Question

Describe differences that exist in how mRNA is synthesized in prokaryotes vs eukaryotes. Be sure to include differences that exist in the polymerases, regulatory sequences, protein subunits, initiation and termination

please be detailled and complete!

Answer 1

Difference between prokaryotic and eukaryotic transcription Sl. No. Prokaryotic Transcription Eukaryotic Transcription 1 Transcription and translation are continuous process and occurs simultaneously in the cytoplasm They are two separate processes, transcription occurs in the nucleus whereas translation occurs in the cytoplasm 2 Transcription initiation machinery is simple since DNA is not associated with any histone proteins Transcription initiation machinery is very complex since the genetic material is associated with proteins 3 Only one type of RNA polymerase enzyme, which synthesize all types of RNA in the cell (mRNA, rRNA and tRNA) Three types of RNA polymerase in the cell. RNA Polymerase I for rRNA synthesis RNA Polymerase II for mRNA synthesis. RNA polymerase III for tRNA and 5S rRNA synthesis 4 RNA polymerase with 5 subunits, Two α subunits, One β subunit, One β’ subunit, One ω subunit. Functional RNA polymerase is 2α1β1β’ω RNA polymerase I with 14 subunits, RNA polymerase II with 10 -12 subunits, RNA polymerase II with 12 subunits 5 σ factor present, which is essential for transcription initiation σ absent and it is not required for transcription initiation. Initiation of transcription is facilitated by initiation factors 6 RNA polymerase can recognize and bind to the promoter region with the help of σ factor RNA polymerase cannot recognize the promoter region directly unless the promoter is pre-occupied by transcription initiation factors. 7 Promoter region always located upstream to the start site Promoter region usually located upstream to the start site, but rarely as in the case of RNA polymerase III, promoter is located downstream to start site 8 Promoter region contain pribnow box at -10 positions. TATA box and CAT box are absent in the promoter region of prokaryotes Promoter region contains; TATA box located 35 to 25 upstream; CAT box located ~ 70 nucleotide upstream; GC box located ~ 110 nucleotide upstream. Pribnow box absent in eukaryotes 9 Termination of transcription is done either by rho dependent mechanisms or rho independent mechanisms A termination mechanism of transcription is not completely known. It may be direct by the poly A signal or by the presence of termination sequence in the DNA 10 Usually there is no post transcriptional modification of the primary transcript Primary transcript undergo post transcriptional modifications (RNA editing) 11 RNA capping absent, mRNA is devoid of 5’ guanosine cap RNA capping present, capping occurs at the 5’ position of mRNA 12 Poly A tailing of mRNA is absent Mature mRNA with a poly A tail at the 3’ position. Poly A tail is added enzymatically without the complementary strand 13 Introns absent in the mRNA Introns present in the primary transcript 14 Splicing of mRNA absent since introns are absent Splicing present, introns in the primary transcript are removed and exons are rejoined by a variety of splicing mechanisms 15 Genes usually polycistronic and hence single transcript may contain sequence for many polypeptides Genes are monocistronic thus single transcript code for only one polypeptide 16 SD sequence (Shine-Dalgarno sequence) present about 8 nucleotide upstream of start codon in the mRNA, SD sequence act as the ribosome binding site SD sequence is absent in mRNA of prokaryotes