Helicase enzymes are known for unzipping the DNA double helix during DNA replication, where the entire genetic code is duplicated. This raises the question of whether helicase performs the same function during transcription, the process of creating an RNA copy of a gene. Although both processes require separating DNA strands, the primary enzymes and mechanisms involved are different.
The Role of Helicase in DNA Replication
During DNA replication, a cell duplicates its entire genome. For this to happen, the two strands of the DNA double helix must be separated, which is the primary job of DNA helicase. This enzyme moves along the DNA, breaking the hydrogen bonds between base pairs. This unwinding creates a replication fork, providing the single-stranded templates for DNA polymerase to synthesize new strands.
The action of helicase in replication is extensive, as it unzips large portions of the chromosome. This ensures each new DNA molecule is an identical copy of the original, containing one old and one new strand. The continuous separation of the DNA strands is driven by the ATP-powered action of DNA helicase.
Unwinding DNA for Transcription
Transcription copies a specific gene into a messenger RNA (mRNA) molecule. The primary enzyme, RNA polymerase, is responsible for both synthesizing RNA and unwinding the DNA. Unlike replication, a separate helicase is not the main enzyme used, as RNA polymerase has its own intrinsic helicase-like activity to separate the DNA strands.
As RNA polymerase moves along a gene, it unwinds a small portion of the DNA, creating a temporary “transcription bubble” about 10-20 base pairs long. The enzyme uses one DNA strand as a template to build the RNA molecule. As the polymerase advances, the DNA double helix rewinds behind it, so only a small segment is unwound at any moment.
This localized unwinding is a major distinction from the extensive separation seen in DNA replication. This method ensures the rest of the chromosome remains stable while allowing targeted access to genetic information.
The Function of Transcription Factors
RNA polymerase does not work alone during transcription. To begin, it requires help from proteins called general transcription factors, which assemble at the gene’s starting point, or promoter. One of these complexes, Transcription Factor II H (TFIIH), has a subunit with helicase activity.
The helicase component of TFIIH functions at the start of transcription. It pries open the DNA double helix at the promoter, creating the initial opening for RNA polymerase to bind to the template strand. This ATP-dependent activity allows RNA synthesis to begin.
Once RNA polymerase begins moving and creating the RNA strand, the unwinding function is taken over by the polymerase itself. The helicase activity of TFIIH is therefore confined to the initiation phase of transcription.
Comparing the Unwinding Processes
The unwinding of DNA in replication and transcription differs in the enzymes involved. Replication uses a dedicated DNA helicase, while transcription primarily relies on the intrinsic activity of RNA polymerase, with an initial assist from TFIIH.
The scale of unwinding is another distinction. Replication unwinds vast sections of DNA to prepare the entire genome for duplication. Transcription involves creating a small, transient bubble that moves along a single gene, with the DNA reforming immediately after.
The purpose also differs. Replication aims to create two complete copies of the genome for cell division. Transcription’s goal is to produce an RNA copy of a single gene to be used for protein synthesis.
Additional Helicase Functions in Gene Expression
Proteins with helicase activity are involved in more than just initiating transcription. For instance, the bacterial helicase protein Rho helps terminate transcription. Rho moves along the new RNA transcript and helps dissociate it from the DNA template, ending the process.
Helicase domains are also found in proteins that perform chromatin remodeling, which is the loosening of the DNA-protein complex. For a gene to be transcribed, the chromatin must be opened to allow access for enzymes. Chromatin remodeling complexes use helicase-like activity to reposition parts of the chromatin, regulating gene accessibility.