In the first step of our process, we see a pre-mRNA molecule. This molecule consists of two intron regions shown in orange, flanking a single exon region shown in green. The entire structure is labeled as mRNA.
In the next step, we observe the beginning of the splicing process. The intron region starts to form a loop structure, which brings the two exon regions closer together. This looping is a critical part of the self-splicing mechanism.
As splicing continues, the intron is completely excised from the mRNA molecule. It forms a characteristic lariat structure. Meanwhile, the two exon regions are ligated together, creating a continuous mature mRNA molecule.
After being excised from the mRNA, the intron does not immediately leave the cell. Instead, it remains within the nucleus as a lariat structure. This is where it will eventually be degraded.
Now we turn to the DNA template. The gene from which our mRNA was transcribed contains similar structural elements: exons shown in green and introns shown in orange. This DNA serves as the template for RNA polymerase II to create mRNA.
RNA Polymerase II, shown in purple, binds to specific regions of the DNA template. It typically associates with the boundaries between exons and introns, preparing to transcribe the gene into pre-mRNA.
Multiple RNA Polymerase II complexes can bind to a single gene. Here we see three polymerase complexes: two are positioned at the exon-intron boundaries, and one is free, ready to initiate transcription.
With the polymerase complexes properly positioned, transcription initiation begins. The DNA template is now being transcribed into a nascent RNA molecule, which will eventually become our pre-mRNA.
Splicing doesn't wait for transcription to finish. It can occur co-transcriptionally, meaning while RNA Polymerase II is still synthesizing the RNA. Here we see the intron beginning to form a loop structure during ongoing transcription.
The intron continues to form its characteristic lariat structure. This happens while the RNA Polymerase II complexes are still engaged with the DNA template, demonstrating co-transcriptional splicing.
As splicing completes, the exons are ligated together to form mature mRNA molecules. The intron sequences are excised as lariat structures. Here we see multiple mature mRNA molecules being produced from a single DNA template.
Once the mRNA molecules are mature, they are exported from the nucleus to the cytoplasm. This transport is essential for the mRNA to reach the ribosomes where protein synthesis occurs.
Interestingly, mRNA molecules can also re-enter the nucleus. This re-entry allows for additional regulatory processes and feedback mechanisms in gene expression.
When mRNA re-enters the nucleus, it can bind to specific DNA sequences. This binding is part of regulatory mechanisms that control gene expression, potentially influencing the transcription of other genes.
The bound mRNA can attract RNA Polymerase II complexes to the DNA. This attraction facilitates the re-initiation of transcription, creating a feedback loop in gene regulation.
With multiple RNA Polymerase II complexes working on a single DNA template, several mRNA molecules can be produced simultaneously. This amplification mechanism allows cells to rapidly increase the production of specific proteins when needed.