<Tr> <Td> Polycistronic operon Regulatory sequence Regulatory sequence Enhancer Enhancer / silencer / silencer Operator Promoter 5'UTR ORF ORF UTR 3'UTR Start Start Stop Stop Terminator Transcription DNA RBS RBS Protein coding region Protein coding region mRNA Translation Protein The structure of a prokaryotic operon of protein - coding genes . Regulatory sequence controls when expression occurs for the multiple protein coding regions (red). Promoter, operator and enhancer regions (yellow) regulate the transcription of the gene into an mRNA . The mRNA untranslated regions (blue) regulate translation into the final protein products . </Td> </Tr> <P> The pre-mRNA molecule undergoes three main modifications . These modifications are 5' capping, 3' polyadenylation, and RNA splicing, which occur in the cell nucleus before the RNA is translated . </P> <P> Capping of the pre-mRNA involves the addition of 7 - methylguanosine (m G) to the 5' end . To achieve this, the terminal 5' phosphate requires removal, which is done with the aid of a phosphatase enzyme . The enzyme guanosyl transferase then catalyses the reaction, which produces the diphosphate 5' end . The diphosphate 5' end then attacks the alpha phosphorus atom of a GTP molecule in order to add the guanine residue in a 5'5' triphosphate link . The enzyme (guanine - N -) - methyltransferase ("cap MTase") transfers a methyl group from S - adenosyl methionine to the guanine ring . This type of cap, with just the (m G) in position is called a cap 0 structure . The ribose of the adjacent nucleotide may also be methylated to give a cap 1 . Methylation of nucleotides downstream of the RNA molecule produce cap 2, cap 3 structures and so on . In these cases the methyl groups are added to the 2' OH groups of the ribose sugar . The cap protects the 5' end of the primary RNA transcript from attack by ribonucleases that have specificity to the 3'5' phosphodiester bonds . </P> <P> The pre-mRNA processing at the 3' end of the RNA molecule involves cleavage of its 3' end and then the addition of about 250 adenine residues to form a poly (A) tail . The cleavage and adenylation reactions occur if a polyadenylation signal sequence (5' - AAUAAA - 3') is located near the 3' end of the pre-mRNA molecule, which is followed by another sequence, which is usually (5' - CA - 3') and is the site of cleavage . A GU - rich sequence is also usually present further downstream on the pre-mRNA molecule . After the synthesis of the sequence elements, two multisubunit proteins called cleavage and polyadenylation specificity factor (CPSF) and cleavage stimulation factor (CStF) are transferred from RNA Polymerase II to the RNA molecule . The two factors bind to the sequence elements . A protein complex forms and contains additional cleavage factors and the enzyme Polyadenylate Polymerase (PAP). This complex cleaves the RNA between the polyadenylation sequence and the GU - rich sequence at the cleavage site marked by the (5' - CA - 3') sequences . Poly (A) polymerase then adds about 200 adenine units to the new 3' end of the RNA molecule using ATP as a precursor . As the poly (A) tail is synthesised, it binds multiple copies of poly (A) binding protein, which protects the 3'end from ribonuclease digestion . </P>

Which of the following is not a step prior that occurs in the nucleus