Eukaryotic gene transcription is the fundamental biological process by which genetic information encoded in DNA is accurately copied into RNA molecules. This process serves as the initial step in gene expression, where the instructions within our genes are used. It forms the basis for all cellular activities, from the growth and development of an organism to its responses to environmental cues. Understanding how transcription occurs in eukaryotic cells, which include all animals, plants, fungi, and protists, is important for comprehending how these organisms function.
Key Players in Transcription
Transcription in eukaryotic cells involves several molecular components. At its core is the DNA template, specifically the gene containing the instructions to be copied. The main enzyme responsible for synthesizing the RNA strand is RNA polymerase, with RNA polymerase II specifically transcribing protein-coding genes. This large, complex enzyme, composed of multiple subunits, moves along the DNA, reading its sequence.
For RNA polymerase II to initiate transcription, it requires the assistance of various proteins known as general transcription factors. These factors assemble at specific DNA sequences called promoter regions, which act as start signals for transcription. A common promoter element for RNA polymerase II is the TATA box, found upstream of the transcription start site. Beyond the promoter, other regulatory DNA sequences called enhancer regions can also influence gene expression. These enhancers can be located at varying distances from the gene and bind additional transcription factors to regulate transcription.
How Genes Are Copied
The copying of genes into RNA in eukaryotes proceeds through three distinct stages: initiation, elongation, and termination. Initiation begins with the assembly of RNA polymerase II and general transcription factors at the promoter region on the DNA. This assembly forms a pre-initiation complex, which unwinds the DNA double helix, making the template strand accessible.
During elongation, RNA polymerase II travels along the DNA template strand, synthesizing a complementary RNA molecule in the 5′ to 3′ direction. As the polymerase moves, it unwinds the DNA ahead and re-winds it behind. The enzyme accurately adds ribonucleoside triphosphates, ensuring that the new RNA strand reflects the genetic code of the DNA template.
Transcription concludes with termination, where the newly synthesized pre-mRNA molecule is released from the DNA template. Eukaryotic termination mechanisms are often coupled with subsequent RNA processing events. For protein-coding genes transcribed by RNA polymerase II, termination is linked to the recognition of a poly(A) signal sequence in the nascent RNA, which triggers cleavage of the RNA and dissociation of the polymerase.
Making RNA Ready for Action
Once the pre-mRNA molecule is transcribed in eukaryotic cells, it undergoes a series of post-transcriptional modifications within the nucleus to become a mature messenger RNA (mRNA) ready for protein synthesis. These modifications are a distinguishing feature of eukaryotic gene expression, ensuring the mRNA’s stability, proper transport, and efficient translation.
One of the first modifications is the addition of a 5′ cap to the beginning of the RNA molecule. This cap is added early in transcription. The 5′ cap plays a protective role, shielding the mRNA from degradation by enzymes and aiding in its export from the nucleus to the cytoplasm. It also helps ribosomes recognize the mRNA for the initiation of protein synthesis.
Another modification is splicing, the process of removing non-coding regions called introns from the pre-mRNA and joining the remaining coding regions, called exons. Introns are “intervening sequences” that do not carry information for protein synthesis, while exons are “expressed sequences.” This removal and joining are carried out by a complex machinery called the spliceosome. Splicing allows for alternative splicing, where different combinations of exons can be joined from a single pre-mRNA, leading to the production of multiple protein variants from one gene.
The final major modification is 3′ polyadenylation, which involves adding a string of adenine nucleotides, known as a poly-A tail, to the 3′ end of the pre-mRNA. This tail is added after the RNA molecule is cleaved at a specific site. The poly-A tail contributes to mRNA stability, protecting it from degradation, and is also involved in its nuclear export and efficient translation.
Controlling Gene Activity
Not all genes in a eukaryotic cell are actively transcribed at all times; instead, gene activity is regulated to ensure proper cellular function and response to environmental changes. This control is achieved through various mechanisms that influence whether and when a gene is copied into RNA. A control point for many genes is at the level of transcription itself.
One regulatory mechanism involves transcription factors, which are proteins that bind to specific DNA sequences near a gene to influence its transcription. These factors can act as activators, promoting the recruitment of RNA polymerase and general transcription factors to increase gene expression. Conversely, repressors can bind to DNA and decrease transcription by inhibiting the binding or function of activators or the transcriptional machinery. The specific combination of activators and repressors present in a cell helps determine which genes are active.
Beyond specific transcription factor binding, the packaging of DNA into chromatin also plays a role in gene regulation. DNA in eukaryotic cells is wrapped around proteins called histones to form nucleosomes. The accessibility of a gene for transcription can be altered by chromatin remodeling, where the arrangement or chemical modification of histones changes how tightly the DNA is wound. For instance, histone acetylation tends to “open up” the chromatin, making genes more accessible for transcription, while histone methylation can lead to a more condensed, transcriptionally inactive state.