All organisms, whether prokaryotes or eukaryotes, must regulate or control which genes are expressed at any given time. Regulation is essential for cell specialization in multicellular organisms, which are made up of different types of cells. Here are the key stages in the expression of a protein-coding gene and the many possible ways a eukaryotic cell can control gene expression. Let's look at the different control components and the roles that each one plays. A nucleosome is the basic unit of chromatin packing in a eukaryotic cell. One way nucleosomes control gene expression is through the unpacking and packing of the DNA, which causes a region of DNA to become either more or less accessible to the transcription machinery. The initiation of transcription is the most important stage for regulating gene expression. Transcription factors, some of which are activators, turn genes on by binding to specific regions of DNA and stimulating gene transcription. RNA processing in the nucleus also provides opportunities for regulating gene expression. The addition of nucleotides to the ends of the RNA, with the 5 prime cap and 3 prime tail, are important for ribosome binding and translation. Alternative RNA splicing, where spliceosome components splice out introns and join together the remaining exons, enables different proteins to be produced from the same mRNA transcript in different cells. Transporting the mRNA transcript across the nuclear membrane to the cytoplasm is an active process that requires the transcript to be recognized by receptors lining pores in the nuclear membrane. Even after a eukaryotic mRNA is fully processed and transported to the cytoplasm, there are several additional opportunities for regulation. The process of translating the mRNA into a polypeptide also offers opportunities for regulation. Among the molecules involved in translation are a great many proteins that control the start of polypeptide synthesis. RNA binding regulatory proteins play a key role in this process. Molecules of mRNA do not remain intact forever. RNA-degrading enzymes in the cytoplasm eventually break them down, and the timing of this event is an important factor regulating the amounts of various proteins that are produced in the cell. After translation is complete, some polypeptides require alterations before they become functional. Protein-phosphorylating enzyme plays an important role in the cleavage, chemical modification, and transport of proteins after translation. The final control mechanism after translation is the selective breakdown of proteins. The length of time each protein functions in the cell is strictly regulated by means of selective degradation.