mRNA's stability in the cytoplasm is crucial for effective protein synthesis. Several mechanisms prevent rapid mRNA degradation. The 5' cap and 3' poly-A tail, added during post-transcriptional modifications, play a significant role. The 5' cap protects the mRNA's front end, while the poly-A tail at the back end serves as a buffer against exonucleases, enzymes that degrade RNA. Additionally, specific RNA-binding proteins associate with the mRNA, shielding it from ribonucleases and increasing its lifespan. These protective measures ensure mRNA remains intact long enough for ribosomes to translate it into proteins multiple times.

Cells can control gene expression at multiple levels, even with the separation of transcription and translation. In the nucleus, gene transcription can be regulated by transcription factors that bind to specific DNA regions, influencing RNA polymerase activity. Post-transcriptional modifications, such as alternative splicing, can produce different mRNA variants from a single gene. Once the mRNA is in the cytoplasm, its stability, rate of degradation, and accessibility to ribosomes can be controlled. Additionally, translation initiation, elongation, and termination can be regulated by various factors. This multi-tiered regulation ensures cells can swiftly respond to changes in their environment or needs by modulating protein synthesis.

If transcription and translation weren't compartmentalised, several issues could arise. Firstly, simultaneous transcription and translation could result in ribosomes attaching to incomplete mRNA, producing incomplete proteins. Secondly, without nuclear protection, DNA could be more exposed to cytoplasmic factors or reactions that might damage it. Thirdly, without compartmentalisation, the rigorous quality control checks, like splicing, capping, and polyadenylation of mRNA before translation, might be compromised. Overall, the lack of compartmentalisation would disrupt the efficiency, accuracy, and regulation of gene expression, potentially leading to non-functional proteins and cellular dysfunctions.

The nucleus has a system to ensure only matured and appropriately processed mRNA exits for translation. After transcription, the pre-mRNA undergoes several modifications: 5' capping, 3' polyadenylation, and splicing. These modifications act as signals for export proteins to recognise and bind to the mature mRNA. The nuclear pore complex (NPC) serves as a gateway, allowing only mRNA with correct modifications to pass through. This checkpoint system ensures that only fully processed and functional mRNA reaches the cytoplasm, preventing potential translation of faulty or incomplete proteins.

The nucleus is enveloped by a double membrane known as the nuclear envelope, which establishes a barrier between the nuclear contents and the cytoplasm. This separation is critical for preserving the integrity of genetic material and controlling gene expression. The double membrane has nuclear pores, intricate structures that regulate the passage of molecules. Larger molecules, like proteins and RNA, require specific signals to pass through, while smaller molecules can diffuse freely. The selectivity ensures that only appropriate molecules, such as transcription factors and RNA, enter or exit the nucleus, maintaining the distinct environments and functions of the nucleus and cytoplasm.