Question

Compare and contrast the process of gene transcription in prokaryotes and eukaryotes. Make specific reference to differences in transcription initiation, elongation and termination

Answer 1

Similarities between prokaryotic and eukaryotic transcription
1. In both groups DNA acts as the template for RNA synthesis
2. In both groups transcription produces RNA molecule
3. Chemical composition of transcript is similar in both groups
4. Transcription is facilitated by the enzyme RNA polymerase in both groups
5. In both groups, one strand of the DNA duplex acts as the template.

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 ? subunits, One ? subunit, One ?’ subunit, One ? subunit. Functional RNA polymerase is 2?1?1?’?	RNA polymerase I with 14 subunits, RNA polymerase II with 10 -12 subunits, RNA polymerase II with 12 subunits
5	? factor present, which is essential for transcription initiation	? 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 ? 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 tothe start site, but rarely as in the case of RNA polymerase III, promoter is located downstream to start site
8	Promotor region contain pribnow box at -10 positions.TATA box and CAT box are absent in the promoter of prokaryotes.	Promoter region contains : TATA box located 35 to 25 upstream: CAT box located nearly 70 nucleotide upstream: GC box located nearly 110 nucleotide upstream.Pribnow box is 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