What are the stages where gene expression is regulated?

Regulation of gene expression in eukaryotes involves the processes of transcription and RNA processing, both occurring within the nucleus. Additionally, regulation extends to protein translation in the cytoplasm. Further modifications to proteins through post-translational processes also contribute to regulatory mechanisms. Gene expression regulation in eukaryotes is categorized into five levels: epigenetic, translational, transcriptional, post-transcriptional, and post-translational. 

• The initial level of gene expression control is epigenetic regulation, which involves modifications of the genetic code. Epigenetics focuses on changes to genes that don't modify the DNA sequence and are not permanent. Rather than altering the nucleotide sequence, these changes affect the chromosomal structure, allowing genes to be activated or deactivated. This type of regulation is accomplished through enduring chemical modifications to DNA and/or proteins associated with chromosomes. 
• Transcriptional regulation determines whether or not mRNA is transcribed from a gene within a specific cell. Eukaryotic gene transcription relies on the binding of RNA polymerase to a promoter to initiate the process. However, in eukaryotes, transcription factors are needed to assist in the initiation of transcription by RNA polymerase. 
• The regulation of translation is primarily influenced by the mRNA molecule itself. The mRNA's stability significantly affects its translation into a protein. Additionally, regulation can occur at the level of the mRNA binding to the ribosome. Even after binding, the speed and extent of translation are still able to be controlled. 
• Post-transcriptional regulation takes place after the transcription of mRNA but before the initiation of translation. This form of regulation can impact mRNA through processes such as mRNA processing, attaching to ribosomes, or transportation from the nucleus to the cytoplasm. 
• The final level of gene expression control in eukaryotes is post-translational regulation. Modifications are made to the protein after its synthesis to influence its activity. An example of post-translational regulation is enzyme inhibition. When an enzyme is no longer required, it undergoes inhibition through an allosteric or competitive inhibitor, preventing it from binding to its substrate. Proteins' stability and function can also be controlled by introducing functional groups like  phosphate, methyl, or acetyl groups. 

Gene expression in prokaryotic cells can be regulated solely by controlling the level of transcription. Regulating the transcription of an operon in prokaryotes involves three mechanisms: repressive control, inducible control, and activator control. 

• Repressive control (e.g. trp operon) uses proteins attached to the operator sequence. These proteins physically block the binding of RNA polymerase, preventing the initiation of transcription. 
• Activator control (e.g. CAP) enhances the affinity of RNA polymerase for the promoter when CAP is attached. 
• Inducible operons contain proteins that bind to either activate or repress transcription based on the local environment and the cellular requirements. The lac operon serves as a typical example of an inducible operon.