Genetic information, stored within DNA molecules, directs the production of proteins, which carry out most cellular processes. Transcription is the initial step, where a gene’s DNA sequence is copied into a messenger RNA (mRNA) molecule. This mRNA then carries the genetic instructions out of the cell’s nucleus for protein synthesis.
Understanding Primers
In molecular biology, a primer is a short nucleic acid sequence that provides a starting point for the synthesis of a new DNA strand. This segment, typically composed of RNA, offers a free 3′-hydroxyl group. This chemical group is necessary for certain enzymes to begin adding new nucleotides, extending the growing nucleic acid chain. Without this point, these enzymes cannot initiate synthesis.
Transcription: No Primer Needed
Transcription, the process of synthesizing an RNA molecule from a DNA template, does not require a primer. The enzyme responsible, RNA polymerase, can initiate the formation of a new RNA strand without needing a pre-existing segment. This allows RNA polymerase to begin synthesis directly on the DNA template.
The Mechanism of RNA Polymerase
RNA polymerase initiates RNA synthesis from scratch, a process known as de novo synthesis. The enzyme begins by recognizing and binding to specific DNA sequences called promoters, located upstream of the genes. In eukaryotic cells, transcription factors aid this binding, positioning the RNA polymerase on the promoter.
Once bound, RNA polymerase unwinds a section of the DNA double helix, creating a transcription bubble. The enzyme accesses the single-stranded DNA template within this region. It incorporates the first ribonucleotide, followed by subsequent nucleotides, forming the RNA chain. This direct initiation, without a primer’s free 3′-hydroxyl group, is a characteristic of RNA polymerase’s action.
DNA Replication: Where Primers Are Essential
In contrast to transcription, DNA replication, the process of copying DNA, requires primers. DNA polymerase, the enzyme responsible for synthesizing new DNA strands, cannot initiate a new DNA strand on its own. It can only add new nucleotides to an existing 3′-hydroxyl group on a pre-existing strand.
To overcome this limitation, primase, a type of RNA polymerase, synthesizes short RNA primers. These RNA primers, about 10-12 nucleotides long in eukaryotes, provide the necessary free 3′-hydroxyl group for DNA polymerase to begin synthesizing the new DNA strand. Once in place, DNA polymerase extends this primer, adding deoxyribonucleotides to build the new DNA chain.
DNA replication proceeds bidirectionally from origins of replication, creating two replication forks. Due to the antiparallel nature of DNA strands and DNA polymerase’s 5′ to 3′ synthesis direction, the leading strand is synthesized continuously with one initial primer. The lagging strand is synthesized discontinuously in short segments called Okazaki fragments. Each Okazaki fragment requires its own RNA primer. After synthesis, these RNA primers are removed and replaced with DNA nucleotides, and fragments are joined to form a continuous DNA strand.