Polymerases are enzymes that build long chains of nucleic acids, the fundamental molecules of life. They assemble DNA and RNA by linking together smaller building blocks called nucleotides. This process, known as polymerization, copies and expresses genetic information within cells. Polymerases create the genetic material that defines every living organism.
The Role in DNA Replication
DNA polymerases are responsible for copying a cell’s genome before cell division. This copying process involves the polymerase reading an existing DNA strand, known as the template, and then adding complementary nucleotides to construct a new strand. The enzyme works by adding new nucleotides to the 3′ end of the growing strand, elongating it in a 5′ to 3′ direction.
DNA polymerase cannot begin synthesizing a new chain on its own; it requires a short pre-existing segment called a primer. This primer provides the necessary starting point for the polymerase to attach and begin adding nucleotides. In prokaryotes, DNA polymerase III is the primary enzyme for replication, synthesizing new DNA strands.
DNA polymerase I in prokaryotes plays a different role, removing the RNA primers from the newly synthesized strands and filling in the resulting gaps with DNA. In eukaryotes, various DNA polymerases specialize in different aspects of replication.
The Role in Gene Expression
RNA polymerases facilitate transcription, the initial step in gene expression where genetic information from DNA is converted into RNA. This process allows the cell to produce proteins and other functional RNA molecules. RNA polymerase binds to specific regions of DNA called promoters, then unwinds a small section of the DNA double helix.
Unlike DNA polymerases, RNA polymerases do not require a primer to initiate RNA synthesis. The enzyme synthesizes a messenger RNA (mRNA) copy of a specific gene by adding nucleotides that are complementary to the DNA template strand. This newly formed RNA molecule grows in the 5′ to 3′ direction, transcribing the genetic code.
RNA polymerase ensures that only one of the two DNA strands serves as a template for transcription. This selective copying results in a single-stranded RNA molecule that carries the genetic message. The production of mRNA by RNA polymerase guides cellular processes.
Essential Proofreading and Repair Functions
Maintaining the integrity of genetic information is important, and DNA polymerases contribute to this through their proofreading capabilities. DNA polymerases possess a 3’→5′ exonuclease activity, which acts as an immediate error-correction mechanism. If an incorrect nucleotide is inserted during DNA synthesis, the polymerase can detect the mismatch.
Upon detecting a mispaired base, the polymerase pauses and moves backward, removing the wrongly added nucleotide from the growing DNA strand. This “backspace” function ensures high fidelity in DNA replication, greatly reducing the error rate.
Beyond real-time proofreading, DNA polymerases also participate in broader DNA repair pathways. After other enzymes remove damaged or incorrect DNA segments, DNA polymerases fill the resulting gaps with the correct nucleotides. This combined action of immediate proofreading and participation in repair mechanisms safeguards the genetic code.
Applications in Research and Medicine
The properties of polymerases have been harnessed in scientific research and medical diagnostics. The Polymerase Chain Reaction (PCR) is a widely used technique that makes millions of copies of a specific DNA segment. This process relies on a heat-stable DNA polymerase.
This polymerase can withstand the high temperatures required to separate DNA strands during PCR, allowing the reaction to proceed through multiple cycles of denaturation, primer annealing, and extension. PCR is employed in various fields, including genetic testing, forensic science for DNA fingerprinting, and diagnosing infectious diseases by amplifying viral DNA.
Reverse transcriptase is a specialized polymerase found in retroviruses. This enzyme synthesizes DNA from an RNA template, a process known as reverse transcription. In laboratories, reverse transcriptase is utilized in techniques like reverse transcription PCR (RT-PCR) to convert RNA samples into DNA, enabling the study of gene expression and the detection of RNA viruses.