In eukaryotes , the process of termination depends on the exact type of polymerase used during transcription . Termination can occur as soon as the RNA polymerase reaches the termination sequence - a specific series of nucleotides along the DNA template . In other instances , the presence of a special protein , a termination factor , is also required for termination to occur . Once termination is complete , the RNA molecule falls off the DNA template .
STEP 4 mRNA is the RNA destined to be translated for protein synthesis . In this case , the RNA produced near the end of transcription is called pre-mRNA . In eukaryotic cells , pre-mRNA undergoes extensive processing before it is ready to be translated . There are three main processing mechanisms :
● RNA splicing : Eukaryotic genes are composed of exons ( proteincoding nucleotide sequences ) and intervening sequences called introns ( non-coding nucleotide sequences which may be involved in gene regulation ). Newly synthesized pre-mRNA undergoes a process in which the introns are clipped out of the mRNA strand , thereby removing the nucleotides that are not involved in protein production . All of the pre-mRNA ’ s introns must be completely and precisely removed before protein synthesis begins . If the splicing process errs by even a single nucleotide , the resulting protein would be dysfunctional .
● The addition of stabilizing and signaling factors at both ends of the molecule :
• 5 ’ Capping : While the pre-RNA is still being synthesized , a special nucleoside cap is added to the 5 ’ end of the growing strand , protecting it from degradation . In addition , factors involved in protein synthesis recognize the cap and help initiate translation .
• 3 ’ Polyadenylation : Once elongation is complete , a poly-A polymerase enzyme adds a string of approximately 200 adenine nucleotides to the 3 ’ end of the pre-mRNA strand . This ‘ 3 ’ poly-A tail ’ further protects the pre-mRNA from degradation and also signals for the export of mRNA from the nucleus to the cytoplasm of the cell .
At the end of transcription , the mRNA molecule , ready for translation , leaves the nucleus and enters the cytoplasm through pores in the nuclear membrane .