DNA helicases is to unwind double-stranded DNA to form single-stranded DNA. In addition to DNA replication, DNA helicases are also involved in transcription, translation, recombination, and DNA repair. A prokaryotic DNA helicase is shown in figure 1.
Figure 1: Prokaryotic DNA Helicase
How Does the DNA Get Unwind and Stay Unwind
DNA is a double-stranded molecule that serves as the hereditary material in most organisms. The two strands of DNA are held together by hydrogen bonds. New DNA is synthesized by a process known as DNA replication. DNA replication is a semiconservative process where both strands serve as templates. Hence, the two strands have to be unwound in order to initiate DNA replication.
DNA helicases are the enzymes that catalyze the DNA unwinding. The unwinding of DNA initiates DNA replication. Origin recognition, initial DNA melting, and eventual formation of the replication fork are the three steps involved in the initiation of DNA replication.
- Origin recognition – DNA replication is initiated at the origin of replication. Several origins of replication can be found in chromosomes. Circular, double-stranded DNA consists of a single origin of replication. A multi-subunit DNA binding complex known as origin recognition complex (ORC) is responsible for the recognition of the origin of replication.
- Initial DNA melting – MCM (mini-chromosome maintenance) helicase is responsible for the initial melting of origin of replication in eukaryotes. In prokaryotes, it is done by the origin recognition protein, DnaA and a hexameric helicase known as DnaB is then loaded into the melted DNA.
- Formation of replication fork – The helicases continue the unwinding process, forming a structure called replication fork. They break down the hydrogen bonds that hold the two complementary strands together. They use cellular energy in the form of ATP for this process.
Initiation of DNA replication in eukaryotes is shown in figure 2.
Figure 2: Initiation of DNA Replication in Eukaryotes
After the initial melting of the double-stranded DNA, DNA polymerase binds to the origin of replication and starts the replication process. As the replication progresses, the replication fork progresses through the unwound DNA strand. Since these DNA helicases are trapped between the two strands, the reannealing of complementary bases is avoided.
Conclusion
DNA helicases are the enzymes responsible for the unwinding of DNA to form single-stranded DNA required by DNA replication, recombination, and repair. They break down hydrogen bonds between complementary bases of the two strands that hold the two strands together. The trapped DNA helicases between the unwound DNA prevent the reannealing.
Reference:
1. Gai, Dahai, et al. “Origin DNA Melting and Unwinding in DNA Replication.” Current Opinion in Structural Biology, U.S. National Library of Medicine, Dec. 2010, Available here.
Image Courtesy:
1. “Helicase” By Phoebus87 at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “EukPreRC” By Lsanman – Own work (CC BY-SA 3.0) via Commons Wikimedia
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